For homeowners, building owners, and facilities managers in snow country

Snow does not load a roof evenly, and the uneven part is the dangerous one.

All roof types · single-family through light commercial

A depth measurement describes the flat average. Drifts, slides, and ice put several times that much weight in specific places — and FEMA is explicit that an unbalanced load is a greater risk to a roof structure than a uniform one.

30-second answer

How much snow can my roof hold, and what should I actually watch for?

Nobody can answer the capacity question from a website. What is answerable: snow loads a roof unevenly. Wind drifts it against walls, parapets, dormers and roof steps; upper slopes slide onto porch roofs below; ice builds at cold eaves. FEMA states that unbalanced loading poses a greater risk to a roof structure than a uniform load. If the building is showing distress, leave it and call a structural engineer.

Stop — immediate hazard

If a building shows any of these, get everyone out before anything else.

That list is FEMA’s, from the Snow Load Safety Guide, and its instruction after it is unambiguous: if any of those warning signs are observed, “the building should be promptly evacuated and a qualified design professional should be contacted to perform a detailed structural inspection.” Wood and steel roofs usually deflect visibly before they let go. That deflection is the warning, and it is the reason to be outside rather than up a ladder.

The second hazard is the removal work itself. OSHA’s hazard alert on rooftop snow removal records that falls cause the most worker fatalities and injuries in this work — off roof edges, through snow-covered skylights, and from ladders and aerial lifts — and that workers may also be injured or killed by a roof collapse while they are on it. It also records that people standing on the ground, including bystanders, “can become trapped under snow falling from roofs and could suffocate.” Those are trained people with fall protection and a written plan.

The third is the one nobody expects. A snow rake handle is long and frequently metal. OSHA’s guidance is to use extendable non-conductive poles and to keep 10 feet between a snow rake and any overhead power line, and the alert describes a worker electrocuted after contacting a 12,000-volt line during a rooftop snow and ice removal. If a service drop runs anywhere near the eave you were thinking of raking, that is a call to a contractor or to your electric utility, not a job.

Sagging or bowing ceilings, ceiling tiles dropping out of a grid, sprinkler heads pushed down below the ceiling, popping or cracking or creaking noises, a roof line or truss chord that has visibly bent, doors or windows that suddenly will not open or close, new cracks in walls or masonry, or severe new leaks. Leave the building, keep everyone out from under the affected area, and call a licensed structural engineer. Do not go up to look.

Learning paths and saved lessons

At a glance

The short versionSection link

There is no national snow load and no universal rule on this page. Design values are set by the code adopted where the building is, and the numbers below are either published figures with their source named or the variables that decide the answer. None of them is a determination about a specific roof.

Ground snow load: A design value for a location, not a measurement of this winterFEMA: ground snow load is the weight of snow on the ground surface, established from National Weather Service data. Writing in 2013 against ASCE 7-10 and the 2012 IBC, FEMA records that those maps indicated a 2 percent probability of the mapped load being equalled or exceeded in any given year, and do not discount that actual loads may exceed them. ASCE has since revised the basis: ASCE/SEI 7-22 publishes reliability-targeted ground snow loads instead. Which instrument applies is set by the edition your jurisdiction adopted.
Roof snow load: Derived from the ground value, then adjusted for the buildingFEMA lists the inputs: ground snow load, the importance and use of the building, the roof's wind exposure, roof slope, roof shape, roof obstructions, and the thermal condition of the building. OSHA puts the same point more bluntly — ground snow load gives only a rough indication of roof snow load.
Weight of one foot of snow: 3 lb/ft² light and dry, up to 21 lb/ft² wet and heavyFEMA P-957, citing Gooch (1999). A seven-fold spread on the same depth measurement, which is why depth alone is not a load.
Weight of ice: A little under 5 lb/ft² per inch of thickness; roughly 57 lb/ft² per footFEMA P-957, whose wording is that one inch of ice weighs “a little less than 5 pounds per square foot” and one foot approximately 57. The Metal Construction Association gives about 5 lb/ft² per inch independently. Ice is heavier per inch than wet, heavy snow, which is why an ice-loaded eave is a load problem and not only a leak problem.
Where a drift forms: Roof steps, parapets, walls, dormers, chimneys, rooftop equipment, valleysFEMA: wind moves snow from one part of a roof to another, and depth at the obstruction is greater than the overall roof snow depth, so the load at the drift is larger than the load elsewhere.
When drift design entered US codes: Drift loads in BOCA from 1975; unbalanced roof snow load in the UBC from 1988FEMA P-957. Anchorage's building official puts the local consequence plainly: the 1988 UBC was adopted there in January 1989, so buildings constructed before 1990 were likely not designed to carry the weight of snow drifts.
Angle of repose for snow: Roughly a 30-degree roof slope — about 6:12 or 7:12FEMA: above that slope, snow slides. Below it, snow can still slide — the angle of repose is the maximum angle at which snow will not slide, not a guarantee about anything shallower.
An ice barrier: Limits damage from ice at the eave. It does not reduce load.It is a waterproof underlayment beneath the covering. Whether one is required at your address, and how far it must run, is set by your jurisdiction's adopted code and its Table R301.2 designation.

Tradeoffs

This page's advice — plan for the concentrations, not the average — and where it is wrongSection link

The position here is that the useful winter work on a building happens before the snow: know the design load, know where this roof concentrates it, fix the drainage and the ceiling plane, and buy removal rather than doing it. There are real cases where a different order is right.

Best when

The building is a shape that concentrates snow: a roof step, a lower roof under a taller wall, a parapet, dormers, a saw-tooth, or rooftop equipment sitting in the wind shadow of something taller.
There is a lower roof — a porch, a sunroom, an entrance canopy, an addition — directly under a slope that sheds.
The building predates 1990, when drift and unbalanced snow loads were still working their way into US model codes.
Construction drawings exist, so the design snow load can be read off them instead of guessed at.
The covering is slippery — standing seam metal, a single-ply membrane — and something below the eave matters: an entry, a walkway, a meter, a neighbour's property.
Ice reliably builds at the same eave every winter, which means the heat source producing it is a fixed feature of the building.

Think twice if

There is distress in the building right now. Then none of this applies: leave, keep clear, and call an engineer. Planning is for a dry autumn.
The roof is a simple, exposed, steep gable with no steps, no dormers and nothing below the eaves. The concentration story still exists but it is a much smaller share of the risk, and the money is better spent on the ceiling plane.
You are in a marginal-snow region where the design event is rain-on-snow rather than deep accumulation. FEMA notes that code provisions incorporate light rain on snow but not heavy rainfall, and that a short rain can saturate a snow layer and significantly increase the load — that is a drainage problem more than a drift problem.
The roof is low-slope with internal drains. There the dominant failure chain is blocked drains, meltwater ponding, and deflection, which is a maintenance question before it is a snow question.
Adding snow guards is being considered on an old roof of unknown structure. Retaining snow that used to slide off is a decision to keep more load up there, and it is one for a design professional, not a product catalogue.
You rent, or you are selling in the spring. Pre-season structural assessment is the right answer for a building and the wrong answer for a short horizon; say so rather than pretending otherwise.

What changes the answer

The design snow load the building was actually built to, and the code edition in force when it was built.
Roof geometry: every step, parapet, dormer, chimney, valley, and piece of rooftop equipment is a place snow collects.
Wind exposure. FEMA states that wind exposure has the largest effect on roof snow load of the variables it discusses — an open site sheds snow, and then drifts it hard at every obstruction.
Covering material. FEMA groups asphalt shingles and aggregate-surfaced built-up roofs as tactile and slow to shed, and metal panels and single-ply membranes as slippery.
Whether the assembly is a cold vented attic, a correctly designed unvented assembly, or a warm leaky ceiling — because that decides whether snow stays put, melts, or dams.
What is below the eaves, and whether it can be moved or made inaccessible instead of guarded.
The adopted code edition and local amendments, which set the ground snow load, the ice-barrier trigger and extent, and everything else that is a requirement rather than a preference.
Span and framing type. FEMA notes long-span systems may have less structural redundancy than short-span ones, which makes failure more catastrophic when it comes.

The mechanism

Six places the same storm puts different amounts of weightSection link

A depth measurement describes the balanced load — the flat average. Wind, slope and ice redistribute that average, and it is the concentrations, not the average, that FEMA identifies as the greater risk to a structure.

A stepped house in section, wind from the left. Number 1 is the balanced blanket everybody pictures. Numbers 2 to 6 are the places the same storm concentrates the load. Hatching marks snow that is denser as well as deeper. Schematic, not to scale, and not a construction detail.Original diagram, Understanding Roofing.

A snow load is a weight per unit of roof area, in pounds per square foot. It sits on top of the dead load — the permanent weight of the structure and the covering — and it is the largest variable load most cold-climate roofs ever carry. Two feet of snow on a roof is not a load. Two feet of snow of a stated density, distributed the way this particular roof distributes it, is closer.

FEMA’s Snow Load Safety Guide puts the reason plainly. The uniform roof snow load a designer starts from “is a value determined for a flat, wide open roof free of obstructions and protrusions. Rarely is a roof such.” Everything below is what happens on the roofs that are not such.

1 · The balanced load, which is the number people quote

An even blanket over the whole plane. This is what a yardstick in the middle of the roof measures, what a news report means by “eighteen inches,” and what most people are picturing when they ask whether their roof can take it. It is a real load, and it is the one a designer starts from. It is not the whole picture, and this page is about the rest of it.

2 · The drift at a roof step — the most under-rated load on any building

Wind transports snow off one part of a roof and deposits it somewhere sheltered. FEMA: “Snow drifts often form on a lower roof in the wind shadow of the higher portions of the building. Snow can also be blown up against and accumulate next to an obstruction (e.g., high roof framing, rooftop equipment, parapet, adjacent building, dormer windows). The snow depth at this obstruction is greater than the overall roof snow depth and therefore creates a larger load on the roof structure at the location of the drift.”

This is what unbalanced loading means, and FEMA is explicit about why it is worse than a heavier uniform load: “Unbalanced snow load poses a greater risk to the roof structural system than a uniform snow load.” A structure designed for an even load across a span is not automatically fine when the same total weight is stacked over one end of it.

It is also the load with the shortest history in American codes. Drift loads entered the BOCA code in 1975; unbalanced roof snow loads did not appear in the Uniform Building Code until 1988. FEMA draws the conclusion for the reader: “A building constructed 40 years ago may not have been designed for snow loads as they are understood today.”

3 · Snow sliding from an upper roof onto a lower one

Where a steeper roof discharges onto a porch, sunroom, addition, or entrance canopy, the lower roof gets a double penalty. It receives more depth than it would have collected on its own — and it receives it as an impact. FEMA: “the dynamic force of the sliding snow onto the lower roof may produce a significant impact force on the lower roof framing, which can potentially overload the roof structure.”

FEMA singles out exactly these structures as the vulnerable ones: “Entrance canopies and porch roofs are particularly susceptible to drifting and sliding snow because they are adjacent to the main building structure. These structures are often building additions in which design considerations for the main structure may not have been accounted for.” If a porch roof was built by a previous owner in a summer, that sentence is about your house.

4 · The drift downwind of the ridge

On a gable or mono-slope roof, the ridge itself creates what FEMA calls an aerodynamic shade — a pocket of slow air on the leeward side where blown snow drops out. The windward slope scours and the leeward slope loads up. The same effect happens downwind of mechanical penthouses, stair towers, and screen walls.

5 · Ice at the cold eave

An ice dam is usually discussed as a leak. It is also a load, and a concentrated one. FEMA describes freeze-thaw cycles producing “higher concentrated loads at roof low points (if not properly drained) and at eaves on sloped roofs,” and notes the second consequence: ice dams “prevent snow from sliding off the roof. Snow and ice accumulation at the eave creates an undesirable unbalanced snow loading condition.”

The Metal Construction Association makes the same point from the covering side: an ice dam creates unintended, unpredictable snow retention, and “many structures and roof materials are not designed to support an ice build-up of several feet, which has been observed on some roofs.” Anchorage’s building official lists “ice formation can overload structural elements” among the reasons that office publishes snow guidance at all.

The mechanism that produces the ice — heat leaking out of the house, snow melting on the warm part of the deck, meltwater refreezing where the deck is over outdoor air — is worked through properly on the ice dam page. This page is about what the ice weighs and what it stops from sliding.

6 · The drop zone

Everything that leaves a roof arrives somewhere. Anchorage’s handout is the least euphemistic sentence found anywhere in this research: “Falling snow and ice has caused fatalities in Anchorage and represents a significant hazard.” It continues: “Metal roofs can avalanche the entire snow load in an instant, without warning. This typically happens on sloped (pitched) roofs when atmospheric conditions warm sufficiently to break the bond between the roof and snow.”

That is not a metal-roof problem so much as a slippery-surface problem with a temperature trigger, and the trigger is a warm spell, not a storm. The dangerous day is often the mild one after the weather has passed and everybody has relaxed.

The distinction the whole page rests on

Ground snow load is not roof snow load, and neither is a measurementSection link

Two different numbers, both often called 'the snow load', doing two different jobs. Mixing them up is how a reasonable conversation about a roof goes wrong in the first sentence.

Ground snow load (engineers write it p_g) is the weight of snow on the ground surface at a location, expressed in pounds per square foot. FEMA: values are “established using data collected by the National Weather Service.” Writing in 2013 against ASCE 7-10 and the 2012 IBC, FEMA records that the maps in those documents “indicate a 2 percent probability of the indicated load being equaled or exceeded in any given year.” It is a design statistic, not a forecast, and FEMA adds the caution that matters: those loads “do not discount that actual snow loads may exceed them, only that the risk of snow-load-induced failure is reduced to an acceptably low level.”

That instrument has since been rebuilt, which is a reason to check the edition rather than repeat the figure. ASCE lists among the changes in ASCE/SEI 7-22 “revised ground snow loads to reflect more recent snow load data and reliability-targeted values” — a different derivation from the older mapped values, and one that reaches a building only through whichever edition its jurisdiction has adopted. What has not changed is the part that matters to a reader: both are design quantities, and neither is a measurement of the snow on a roof.

Roof snow load is what a designer actually applies to the structure. It is derived from the ground value and then adjusted for the building itself.

The two values people call the snow load, and what each one is. Definitions and factor list quoted from FEMA P-957 and the OSHA rooftop snow removal hazard alert.
	Ground snow load	Roof snow load
What it describes	The weight of snow on the ground surface at a location.	The weight of snow on the roof surface, used to design the structure.
Where it comes from	National Weather Service data, published as maps in ASCE 7 and the IBC — or, in some states, replaced outright by a value set in state rule.	Derived from the ground value by a structural engineer applying the code adopted where the building is.
What it depends on	Location, and the code edition the jurisdiction adopted. FEMA notes that season, altitude and humidity produce a wide range of regional snow characteristics, and that local authorities and structural engineers are the people most familiar with them.	FEMA's list: the ground snow load value; the importance, occupancy, and use of the building; the wind exposure of the roof; roof slope; roof shape; roof obstructions; and the thermal condition of the building.
Whether it is a measurement	No. It is a design value. Under the ASCE 7 editions FEMA cites, one with a stated 2 percent annual exceedance probability; ASCE/SEI 7-22 derives it from a reliability target instead.	No. It is a design value too, and the load on the roof this week is a different question again.
How the two relate	The starting point.	OSHA: ground snow load “can provide a rough indication of roof snow load, but roof snow loads also depend upon factors such as melting and re-freezing of snow and ice, drifting, roof slope, type of roof, and design features.”
Who determines it for a building	The adopted code, via a map or a rule. Not the owner.	A licensed design professional. FEMA: where no drawings exist, retain one before the snow season to determine structural capacity.

Read this table one item at a time

What it describes

Ground snow load: The weight of snow on the ground surface at a location.
Roof snow load: The weight of snow on the roof surface, used to design the structure.

Where it comes from

Ground snow load: National Weather Service data, published as maps in ASCE 7 and the IBC — or, in some states, replaced outright by a value set in state rule.
Roof snow load: Derived from the ground value by a structural engineer applying the code adopted where the building is.

What it depends on

Ground snow load: Location, and the code edition the jurisdiction adopted. FEMA notes that season, altitude and humidity produce a wide range of regional snow characteristics, and that local authorities and structural engineers are the people most familiar with them.
Roof snow load: FEMA's list: the ground snow load value; the importance, occupancy, and use of the building; the wind exposure of the roof; roof slope; roof shape; roof obstructions; and the thermal condition of the building.

Whether it is a measurement

Ground snow load: No. It is a design value. Under the ASCE 7 editions FEMA cites, one with a stated 2 percent annual exceedance probability; ASCE/SEI 7-22 derives it from a reliability target instead.
Roof snow load: No. It is a design value too, and the load on the roof this week is a different question again.

How the two relate

Ground snow load: The starting point.
Roof snow load: OSHA: ground snow load “can provide a rough indication of roof snow load, but roof snow loads also depend upon factors such as melting and re-freezing of snow and ice, drifting, roof slope, type of roof, and design features.”

Who determines it for a building

Ground snow load: The adopted code, via a map or a rule. Not the owner.
Roof snow load: A licensed design professional. FEMA: where no drawings exist, retain one before the snow season to determine structural capacity.

Neither column is a statement about how much snow is on any roof right now. Both are design quantities. The distance between a design quantity and this morning's roof is the reason this page keeps sending structural questions to an engineer.

The values can be set by rule rather than read off a map

Minnesota does exactly this. The residential code’s design-criteria section, Minnesota Rules part 1309.0301, states that “The ground snow loads to be used in determining the design snow loads for buildings and other structures are given in Minnesota Rules, part 1303.1700.” Part 1303.1700 then sets the number by county: 60 pounds per square foot in twenty-nine named counties, and 50 pounds per square foot in every other county in the state.

That is worth sitting with, because it is the clearest available proof that there is no national answer here. The same building, on the same drawings, is designed to a different load on either side of a county line in one state — and to a value derived from an entirely different instrument in the next state over. Whether your authority having jurisdiction reads a map, a rule, or a locally amended table is a question with one correct answer and it is not on this website.

A worked example

Why “we got eighteen inches” is not a loadSection link

Run the same depth measurement through the published density range and the honest answer spans a factor of seven. Then put a drift next to it and the spread gets worse. This is arithmetic to show the size of the uncertainty, not a calculation anyone should act on.

FEMA publishes the range: “The weight of 1 foot of fresh snow ranges from 3 pounds per square foot for light, dry snow to 21 pounds per square foot for wet, heavy snow.” It also publishes the ice figures — about 5 lb/ft² per inch of thickness, roughly 57 lb/ft² per foot — and notes that ice weighs significantly more per inch than heavy wet snow.

Anchorage’s building official supplies a useful anchor at the dense end of that range from the other direction, describing a drift weighing more than 40 lb/ft² as “roughly 2 feet of dense, wind-worked snow” — about 20 lb/ft² per foot, which lands near the top of FEMA’s range and is what wind-packed drift snow behaves like.

Multiply depth in feet by density in pounds per square foot per foot of depth. That is the whole calculation. The interesting part is the width of the answer.

What a depth measurement can mean, using the density figures published in FEMA P-957 and the dense-snow anchor published by the Municipality of Anchorage. Illustrative arithmetic only — not a determination about any roof.
What is on the roof	Depth	Light, dry snow — 3 lb/ft² per foot	Dense, wind-worked snow — about 20 lb/ft² per foot	Wet, heavy snow — 21 lb/ft² per foot
The field of the roof, freshly fallen	18 in (1.5 ft)	4.5 lb/ft²	30 lb/ft²	31.5 lb/ft²
The same snow after a thaw and refreeze	Shallower — perhaps 12 in	Not applicable; a settled blanket is no longer light and dry	20 lb/ft²	21 lb/ft²
A drift at a roof step, in the same storm	4 ft	12 lb/ft²	80 lb/ft²	84 lb/ft²
Ice built up at a cold eave	4 in	About 20 lb/ft², at FEMA's roughly 5 lb/ft² per inch — density range does not apply to solid ice	About 20 lb/ft²	About 20 lb/ft²

Read this table one item at a time

The field of the roof, freshly fallen

Depth: 18 in (1.5 ft)
Light, dry snow — 3 lb/ft² per foot: 4.5 lb/ft²
Dense, wind-worked snow — about 20 lb/ft² per foot: 30 lb/ft²
Wet, heavy snow — 21 lb/ft² per foot: 31.5 lb/ft²

The same snow after a thaw and refreeze

Depth: Shallower — perhaps 12 in
Light, dry snow — 3 lb/ft² per foot: Not applicable; a settled blanket is no longer light and dry
Dense, wind-worked snow — about 20 lb/ft² per foot: 20 lb/ft²
Wet, heavy snow — 21 lb/ft² per foot: 21 lb/ft²

A drift at a roof step, in the same storm

Depth: 4 ft
Light, dry snow — 3 lb/ft² per foot: 12 lb/ft²
Dense, wind-worked snow — about 20 lb/ft² per foot: 80 lb/ft²
Wet, heavy snow — 21 lb/ft² per foot: 84 lb/ft²

Ice built up at a cold eave

Depth: 4 in
Light, dry snow — 3 lb/ft² per foot: About 20 lb/ft², at FEMA's roughly 5 lb/ft² per inch — density range does not apply to solid ice
Dense, wind-worked snow — about 20 lb/ft² per foot: About 20 lb/ft²
Wet, heavy snow — 21 lb/ft² per foot: About 20 lb/ft²

Read across the second row rather than down it. A blanket that settled from 18 inches to 12 has not shed weight — MCA describes the thaw cycle passing meltwater down through the blanket and refreezing it into a crust, so that “the moisture and all of the weight are still present, however much the depth of the blanket is reduced dimensionally.” The roof looks better and is carrying the same load, now in a harder slab that is better bonded and more likely to release all at once.

What the numbers in that table are, and what they are not

They are a spread, not an estimate. The same eighteen inches is 4.5 lb/ft² or 31.5 lb/ft² depending on what fell. Nobody standing on the ground can close that gap by looking, which is the point.
The drift row is the one that should be uncomfortable. Four feet of wind-packed snow at a roof step is in the same range as Anchorage’s entire minimum design load, sitting over one part of a span rather than spread across it — and FEMA is explicit that unbalanced loading is the more dangerous arrangement.
None of it says anything about capacity. Load and capacity are two separate quantities, and this page has deliberately published nothing about the second one.
Nothing here is a reason to go up and measure. Depth read from a window, from a photograph, or off a neighbouring surface is enough for the only decision this arithmetic supports, which is whether to phone somebody.

The under-appreciated factor

Every obstruction on a roof is a place snow collectsSection link

Drifting is the mechanism that turns an ordinary winter into a structural event, and it is almost entirely a function of shape. Read your own roof against this list from the ground.

Where drifting and sliding concentrate snow on a roof, and why. Locations and mechanisms quoted from FEMA P-957 unless noted.
Feature	What the wind or the slope does there	What it looks like from the ground
A lower roof beside a taller section	The classic roof step. Snow drifts on the lower roof in the wind shadow of the higher portion, and can also be blown up against the tall wall from the other direction.	A bank of snow noticeably deeper against the wall than out on the open part of the low roof. The most common and most consequential pattern on this list.
Parapets	FEMA names parapets on flat and low-slope roofs, and at changes in roof elevation on stepped roofs, as common locations for snow drifts.	On a low-slope roof, snow standing higher along the perimeter than in the field. Often visible only from an upper window of a neighbouring building.
The leeward side of a ridge	A geometric feature that blocks blowing snow creates what FEMA calls an aerodynamic shade. On gable and mono-slope roofs this occurs on the leeward side of the ridge.	One slope scoured nearly bare while the other holds a deep cornice just below the ridge. A reliable indicator of the prevailing wind at your site.
Dormers and chimneys	Irregularities on residential roofs — chimneys, dormer windows, porch roofs, skylights — collect drifts against their uphill and sheltered faces.	Snow piled against a dormer cheek or the uphill side of a chimney, and holding there after the open slope has cleared.
Valleys, and the low points of saw-tooth roofs	Valleys of saw-tooth roofs and the intersections of sloping residential roofs accumulate greater snow depth than elsewhere on the roof.	A deep white channel running down a valley long after the planes on either side have shed. Also the place ice tends to build away from the eave.
Rooftop equipment, screens, and solar arrays	Mechanical screen walls, rooftop vents, skylights, heating and cooling units, and solar panels all provide a place for snow to drift, and rooftop protrusions such as mechanical penthouses and stair towers cast their own aerodynamic shade.	Snow standing around and downwind of equipment on a low-slope roof. On houses, snow held against and below an array long after the rest of the slope is clear.
Porch roofs, sunrooms, and entrance canopies	They both drift and receive sliding snow, and FEMA notes the dynamic impact force of a slide onto a lower roof can potentially overload the framing.	A mound of snow on the lower roof much deeper than the main roof above it. Often the single deepest snow on the property.
Cold eaves, where ice builds	Freeze-thaw cycles produce concentrated ice load at eaves and at undrained low points, and the resulting dam prevents snow sliding off, creating an unbalanced load.	A thickening white edge with heavy icicles. Gutters visibly pulling away from the fascia is a load symptom, not a gutter defect.

Read this table one item at a time

A lower roof beside a taller section

What the wind or the slope does there: The classic roof step. Snow drifts on the lower roof in the wind shadow of the higher portion, and can also be blown up against the tall wall from the other direction.
What it looks like from the ground: A bank of snow noticeably deeper against the wall than out on the open part of the low roof. The most common and most consequential pattern on this list.

Parapets

What the wind or the slope does there: FEMA names parapets on flat and low-slope roofs, and at changes in roof elevation on stepped roofs, as common locations for snow drifts.
What it looks like from the ground: On a low-slope roof, snow standing higher along the perimeter than in the field. Often visible only from an upper window of a neighbouring building.

The leeward side of a ridge

What the wind or the slope does there: A geometric feature that blocks blowing snow creates what FEMA calls an aerodynamic shade. On gable and mono-slope roofs this occurs on the leeward side of the ridge.
What it looks like from the ground: One slope scoured nearly bare while the other holds a deep cornice just below the ridge. A reliable indicator of the prevailing wind at your site.

Dormers and chimneys

What the wind or the slope does there: Irregularities on residential roofs — chimneys, dormer windows, porch roofs, skylights — collect drifts against their uphill and sheltered faces.
What it looks like from the ground: Snow piled against a dormer cheek or the uphill side of a chimney, and holding there after the open slope has cleared.

Valleys, and the low points of saw-tooth roofs

What the wind or the slope does there: Valleys of saw-tooth roofs and the intersections of sloping residential roofs accumulate greater snow depth than elsewhere on the roof.
What it looks like from the ground: A deep white channel running down a valley long after the planes on either side have shed. Also the place ice tends to build away from the eave.

Rooftop equipment, screens, and solar arrays

What the wind or the slope does there: Mechanical screen walls, rooftop vents, skylights, heating and cooling units, and solar panels all provide a place for snow to drift, and rooftop protrusions such as mechanical penthouses and stair towers cast their own aerodynamic shade.
What it looks like from the ground: Snow standing around and downwind of equipment on a low-slope roof. On houses, snow held against and below an array long after the rest of the slope is clear.

Porch roofs, sunrooms, and entrance canopies

What the wind or the slope does there: They both drift and receive sliding snow, and FEMA notes the dynamic impact force of a slide onto a lower roof can potentially overload the framing.
What it looks like from the ground: A mound of snow on the lower roof much deeper than the main roof above it. Often the single deepest snow on the property.

Cold eaves, where ice builds

What the wind or the slope does there: Freeze-thaw cycles produce concentrated ice load at eaves and at undrained low points, and the resulting dam prevents snow sliding off, creating an unbalanced load.
What it looks like from the ground: A thickening white edge with heavy icicles. Gutters visibly pulling away from the fascia is a load symptom, not a gutter defect.

Two structural notes ride along with this list. FEMA identifies wind exposure as having the largest effect on roof snow load of the variables it discusses — an open site sheds more but drifts harder at every obstruction. And it warns that secondary structures, including canopies, porches, carports, detached garages and sheds, do not typically perform as well as primary buildings during snow events.

Age is a proxy for whether drift was designed for at all

Drift loads reached the BOCA code in 1975. Unbalanced roof snow loads did not enter the Uniform Building Code until 1988. FEMA’s summary is that “only relatively recently have drifting and sliding snow loads been addressed in building codes,” and that a building constructed forty years ago may not have been designed for snow loads as they are understood today.

Anchorage turns that into an operational rule for its own building stock: because the 1988 UBC took effect there in January 1989, buildings constructed before 1990 “were likely not designed to accommodate the weight of snow drifts,” and its guidance is that such buildings should be periodically inspected for drift formation, with drifting heavier than 40 lb/ft² removed unless the roof can be shown capable of carrying it.

That is one jurisdiction’s threshold for one class of local buildings, and it is not transferable as a number. What is transferable is the reasoning: the older the building, the more likely that drift was never in the design at all, and the more valuable a pre-season structural assessment becomes.

What leaves the roof

Sliding snow, and where snow guards are genuinely neededSection link

Snow retention is a real engineering product with a narrow, well-defined job. It is also sold as a decoration for eaves that had no problem. The difference is whether anyone surveyed what is underneath.

Safety first

A slab of snow leaving a roof does not give a warning, and the trigger is warm weather.

Anchorage’s building official records that falling snow and ice has caused fatalities there, and that metal roofs “can avalanche the entire snow load in an instant, without warning” when conditions warm enough to break the bond between the panel and the snow. MCA records that serious accidents and even fatalities have resulted from rooftop avalanches. OSHA records that people on the ground — workers and bystanders — can be trapped under snow falling from roofs and suffocate.

The instruction from Anchorage is two options, not three: install snow guards to retain the snow on the roof, or block access to the area. For ice and icicles hanging over areas people and pets reach: remove the ice, or block access. Deciding to hope is not on the list.

After a heavy fall, keep entries, walkways, parking spaces, and anything below a sloped eave clear during the first thaw — not during the storm. If ice or snow is overhanging somewhere people or pets go, block the area off and have it removed by a contractor. Do not stand under it to knock it down.

What a snow guard actually is

A snow guard or snow retention system is a set of devices fixed to a roof to hold the snow blanket in place so it leaves slowly by melting and evaporating rather than suddenly as a slide. MCA’s snow-retention bulletin — a separate document from its cold-climate one — describes every such system as “a load chain to transfer the sliding forces of snow into the building structure,” and adds the consequence: “For proper design, each link in this chain must be proven to withstand the forces to which it will be exposed.”

That framing does two useful things. It explains why snow guards are engineered rather than chosen — the loads are calculated, the product’s tested capacity is known, and the spacing follows from dividing one by the other. And it explains why retaining snow is a structural decision: a guard’s whole purpose is to keep weight on the roof that used to leave it.

Where retention or exclusion is genuinely warranted, in the order the sources put them. Design hierarchy from the Metal Construction Association's cold-climate bulletin; the two-option instruction from the Municipality of Anchorage.
What is below the shedding slope	First choice	If that is not possible	Source language
An entry, doorway, or exit	Put the entrance under a gable end rather than under an eave.	Retain the snow above it — and size the tributary area as a wedge, not a rectangle.	MCA: “Building entrances should be beneath gables, not eaves.” On isolated assemblies: the retained bank “will generally resemble a wedge, not a rectangle, hence tributary areas may be much larger than first anticipated.”
A walkway, path, or steps	Route pedestrian traffic out of the shed zone.	Make the drop zone inaccessible, or retain.	MCA: “Pedestrian and vehicular traffic patterns must be routed away from potentially dangerous snow-shed zones… Drop zones should be made inaccessible to pedestrian and vehicular traffic.”
A gas meter, regulator, flue, or vent terminal	Retain the snow above it, or relocate the fixture.	Block access is not an option here — the fixture cannot move out of the way.	MCA: any permanent structure or fixture within the trajectory of sliding snow “must be designed to withstand the anticipated impact,” including “incidental mechanicals.” Anchorage names ice over accessible areas as a significant hazard.
A lower roof — porch, sunroom, addition, canopy	Design the lower roof for the discharge it will receive.	Retain the snow on the upper slope so the lower roof is not the catcher.	MCA: “When a lower roof has an eave above that is not protected with snow guards, the discharge of sliding snow can also cause increased loading to the lower roof. Design roof snow loads will be increased in such areas.”
Parking, vehicles, and equipment	Design parking layout around the shed zones.	Mark and exclude, seasonally if not permanently.	MCA: “Locations of ingress and egress, as well as parking area design should anticipate this snow slide.”
A neighbour's property, or a lot line	Retain. The area cannot be excluded because it is not yours to exclude.	There is no second option; this is the case where retention is the only mechanism available.	MCA lists landscaping and vegetation among what shedding snow damages, and states that where natural shed cannot be accommodated in the design, snow retention devices should be employed.
Nothing — an open slope over ground nobody uses	Let it shed, and keep it that way.	Retaining here adds load to the roof and buys nothing.	MCA permits shed-at-will “under limited circumstances,” but warns that “due to higher unpredictability and the increased hazards associated with the ‘shed-at-will’ philosophy, only in carefully selected situations should snow retention devices be omitted.”

Read this table one item at a time

An entry, doorway, or exit

First choice: Put the entrance under a gable end rather than under an eave.
If that is not possible: Retain the snow above it — and size the tributary area as a wedge, not a rectangle.
Source language: MCA: “Building entrances should be beneath gables, not eaves.” On isolated assemblies: the retained bank “will generally resemble a wedge, not a rectangle, hence tributary areas may be much larger than first anticipated.”

A walkway, path, or steps

First choice: Route pedestrian traffic out of the shed zone.
If that is not possible: Make the drop zone inaccessible, or retain.
Source language: MCA: “Pedestrian and vehicular traffic patterns must be routed away from potentially dangerous snow-shed zones… Drop zones should be made inaccessible to pedestrian and vehicular traffic.”

A gas meter, regulator, flue, or vent terminal

First choice: Retain the snow above it, or relocate the fixture.
If that is not possible: Block access is not an option here — the fixture cannot move out of the way.
Source language: MCA: any permanent structure or fixture within the trajectory of sliding snow “must be designed to withstand the anticipated impact,” including “incidental mechanicals.” Anchorage names ice over accessible areas as a significant hazard.

A lower roof — porch, sunroom, addition, canopy

First choice: Design the lower roof for the discharge it will receive.
If that is not possible: Retain the snow on the upper slope so the lower roof is not the catcher.
Source language: MCA: “When a lower roof has an eave above that is not protected with snow guards, the discharge of sliding snow can also cause increased loading to the lower roof. Design roof snow loads will be increased in such areas.”

Parking, vehicles, and equipment

First choice: Design parking layout around the shed zones.
If that is not possible: Mark and exclude, seasonally if not permanently.
Source language: MCA: “Locations of ingress and egress, as well as parking area design should anticipate this snow slide.”

A neighbour's property, or a lot line

First choice: Retain. The area cannot be excluded because it is not yours to exclude.
If that is not possible: There is no second option; this is the case where retention is the only mechanism available.
Source language: MCA lists landscaping and vegetation among what shedding snow damages, and states that where natural shed cannot be accommodated in the design, snow retention devices should be employed.

Nothing — an open slope over ground nobody uses

First choice: Let it shed, and keep it that way.
If that is not possible: Retaining here adds load to the roof and buys nothing.
Source language: MCA permits shed-at-will “under limited circumstances,” but warns that “due to higher unpredictability and the increased hazards associated with the ‘shed-at-will’ philosophy, only in carefully selected situations should snow retention devices be omitted.”

Two practical cautions from the same bulletin. In highly critical applications MCA advises a minimum of two rows even where calculations show one is adequate, because a snow blanket lacking compressive strength can buckle and fold over a single row. And adhered snow guards “should not be considered for use as primary system components,” because adhesive holding strength diminishes with age and exposure, which makes an out-of-the-box test result irrelevant to how the device behaves in year twelve.

The thing a snow guard cannot do

It cannot make a roof stronger. Everything a retention system holds up there is load the structure now carries for longer, in a place it might otherwise have left. On a modern roof designed for the full balanced load that is a non-event. On an old, unknown, or already marginal structure — particularly the pre-1990 truss construction Anchorage flags — it is a change to the loading assumption, and it belongs in front of a design professional before it belongs in front of a supplier.

There is also a site-drainage consequence that gets missed. MCA notes that in heavy snow areas, snow shed “can also inhibit proper site drainage, directing roof run-off and snowmelt-water towards, rather than away from, building walls and foundations.” Where the snow lands is a wet-basement question as well as a safety one.

Where the code guardrail sits

Ice barrier extent is a jurisdictional question, not a rule of thumbSection link

An ice barrier is a waterproof underlayment under the covering. It limits what ice at the eave does to the building. It does not stop the ice forming and it does not reduce the load.

An ice barrier — also called ice-and-water shield, or by a manufacturer’s brand name — is a self-adhering membrane or a doubled, cemented underlayment installed on the deck at the eave before the covering goes on. Its job is to stay watertight when water held behind a dam stops running and starts standing.

Here is one jurisdiction’s adopted wording, quoted in full so the shape of the provision is visible. This is the 2021 Seattle Residential Code, Section R905.1.2 — the edition Seattle’s Department of Construction and Inspections put into effect on 15 November 2024, and the one it says will stand until the 2024 Seattle codes arrive, which SDCI does not expect before May 2027:

“In areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2, an ice barrier shall be installed for asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. The ice barrier shall consist of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet shall be used in place of normal underlayment and extend from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line of the building. On roofs with slope equal to or greater than 8 units vertical in 12 units horizontal (67-percent slope), the ice barrier shall also be applied not less than 36 inches (914 mm) measured along the roof slope from the eave edge of the building.”

Three things in that paragraph deserve attention, and none of them is the number.

It is conditional, and the condition is filled in locally

The requirement applies “in areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2.” Table R301.2 is the climatic and geographic design criteria table each adopting jurisdiction completes for itself — the same table Minnesota’s residential code populates with its own ground snow loads. Whether an ice barrier is required at your address is a designation your authority having jurisdiction made, not something the code text alone can tell you.

The dimension is measured inward from the wall, not outward from the eave

“24 inches inside the exterior wall line” is a plan dimension referenced to the wall. On a house with a deep overhang, the membrane has to cover the whole overhang plus that 24 inches, measured along the slope — which is routinely more than one 36-inch course. That arithmetic, with the pitch-factor conversion and three worked roofs, is on the ice dam page and is not repeated here.

It is worth knowing that trade guidance sometimes asks for more than the code minimum. MCA’s cold-climate bulletin recommends that at cold icing eaves the membrane “should be extended from the outer extremity of the eave to a distance of at least 30″ inside the heated building envelope,” and gives the worked case: a 24-inch overhang means at least 54 inches of coverage. That is a trade recommendation, not a requirement anywhere, and it is referenced to the heated envelope rather than the wall line — a different datum from the code’s.

It is mitigation, not prevention, and it is not load relief

The membrane keeps water out of the building. The dam still forms, the ice still weighs what it weighs, the gutter and fascia still take the strain, and the roof still stops shedding snow. Nothing in this section reduces a pound of load. Treat the ice barrier as the last line of defence for a problem solved somewhere else.

The Seattle text above is the law in Seattle. Other jurisdictions adopt different editions, amend the extent and the trigger, and may enforce differently on a re-roof permit than on new construction. Confirm the adopted edition, its amendments, its effective date, and the Table R301.2 designation with your authority having jurisdiction before treating any dimension here as a requirement for your building.

The tension nobody mentions

A better-built roof holds more snow, and that is the right tradeSection link

Here is a fact that sounds like an argument against insulation, and is not. FEMA: “A well-insulated or well-ventilated roof typically retains more snow than a poorly insulated roof or a roof over a poorly ventilated attic. Well-insulated roofs do not permit heat from within a building to melt roof snow from beneath.” It compares a properly performing cold roof to a carport, where the top and underside are the same temperature and so the melt rate is unaffected.

The corollary is that a leaky, poorly insulated house melts some of its own snow load away. That is not a benefit and nobody should design for it. The melt has to go somewhere, and on a sloped roof with a cold eave it goes into an ice dam — which, as FEMA notes, then prevents snow sliding off the roof and creates an unbalanced eave load of its own. A building that sheds load by leaking heat has swapped a distributed load it was designed for and a heating bill it was not, for a concentrated load at the eave and water inside the wall.

FEMA also draws the low-slope distinction: “A properly designed, internally drained flat or low slope roof is not affected by building insulation characteristics to the degree sloped structures are,” because meltwater on an uninsulated low-slope roof does not meet the cold-eave temperature differential that produces dams. On low-slope roofs the question moves to whether the drains work.

What the building-science guidance actually asks for

The Department of Energy’s Building America Solution Center puts the cold-roof target in three parts, and the order matters more than any single number:

Stop the air. BASC identifies heat loss into the attic through air leakage warming the roof deck as “one of the greatest sources of ice dams in residential construction.” A continuous air barrier at the ceiling plane is the first move, not the third.
Insulate. For roofs built against ice damming, BASC recommends R-60 or greater in climate zones 5 and above. That is a recommendation in a technical guide, not the minimum in any jurisdiction’s adopted energy code, and the two are not the same number.
Keep the intake open. BASC calls for room above the insulation at the exterior walls for a minimum of two inches of ventilation space — the gap a baffle exists to protect. Blown insulation slumped into the eave closes the intake and is one of the cheapest faults on any roof to create and to fix. BASC frames the purpose of ventilation rates and ratios as ensuring enough cold air is present to prevent heat losses through the deck from melting roof snow.

The ventilation guide works through intake and exhaust balance and the net free area arithmetic properly. Two things belong here rather than there.

Unvented assemblies are legitimate, and they change the questions

BASC treats vented and unvented assemblies as two valid ways to locate the thermal boundary, chosen on climate zone, the intended use of the space, building design and configuration, and where the HVAC equipment sits. There is no version of this site that says more ventilation is always better.

What changes in an unvented assembly is the failure mode, not the physics of load. BASC’s cold-climate condition is that air-impermeable insulation be maintained at 50 percent or more of the total R-value of the roof system for condensation control — on an R-80 assembly, at least R-40 of it air-impermeable. An improvised unvented conversion in snow country is a condensation problem waiting for a cold January, and it is a design decision for a qualified professional rather than a retrofit to attempt by analogy.

And the reason none of this resolves into a formula

Snow is not a passive blanket sitting on top of the problem. It is itself an insulator, and it lets melt start from the roof surface rather than from the ceiling below: MCA notes that “due to insulating characteristics of snow, solar thaw can occur even on cloudy days and when ambient air temperatures are well below freezing.” The same bulletin allows the opposite trade where a vaulted ceiling makes a vented cold roof impractical, saying that extreme levels of ceiling insulation together with a solar-absorptive roof colour “will go a long way toward forgiving the absence of attic ventilation and ‘cold roof’ design.”

Those are trade recommendations for metal roofs rather than rules, and they are quoted here to make one point: the thermal side of a snow roof has more than one legitimate answer, and which one is right in genuinely deep-snow country is a design question for someone who designs roofs in that climate. It is the clearest case on this page of general guidance being the wrong instrument.

If the load genuinely has to be reduced

Removal is a service to buy, and there is a right order to do it inSection link

Stop — immediate hazard

Do not remove snow from a roof yourself, and stop immediately if anyone suspects the roof is failing.

FEMA recommends removal be performed by a licensed, insured professional roofing contractor with experience removing snow from roofs, specifically because of that contractor’s familiarity with safety protocols. Anchorage’s building official recommends licensed and bonded roofing companies, “given the hazardous nature of working in high places.” University of Minnesota Extension’s guidance for farm buildings begins “hire a professional if possible.” Three independent sources, one answer.

The reason is not squeamishness about ladders. OSHA requires that a surface weighed down by snow be inspected by a competent person to determine whether it is structurally safe to access at all, because it may be at risk of collapsing — and a snow-covered roof hides skylights, drains, and vents that a person can go through.

FEMA’s own instruction, printed as a warning box in its guide: “If there is any concern that snow loads may cause a collapse of the roof structure, cease all removal activity and evacuate the building.” Then call a licensed structural engineer, and a licensed, insured roofing contractor who does winter work.

The order a competent crew works in, and why each step exists

This is FEMA’s method, and it is worth reading even if you will never do it, because it is the thing to listen for when a contractor explains what they are going to do:

Drifted snow first, at building elevation changes, parapets, and around equipment. The concentrations come off before the field does, because the concentrations are the problem.
Then work out from the centre of the roof, removing snow in the direction of the primary structural members. FEMA states plainly that this is what prevents creating an unbalanced snow loading condition during the removal itself.
Leave at least two inches on the roof. Removing snow completely “can result in serious damage to the roof covering and possibly lead to leaks and additional damage.”
No mechanical equipment, no sharp tools. FEMA: the risk of damaging the membrane or other rooftop items outweighs the advantage of speed; use plastic rather than metal shovels; avoid metal snow rakes, which can damage roofing material.
Do not stockpile snow on the roof. A pile is a drift you built on purpose. OSHA gives the same instruction from the worker-safety side: remove snow uniformly, avoid making snow piles.
Keep the discharge clear of exits, fire escapes, downspouts, ventilation openings, and equipment, and dispose of removed snow in designated ground areas.
Hold the ground. FEMA: always have someone below to keep foot traffic away from where falling snow or ice could cause injuries, and confirm the area below is clear of equipment that could be damaged. OSHA suggests keeping people back 10 feet from where snow is expected to fall.
Inspect the covering afterwards for damage, and consider a structural inspection after a particularly large event.

What a homeowner can legitimately do

Watch the building from inside and from the ground for the warning signs at the top of this page, and act on them by leaving rather than by investigating.
Keep ground-level drainage working: downspout discharge points clear of ice and snow, so meltwater has somewhere to go. FEMA lists this among the minor corrective actions a homeowner can reasonably handle.
Keep the drop zones empty. Move the car, move the bins, keep the path away from the eave, and keep children and pets out from under overhanging ice.
Keep the gas meter and any vent terminals clear of snow, and — this is the part that matters — clear of anything that could fall on them from above. If ice is building over a meter, that is a contractor call, and a gas utility call if the meter or its vent is buried or damaged.
Book the pre-season assessment for next autumn while this winter is still fresh. FEMA’s advice is a building inspection and vulnerability assessment before the snow season, with a licensed design professional retained where the drawings do not record the design snow load or the structure is in question. The only good time to arrange that is when nobody else is asking.

What a homeowner should not do is the entire remainder of the list: climbing, raking near power lines, chipping ice, standing under an eave, or clearing one half of a roof because that is the half they can reach.

Considerations

What changes this on a real buildingSection link

Structural weight

Whether a specific roof can carry a specific snow condition is a structural determination about that building. It depends on the design load it was built to, the code edition in force at the time, the framing type and span, the condition of the members and connections now, and any alteration since. FEMA’s instruction where no drawings exist is to “retain the services of a licensed design professional before the snow season to determine structural capacity.”

Anchorage’s handout is a useful reminder that a code minimum is not a guarantee. Buildings there must be designed for at least 40 lb/ft², and the handout goes straight on: “We know, however, from numerous roof failures that many buildings in our community cannot safely support this much weight. Roof failures, including collapse, have occurred when roofs were supporting substantially less snow weight than the code minimum.” The failures it names are concentrated in pre-1990 wood trusses with metal gang-nail plates.

This page does not publish snow-load determinations and will not. Capacity, drift design, and whether an existing roof is adequate are questions for a licensed structural engineer looking at that building and its drawings — or at that building in the absence of drawings.

Code and jurisdiction

There is no nationwide building code for site-built construction in the United States. Snow provisions reach a building through whatever edition the state or local government adopted, as amended, on that government’s own effective date.

Minnesota is a clean worked example of how far that can depart from a map. Minnesota Rules part 1309.0301 — the state residential code’s design-criteria section — states that “The ground snow loads to be used in determining the design snow loads for buildings and other structures are given in Minnesota Rules, part 1303.1700.” That rule then sets the value by statute-backed rule rather than by reading a national figure: 60 pounds per square foot in twenty-nine named counties, and 50 pounds per square foot in every other county in the state. A designer in Minnesota does not interpolate a contour. They look up the county.

Record the jurisdiction, the adopted edition, the amendments, the effective date, and the official URL for any code claim, and confirm it with the authority having jurisdiction. Nothing on this page is a code determination for your building, and Minnesota's values are Minnesota's.

Slope and drainage

Slope decides whether snow stays or goes, and both outcomes have costs. FEMA: “Low slope roofs retain snow more so than pitched roofs. However, roof pitches as low as 10 degrees have been observed to shed snow” — and “Roof pitch that exceeds the angle of repose of snow results in snow sliding; the angle of repose is the maximum angle at which snow will not slide, approximately a 30 degree roof slope, often referred to as 6:12 or 7:12.” A steep roof trades a load problem for a discharge problem. If you want the arithmetic that converts between slope expressions, it is on the roof pitch page.

Moisture and ventilation

Snow load and moisture control are the same conversation on a cold roof, and they pull in opposite directions. Meltwater that cannot leave becomes ice at the eave or ponding at a low-slope drain, and FEMA flags both: blocked or poorly designed drainage turns snow melt from a load reduction into a concentrated load. Gutters, downspouts, and internal drains are structural components in winter whether or not anyone thinks of them that way.

Vented attics and correctly designed unvented assemblies are both legitimate. There is no universal ventilation ratio on this site, and there is no version of this page that says more ventilation is always better.

Maintenance

FEMA’s pre-season list is short: gutters and downspouts in repair and free of debris, internally drained roof downspouts clear, seals around rooftop penetrations intact, soffit and ridge ventilation open, trusses not leaning out of plane, gang-nail plates and lateral bracing sound, attic areas dry. Most of that list is written for someone who already has safe roof and attic access — a maintenance department or a contractor, not a homeowner with a ladder. The share FEMA hands to a homeowner is deliberately narrow: “minor mitigation or corrective actions like ensuring gutters and downspouts are not blocked at the discharge point.” For the structural items — out-of-plane trusses, corroded metal brackets — it says “the skills of a professional are required.” That division is worth respecting rather than splitting the difference.

Access and site conditions

The single most effective safety decision on this whole subject is to design the ground so nothing needs guarding. MCA’s cold-climate bulletin says it in design terms: “Locations of ingress and egress, as well as parking area design should anticipate this snow slide. Building entrances should be beneath gables, not eaves. Pedestrian and vehicular traffic patterns must be routed away from potentially dangerous snow-shed zones.” Moving a walkway is cheaper, more reliable, and less loaded than any hardware.

Warranty and repair

What attaching things to a roof does to its warranty, and what repairs after a snow year look likeSection link

Snow guards are an attachment to a finished roof: Anything fixed to a covering is a warranty question for both the covering manufacturer and the installer, and it is a question to settle in writing before the work, not after a claim. MCA’s position on one common shortcut is unusually direct: “Based on the significant reports of failure for adhered devices and a lack of scientific foundation for their use, adhered snow guards should not be considered for use as primary system components,” because adhesive holding strength diminishes with age and environmental exposure, which makes an out-of-the-box test result irrelevant to aged performance.
Seam-clamped systems on standing seam: On standing seam metal, MCA describes non-penetrating clamps as the more desirable alternative when appropriately tested, and adds two practical cautions: clamps with cup-point set screws or sharp nodes are prone to tearing the seam material under load or damaging coatings, and clamp-on devices should avoid attaching at clip locations so the panel can still move thermally. A snow-guard specification that names a product and its tested load is a different document from one that names a category.
Damage from removal work: FEMA’s removal guidance exists partly because removal damages roofs: “Removing snow completely from a roof surface can result in serious damage to the roof covering and possibly lead to leaks and additional damage. At least 2 inches of snow should be left on the roof.” It also says not to use mechanical snow removal equipment, not to use sharp tools such as picks, to use plastic rather than metal shovels, and to avoid metal snow rakes because they can damage roofing material. Damage caused by a homeowner or an uninsured helper is not usually anybody else’s problem.

Repairability

After a heavy season, the repairs cluster in predictable places: edge metal and gutters pulled by ice and sliding snow, fasteners and clips worked loose along the discharge path, covering scarred where a rake or a shovel ran, and whatever was standing in the drop zone. FEMA recommends inspecting the roofing material for signs of damage after removal work, and adds that “a quick inspection of the structural system may be prudent after particularly large snow events.” Spring, not the following December, is when that inspection is worth buying.

A warranty is a contract between a reader and whoever wrote it. What it covers, what voids it, whether it transfers, and how it is enforced are set by that document and by the law where the reader lives. Read the actual warranty for the product and the installer in front of you — not a summary of one, including this one.

Ask before you sign

Questions to ask an installerSection link

These are questions for a roofing contractor, a snow-removal contractor, and — for anything structural — an engineer. Some of them are deliberately questions a weak answer cannot survive.

What design snow load was this roof built to, and where did you get that number?
The honest answers are “it is on the drawings” or “there are no drawings, so it would take an engineer to establish it.” FEMA says exactly that second thing. A contractor who produces a confident figure with no document behind it has told you something useful about the rest of the proposal.
Which parts of this roof drift, and which parts receive snow sliding off something above?
A competent answer walks the building and names features: this step, that dormer cheek, the wall above the sunroom, the equipment screen. An answer about the roof as a single average surface is the answer this page exists to argue with.
What is directly below every eave that sheds, and what are we doing about each one?
MCA’s hierarchy is worth borrowing: design the shed zone so nothing is in it, make the drop zone inaccessible, or retain the snow. Anchorage’s is the same in two lines — install snow guards to retain the snow on the roof, or block access to the area. “We’ll put some guards on” without that survey is hardware in search of a problem.
If you are proposing snow retention, what load did you calculate, and for which specific product?
MCA: the first step in snow retention design is to determine the loads to be resisted, and because system design is site- and product-specific, the specification should be product-specific, with any substitution required to demonstrate equivalence by testing or calculation. Spacing follows from that calculation. A quote that names a quantity of guards but no load is a quote for parts.
Is anything being retained above an entry or a flue, and how did you size the tributary area there?
MCA flags this specific case: at an isolated assembly the retained bank “will generally resemble a wedge, not a rectangle, hence tributary areas may be much larger than first anticipated.” It is the point where a well-meant single row over the front door is most likely to be undersized.
If you remove snow, in what order, with what tools, and who is keeping the ground clear?
FEMA’s method is specific: drifted snow first at elevation changes, parapets and around equipment; then work outward from the centre; remove in the direction of the primary structural members to avoid creating an unbalanced load; leave at least two inches; no stockpiling on the roof; keep snow away from exits, fire escapes, downspouts and vents. It also says to have someone below keeping foot traffic away. A contractor who can describe this is a contractor who has done it.
Are you licensed, insured, and OSHA-compliant for rooftop work, and can I see the certificates?
FEMA recommends a licensed, insured professional roofing contractor experienced in removing snow from roofs, specifically because of familiarity with safety protocols. Anchorage’s building official recommends licensed and bonded roofing companies. This is the one trade question where the paperwork is the point.

Require these in writing

The specific roof areas to be cleared, in order, and the areas deliberately not cleared
The minimum depth to be left on the covering, stated as a number
Tools: non-metallic rakes, plastic shovels, no mechanical equipment, no picks or chisels
Where removed snow will be placed on the ground, and how exits, fire escapes, downspouts and vents will be kept clear
Ground control: who marks and holds the exclusion zone below the work
For snow retention: the product by name, its tested allowable load, the calculated design load resisted, the row spacing, and the attachment method
A post-work inspection of the covering, with photographs, and who pays if damage is found
Certificates of general liability and workers' compensation, current and naming the entity doing the work

What goes wrong

Misconceptions and failure modesSection link

Common misconceptions

Common belief
My roof is rated for X inches of snow.
What is actually true
Roofs are not designed in inches. They are designed in pounds per square foot, because the same depth can weigh wildly different amounts — FEMA’s published range for one foot of snow runs from 3 lb/ft² for light dry snow to 21 lb/ft² for wet heavy snow. Anyone quoting a depth rating has converted a load using an assumed density and then dropped the assumption on the floor.
Common belief
The ground snow load map tells me what is on my roof.
What is actually true
It does not, twice over. Ground snow load is a design value with a stated design basis, not a measurement of any particular winter — FEMA, writing to ASCE 7-10, notes the mapped values indicate a 2 percent probability of being equaled or exceeded in any given year, and that they do not discount that actual loads may exceed them. ASCE/SEI 7-22 sets the value on a reliability target instead; either way it is a design quantity. And the roof value is derived from it, not equal to it. OSHA: “Snow load on the ground can provide a rough indication of roof snow load, but roof snow loads also depend upon factors such as melting and re-freezing of snow and ice, drifting, roof slope, type of roof, and design features.”
Common belief
The snow settled, so there is less weight up there now.
What is actually true
Settling is densification, not disappearance. MCA describes the thaw cycle: meltwater migrates down through the blanket, the outer layers refreeze into a crust, and “except for a small amount of water that may evaporate during this process, the moisture and all of the weight are still present, however much the depth of the blanket is reduced dimensionally.” A shallower roof after a thaw can be carrying the same load, in a harder, better-bonded slab that is now primed to release all at once.
Common belief
A steep roof cannot have a snow problem.
What is actually true
A steep roof trades retention for discharge. It sheds — onto the porch roof, the entry, the walkway, the meter, or the neighbour’s fence. MCA records that “serious accidents and even fatalities have resulted from rooftop avalanches” and that shedding snow does serious damage to landscaping, other building elements, and roofs themselves. Steep roofs also drift on the leeward side of the ridge, and they still ice at the eave.
Common belief
Shovelling the roof is the safe, responsible thing to do when it gets deep.
What is actually true
Removal is a legitimate response when the load genuinely approaches capacity, and it is a job to buy rather than attempt. Beyond the fall risk, doing it wrong makes the structural problem worse: OSHA states that “shoveling or raking a roof without using the proper procedures can also increase the risk of roof collapse by creating an unbalanced load on the roof,” and its remedy is to remove snow uniformly and avoid making piles on the roof. Clearing one half of a span is a way of inventing a drift.
Common belief
Heat cable will take care of the ice.
What is actually true
Cable manages a drainage channel; it does not remove a load or change the deck temperature that made the ice. It also has to survive the snow moving over it: MCA notes that where cables are used, “snow retention systems must also be incorporated to prevent migrating snow from tearing the heating cables from the roof surface.” The trade view of heat cable as a primary remedy, and the reasons for it, are set out on the ice dam page.
Common belief
If snow guards are good, more snow guards in one row is better.
What is actually true
Rows matter more than count. MCA advises that “in highly critical applications a minimum of two rows of parts or assemblies should be used, even though calculations may show one row to be adequate,” because a snow blanket lacking compressive strength can buckle, and “the loop of the buckled blanket may fold over the single row and potentially fall to the ground.” A single dense row at the eave can be overtopped by the thing it was installed to stop.

How it actually fails

Drift overload at a roof step or parapet: Wind deposits snow in the aerodynamic shade of a taller section, against a parapet, or beside rooftop equipment, concentrating far more weight over one part of a span than the balanced load. FEMA identifies unbalanced loading as a greater risk to the structural system than a uniform load, and notes drift design entered US model codes only in 1975 (BOCA) and 1988 (UBC).What you can see: A visibly deeper bank against a wall, parapet, or equipment screen on a lower roof. Snow depth on one part of a roof several times what it is elsewhere. Inside: new deflection, doors binding, or cracking noises — all of which mean leave the building.
Impact and overload on a porch, sunroom, or entrance canopy: Snow slides off an upper slope onto a lower roof that was often added later and detailed without reference to the main structure. FEMA describes both the accumulation and “the dynamic force of the sliding snow onto the lower roof” producing a significant impact force on the lower roof framing.What you can see: A mound of snow on a porch roof much deeper than the main roof beside it. Debris, granules, or bent edge metal on the lower roof after a thaw. Posts or a ledger that have moved.
Rooftop avalanche onto people, vehicles, or a gas meter: A warm spell breaks the bond between a slippery covering and the slab of snow on it. Anchorage: metal roofs “can avalanche the entire snow load in an instant, without warning,” when conditions warm enough to break that bond, and falling snow and ice has caused fatalities there.What you can see: Snow that has crept downslope past the eave line and is overhanging. A hard crust after a thaw-refreeze cycle. Icicles above a doorway, walkway, meter, or parking space. Rising temperatures forecast after a heavy fall — which is the warning, not the aftermath.
Concentrated ice load and blocked drainage at the eave: Melt and refreeze cycles build ice at a cold eave or at a low point that will not drain. FEMA describes higher concentrated loads at eaves and at undrained low points, and notes the ice dam also stops snow sliding off, converting a shedding roof into a retaining one.What you can see: A thickening white edge and heavy icicles. Gutters pulling away from fascia. Water staining at the junction of a ceiling and an exterior wall. Standing water on a low-slope roof that does not clear.
Ponding after the melt, on a low-slope roof: Meltwater between storms reaches a blocked, poorly designed, or poorly maintained drainage system. FEMA: on flat or low-slope roofs melt “may accumulate in low areas,” and “ponding creates a concentrated load on the roof structural system and a potential hazard.” Deflection deepens the low point, which collects more water.What you can see: Water that is still standing days after a thaw. Dirt rings and debris lines marking a former pond. Deflection visible along a parapet or a roof edge.
Damage caused by the removal itself: Shovels, picks, metal rakes, or mechanical equipment reach the covering through the snow, and clearing one area before another creates the unbalanced condition the work was meant to prevent. FEMA advises leaving at least two inches, using plastic rather than metal, and removing in the direction of the primary structural members; OSHA warns that improper removal can increase collapse risk.What you can see: Gouged or torn covering, displaced flashing, and bent drip edge along the first few feet. Granules in the gutters and on the ground in spring. New leaks that appear after the snow is gone rather than during it.

Sources and further readingSection link

Understanding Roofing / Published August 26, 2026

Scope and limitations

It cannot tell you what your roof can carry.
That is a structural determination about one building, made from its drawings or from an engineer's assessment of it, and this site does not publish snow-load determinations.
It cannot tell you what is on your roof right now.
Every load figure here is either a design value with a stated exceedance probability or a published density range.
Neither is a measurement of this winter at your address.
It cannot tell you whether an ice barrier is required where you live, or how far it must run.
That is set by your jurisdiction's adopted code edition, its amendments, and the values that jurisdiction entered in Table R301.2.
It does not publish a cost figure.
Snow removal is priced in an emergency market during the week a whole region wants it; snow retention is priced per product, per row, per calculated load; and structural assessment is priced per building.
No defensible national dataset separates any of the three, and a national average would be worse than silence.
It cannot tell you whether your insurance responds to snow-load damage, collapse, ice damage, or damage caused by removal work.
Coverage, causation, deductibles, and exclusions are governed by the policy and by the law of the state where the building is.
It does not name a snow-guard product, spacing, or layout.
MCA is explicit that this is site- and product-specific engineering, and a website that named a spacing would be inventing the one number that matters.
The Minnesota, Anchorage, and Seattle provisions cited here are those jurisdictions' own and are not evidence of what applies anywhere else.
They are included as worked examples of how much adopted requirements vary, not as a rule.

Snow Load Safety Guide (FEMA P-957), Risk Management Series
Federal Emergency Management Agency / January 2013
The definition of ground snow load and, as FEMA states it for ASCE 7-10 and the 2012 IBC, the 2 percent annual exceedance probability behind the mapped values; the seven inputs that turn a ground snow load into a roof snow load; that unbalanced loading poses a greater risk than uniform loading; the drifting mechanism and the locations drifts form (lower roofs in a wind shadow, parapets, roof steps, rooftop equipment, dormers, valleys, saw-tooth low points); sliding snow onto porch roofs and entrance canopies and its dynamic impact force; the snow weight range of 3 to 21 lb/ft² per foot of depth; ice at about 5 lb/ft² per inch and roughly 57 lb/ft² per foot; the angle of repose of about 30 degrees, often referred to as 6:12 or 7:12, and that shallower roofs can still shed; tactile versus slippery covering materials; that well-insulated or well-ventilated roofs retain more snow; that ice dams prevent sliding and create an unbalanced eave load; the rain-on-snow and ponding provisions; the warning-sign list and the instruction to evacuate promptly and contact a qualified design professional; the recommendation to use a licensed, insured professional roofing contractor for removal; the removal method list; and the dates drift and unbalanced loads entered the BOCA and UBC codes.
Best-practice guidance for building stakeholders, not adopted law anywhere, and written with commercial and industrial buildings foremost. It references ASCE 7-10 and the 2012 IBC, both since superseded. It explicitly places the process for determining special load conditions beyond its own scope. The copy read for this page was the identical FEMA P-957 PDF mirrored by the National Institute of Building Sciences' Whole Building Design Guide, because fema.gov refused automated retrieval on the verification date; the cover page, pagination, and January 2013 date match the FEMA publication.
Hazard Alert: Falls and Other Hazards to Workers Removing Snow from Rooftops and Other Elevated Surfaces (HA-3513)
U.S. Department of Labor, Occupational Safety and Health Administration / December 2017
That falls cause the most worker fatalities and injuries during rooftop snow removal, off roof edges, through skylights, and from ladders and aerial lifts, and that workers may also be injured or killed by a roof collapse; that snow load on the ground provides only a rough indication of roof snow load and that roof snow loads also depend on melting and re-freezing, drifting, roof slope, roof type, and design features; that shoveling or raking without proper procedures can increase the risk of collapse by creating an unbalanced load, and that snow should be removed uniformly with no piles left on the roof; that employers should use removal methods that do not involve workers going on roofs where possible; that people on the ground can become trapped under falling snow and suffocate, and should be kept back from the fall zone; and the guidance to use extendable non-conductive snow-rake poles and maintain 10 feet from overhead power lines, alongside the 12,000-volt electrocution incident.
A hazard alert addressed to employers under the Occupational Safety and Health Act. It states of itself that it is not a standard or regulation and creates no new legal obligations, and it says nothing about what any particular roof can carry.
Winter Weather — Hazards and Precautions
U.S. Department of Labor, Occupational Safety and Health Administration
That a surface weighed down by snow must be inspected by a competent person to determine whether it is structurally safe to access, because it may be at risk of collapsing; that snow-covered rooftops hide hazards such as skylights; that electrical hazards may exist from overhead power lines or from snow removal equipment; and the preference for removal methods that do not require workers to go onto roofs.
Workplace safety guidance for employers, not building-performance guidance and not a code source.
Handout AG.30 — Snow Removal Guidance
Municipality of Anchorage, Development Services Department, Building Safety (signed by the Acting Building Official) / 14 November 2023
That buildings in Anchorage must be designed to support a minimum 40 lb/ft² snow load, a requirement dating back at least to the 1960s; that roof failures including collapse have occurred there while roofs were supporting substantially less than the code minimum, concentrated in pre-1990 wood truss construction with metal gang-nail plates; that the requirement to design for snow drifting first appeared in the 1988 Uniform Building Code, adopted by the Municipality in January 1989, so buildings built before 1990 were likely not designed for drift; that drifting weighing more than 40 lb/ft², roughly two feet of dense wind-worked snow, should be removed from those buildings unless the roof is shown capable of carrying it; that falling snow and ice has caused fatalities in Anchorage; that metal roofs can avalanche the entire snow load in an instant without warning when a warm spell breaks the bond; that areas below downward-sloped metal roofs require snow guards or blocked access, and ice over areas accessible to people and pets must be removed or the area blocked; that ice formation can overload structural elements; and the recommendation to consult licensed and bonded roofing companies for snow and ice removal.
Guidance published by one authority having jurisdiction and applicable in Anchorage. Its 40 lb/ft² figure is Anchorage's minimum, not a national one, and its drift threshold is stated as guidance for a specific class of older local buildings rather than as a general rule.
ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures — standard overview and summary of changes
American Society of Civil Engineers / Structural Engineering Institute / 2022
That the ground snow load provisions FEMA P-957 describes have since been revised: ASCE lists among the changes in ASCE/SEI 7-22 “revised ground snow loads to reflect more recent snow load data and reliability-targeted values”. Cited only to establish that the 2 percent annual exceedance figure belongs to the earlier editions FEMA was writing against, not to state what any current value is.
A standards developer's own description of its standard. ASCE 7 is not law anywhere until a jurisdiction adopts a code edition that references it, so which derivation applies at any address is a question for the authority having jurisdiction. The standard itself is behind a paywall and was not purchased; nothing on this page quotes its provisions or reproduces any value from it.
Minnesota Rules, part 1303.1700 — Ground Snow Load
Minnesota Office of the Revisor of Statutes (official publication of Minnesota Rules) / Published electronically 11 July 2007
That Minnesota sets ground snow load by state rule rather than by reading a national map: 60 pounds per square foot in twenty-nine named counties, and 50 pounds per square foot in all other counties. Statutory authority is cited as Minn. Stat. §§ 16B.59, 16B.61, 16B.64, 326B.101, 326B.106 and 326B.13.
Minnesota law only. It fixes the ground snow load value; the roof snow load still depends on the building, and local administration and amendments are a separate question for the authority having jurisdiction.
Minnesota Rules, part 1309.0301 — Section R301, Design Criteria
Minnesota Office of the Revisor of Statutes (official publication of Minnesota Rules) / Published electronically 31 March 2020
That the Minnesota Residential Code's design-criteria section directs the ground snow loads used in determining design snow loads to Minnesota Rules part 1303.1700, and that Table R301.2(1), climatic and geographic design criteria, is where roof snow load and the other local design values are recorded for a jurisdiction.
Minnesota law only, and one part of a chapter that adopts a model code by reference with numerous amendments. It is cited here to show the mechanism by which a state replaces a mapped value, not as a requirement anywhere else.
2021 Seattle Residential Code, Chapter 9 — Roof Assemblies, Section R905.1.2 Ice barriers
City of Seattle, Department of Construction and Inspections / 2021 edition; effective in Seattle 15 November 2024, per the Seattle Department of Construction and Inspections, which does not expect the 2024 Seattle codes before May 2027
The verbatim wording of the ice-barrier provision as adopted by one jurisdiction: that it applies in areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2; that it consists of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment; that it extends from the lowest edges of all roof surfaces to a point not less than 24 inches inside the exterior wall line of the building; that on roofs with slope equal to or greater than 8 units vertical in 12 units horizontal it is also applied not less than 36 inches measured along the roof slope from the eave edge; and the exception for detached accessory structures not containing conditioned floor area.
This is the code adopted in the City of Seattle and it is law only there. Whether the requirement is triggered at all depends on that jurisdiction's Table R301.2 designation. Other jurisdictions adopt different editions, amend the extent and the trigger, and enforce differently on a re-roof than on new construction. It is quoted here as an example of adopted wording, not as a requirement anywhere else.
Technical Bulletin: Metal Roof Design for Cold Climates
Metal Construction Association / Version 10/19 V2
That a snow blanket bonds to metal panels and imposes a downslope drag load; that serious accidents and fatalities have resulted from rooftop avalanches and that shedding snow damages landscaping, building elements, and roofs; that building entrances should be beneath gables rather than eaves, that pedestrian and vehicular routes must be kept out of snow-shed zones, that any permanent structure or fixture in the trajectory must be designed for the impact, and that drop zones should be made inaccessible; that snow retention devices should be omitted only in carefully selected situations; that an unguarded eave above a lower roof increases that lower roof's design load and that drift at parapets and walls above lower roofs can increase design roof snow loads significantly; that snow retention design begins by determining the loads to be resisted and is site- and product-specific, so specifications should be product-specific with substitutions required to demonstrate equivalence; that at an isolated assembly such as over an entry the retained bank is wedge-shaped and the tributary area larger than first anticipated; that a minimum of two rows is advised in highly critical applications because a buckling blanket can fold over a single row; that adhered snow guards should not be used as primary system components; that clamp-on devices should avoid clip locations and that cup-point set screws can tear seam material; that thaw densifies a blanket without removing its weight; that ice weighs about 5 lb/ft² per inch of thickness; that an ice dam creates unpredictable snow retention whose sudden release is a hazard below the eave; that heat cabling requires snow retention to keep migrating snow from tearing it off the roof; that the insulating characteristics of snow let solar thaw occur on cloudy days and when ambient air is well below freezing; and that where a vaulted ceiling makes a cold attic impractical, extreme ceiling insulation and a solar-absorptive roof colour go a long way toward forgiving the absence of attic ventilation and cold-roof design.
Trade-association guidance written for metal roofs, and product-adjacent by nature. MCA disclaims any warranty that the information is suitable for any general or particular use. Nothing in it is a code requirement anywhere, and its recommended dimensions — including extending eave membrane at least 30 inches inside the heated building envelope — are trade recommendations, not legal minimums.
Technical Bulletin: Qualifying Snow Retention Systems for Metal Roofing
Metal Construction Association / Version 04/2023
The load-chain framing quoted on this page: that all snow guard systems represent a load chain transferring the sliding forces of snow into the building structure, and that for proper design each link in that chain must be proven to withstand the forces to which it will be exposed, by testing, engineering and quality control rather than guesswork; and that the system's structural capacity is set by the weakest link in that chain.
A second, separate MCA bulletin, written for pitched metal roofs and product-adjacent in the same way as the cold-climate one. It is criteria for evaluating snow guard systems, not a code requirement anywhere, and it publishes no load determination for any roof. It notes that mandatory snow guard requirements are common in alpine Europe and rare in North America.
Construct Roofs and Attics for Ice Dam Prevention
U.S. Department of Energy, Building America Solution Center (PNNL) / Last updated 6 December 2022
That ice dams are typically prevalent where the ground snow load is greater than 60 lb/ft² and are likely anywhere it exceeds 30 lb/ft²; that in climate zones 5 and greater the recommended R-value for roofs built against ice damming is R-60 or greater; that there should be room above the insulation at the exterior walls for a minimum of two inches of ventilation space; that heat loss into the attic through air leakage warming the roof deck is one of the greatest sources of ice dams in residential construction; and that ventilation rates and ratios exist to ensure enough cold air is present to prevent heat losses through the deck from melting roof snow.
Best-practice guidance for builders and designers, not adopted law. Its recommended R-values are guide recommendations and are not the minimum in any jurisdiction's energy code. Its ground snow load thresholds describe where ice dams are common, not whether one will occur on a specific building.
Vented versus Unvented Attic
U.S. Department of Energy, Building America Solution Center (PNNL) / Last updated 5 December 2022
That both vented and unvented attic approaches are legitimate and are chosen on climate zone, the intended use of the space, building design and configuration, and the location of HVAC equipment; and that for cold climates air-impermeable insulation should be maintained at 50 percent or more of the total R-value of the roof system for condensation control.
Guidance, not code. Its code references are to specific IRC and IECC editions and are model provisions rather than the law in any particular jurisdiction.
Prevent roof collapses from snow and ice
University of Minnesota Extension / Reviewed 2026
The advice to hire a professional for roof snow removal where possible; the ground-level checks before any removal work — looking at sidewalls for bulges or failed knee braces, checking whether the roof line is still straight, and looking at ceilings, open trusses, and walls for damage; and the warning to prevent large chunks of ice and snow sliding off a roof from falling on people, animals, or equipment.
Written for farm and agricultural buildings, whose structures and occupancies differ from houses. It is extension guidance, not a code source, and it publishes no load determination.

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