For homeowners, buyers, and residential contractors

A shed roof is the simplest roof to build and the easiest one to under-drain.

Steep-slope and low-slope · additions, accessory structures, and whole houses

One plane. No ridge, no hip, no valley, almost no cut waste. All of that simplicity is real. The bill for it is that everything the roof does — drain, exhaust, shed snow — happens at one of its two edges.

30-second answer

What is a shed roof, and what actually goes wrong with one?

A shed roof is one sloping plane, high at one edge and low at the other, with no ridge, hip, or valley. That makes it cheap to frame, fast to cover, and easy to inspect. Its cost is concentration: the entire roof drains to a single eave, so that gutter, its downspouts, and the ground beneath them carry every gallon a two-sided roof would have split.

Safety first

Everything a shed roof catches lands on one strip of ground

On a gable or a hip, runoff and sliding snow are divided between two or four edges pointing in different directions. A shed roof has one. Every gallon and every pound of snow the plane collects arrives along a single line, and whatever sits below that line receives all of it — repeatedly, for the life of the roof.

That is a water problem at the foundation, a rot problem at the fascia and soffit, and in freezing climates a genuine physical hazard: an unbroken plane has no ridge, hip, valley, or dormer interrupting it, so there is nothing holding a snow load back once it starts to move and it can release as one sheet.

Look at what is under the low edge before you look at anything else: a doorway, a walk, a deck, a parking space, a gas meter, a basement window. If snow or ice has built up on a single-plane roof above any of those, keep people and vehicles out of that strip until it has released or been cleared by someone equipped to do it from the ground. Do not stand under the low edge to look up at it, and do not climb onto the roof to push it off.

Learning paths and saved lessons

At a glance

The short versionSection link

Six things that are true of every shed roof. The second one is the reason this page exists.

The shape: One plane, one high edge, one low eaveAlso sold as a lean-to, a mono-pitch, a mono-slope, or a skillion. Same geometry, different marketing.
The drainage load: Twice the catchment per foot of gutter as a same-footprint gableArithmetic, worked below. A 30 by 40 ft plan area hangs 30 sq ft of roof on every foot of one 40 ft gutter; split into a gable, each of the two gutters carries 15.
Cut waste and accessories: The lowest of any common pitched shapeNo hips, no valleys, no angled cuts through the field, no valley metal, no hip-and-ridge run. A real saving, and a small one against area, covering, and deck.
The ventilation problem: There is no ridge to exhaust fromSeattle's adopted residential code requires cross ventilation for each separate space, and where the reduced 1/300 ratio is used it puts 40 to 50 percent of the vent area in the upper portion of the space, not more than 3 ft below the ridge or highest point. On a single-slope cathedral roof each rafter bay is a separate space and the highest point is a wall.
The slope arithmetic: Available rise in inches ÷ depth in feet = slope in units per 1224 inches of usable rise across a 12 ft addition is exactly 2:12. Across 16 ft it is 1.5:12, and no lapped shingle of any kind is permitted there.
The characteristic failure: Undersized drainage at the low edge, and the ground under itNot the covering. On a shed roof the covering is the easiest part of the job.

Tradeoffs

This page says size the low edge for the whole roof. Here is when that is not the binding constraint.Section link

The position taken here is that a shed roof is an excellent, cheap, honest shape whose one systematic risk is drainage concentration. That position has real limits, and on some buildings something else decides the outcome first.

Best when

The building is small, simple, and rectangular — a garage, a workshop, a porch, a rear addition. The shape is at its most efficient exactly where a gable's second plane buys the least.
The low edge discharges somewhere that can take it: away from the foundation, away from an entrance, onto ground that already drains, or into a leader that goes somewhere on purpose.
There is enough vertical room to keep the slope in steep-slope territory. Above about 3:12 you have the whole covering catalogue and a straightforward assembly.
The high edge is a free-standing wall rather than a junction with an existing house. A shed roof that terminates in its own parapet or fascia has no headwall flashing to get wrong.
The roof is going to be metal. A single unbroken plane with no valleys is the best case standing-seam ever gets: full-length panels, no cross-cuts, no valley detailing.
You want the roof to be inspectable. One plane visible from the ground on one side of the building is the easiest roof in residential construction to look at honestly.

Think twice if

The low edge lands over a doorway, a walkway, a driveway, or a deck. In a snow climate that is a hazard, and in any climate it is a maintenance argument you will have every autumn.
The addition is deep and the available rise is small. Past a certain depth the shape quietly stops being a shingle roof and becomes a membrane roof — with a different assembly, a different trade, and different detailing, none of which most quotes make explicit.
The high edge dies into an existing wall on a house built before about 1990. Cutting into siding, felt, or mastic to set a counterflashing can disturb asbestos-containing material, and testing comes before disturbance.
The high edge is against a taller wall in snow country. Snow drifts against walls, and the drift surcharge on a low roof beside a high one is a structural question for a design professional, not a shape question.
The interior is a cathedral ceiling and the roof framing is shallow. There is no attic to hide anything in, the insulation depth and the required clear air space compete for the same rafter, and something has to give.
The building is tall and exposed. A single large plane presents one continuous windward or leeward surface, and what the wind does to it is a site- and building-specific calculation, not a property of the shape.

What changes the answer

How deep the roof is from the high edge to the low one — the single number that decides whether the slope survives.
How much vertical room actually exists at the high edge after the covering, the flashing turn-up, and the top of the wall framing are taken out of it.
The local rainfall rate. The same roof in Phoenix and in Houston puts very different flows into the same gutter, and the sizing follows the rain, not the roof.
What is directly under the low edge, and where the downspouts can legally and practically discharge.
Whether the assembly is vented or a correctly designed unvented one. Both are legitimate; they impose completely different requirements on the high edge.
Whether the roof is new construction or a re-cover on an existing plane, which changes what can be altered and what has to be lived with.
The adopted code edition and amendments where the building is, and what the authority having jurisdiction will require on the permit.
Snow. In deep-snow country a single plane is a sliding-snow design problem, a drift problem at the high wall, and possibly an ice-dam problem, all at once.

The mechanism

There is nothing wrong with one plane. The problem is that one plane has one edge.Section link

A shed roof removes the ridge, the hips, and the valleys — and with them, every place a conventional roof divides its work. What is left arrives at the bottom.

Section through a shed-roofed addition against an existing house wall. Numbers key to the list directly below, which is the full text equivalent of the drawing. Schematic, not to scale, and not a construction detail.Original diagram, Understanding Roofing.

The high edge. On a free-standing shed roof this is just a tall fascia. On an addition it is a headwall: the roof runs into an existing wall, and a base flashing has to turn up that wall with a counterflashing let in above it so water cannot get behind. The adopted code text quoted on this page requires flashing “at wall and roof intersections, wherever there is a change in roof slope or direction and around roof openings.” This edge is also, as it happens, the only place a single-slope roof can exhaust ventilation air — which is why two entirely separate trades have a claim on the same eight inches of wall.
The single plane. Every arrow points the same way and they get longer toward the bottom, because the water crossing any line on the roof is everything that fell above that line. On a gable the same building has two planes each collecting half as much; on a hip, four planes collecting less still, in four directions. The shed roof does not divide.
The ventilation cavity. Where the ceiling is applied to the underside of the rafters, each rafter bay is its own sealed tube. Seattle’s adopted residential code requires “cross ventilation for each separate space,” and where eave or cornice vents are installed, “not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.” The Department of Energy’s Building America assembly for a vented cathedral ceiling puts it the same way and goes further: only 1 inch of ventilation space above the cavity insulation is required by code, but 2 inches is recommended where ice damming could be an issue or where rafter spans are long. A shed roof over an addition is frequently a long span.
The slope arithmetic. The rise available at the high edge, divided by the depth of the roof, is the slope. It is worked out in full below, and it is the number that quietly decides which coverings are even permitted — because on a real addition the rise is capped by something that was built before anyone thought about the roof, usually a window sill.
The intake. The low openings that feed every one of those rafter bays, normally in the soffit under the eave overhang. If the roof has no overhang, the intake has to come from somewhere else, and that is a design decision made before the framing, not after.
The low eave. One gutter and its downspouts, carrying the runoff of the whole roof. This is the component most often specified by habit — the same profile and the same two downspouts that went on the gabled house next door — and habit is wrong here by a factor of two.
The strip of ground. Everything the downspouts discharge lands in one narrow band along one side of the building. Water repeatedly delivered to the same few square feet of soil beside a foundation is a foundation problem, and in a freezing climate it is a sheet of ice on whatever is underneath.

What the shape genuinely gives you

It is worth being clear that the simplicity is not marketing. A shed roof has no valley — one of the details most vulnerable to water intrusion on a pitched roof, and one of the more expensive ones to detail properly. It has no hip, so there are no angled cuts through the field and no hip-and-ridge accessory run. It has no ridge, so there is no ridge cap to buy or to fail. Every course runs from one end of the building to the other. A crew can stage it from one side, and an inspector — or an owner with binoculars — can see the entire roof surface from one position on the ground.

That last point deserves more weight than it usually gets. Most of what this site says about inspection is a series of workarounds for the fact that you cannot see a roof. On a shed roof over a single-storey addition, you frequently can. See what a real roof inspection covers for what to do with that view.

And where the simplicity stops

Two places. The first is the low edge, which is the whole of the next section. The second is the high edge, which has to be simultaneously watertight against a wall, structurally connected to something, and open enough to exhaust air. Those three requirements are in tension, they are usually detailed by different people, and on a shed roof they all land within a few inches of each other.

The arithmetic

One edge, twice the load: what the concentration actually amounts toSection link

This is the calculation a shed-roof proposal should already contain. It takes three lines, and it is the difference between a gutter that works and the failure this shape is known for.

Step one: the catchment is the plan area, not the roof area

Rain falls close enough to vertically that the area a roof collects from is its horizontal footprint — its plan area — not the larger sloped surface area you order material by. Tilting a plane does not put more sky above it. This trips people up in both directions: the material takeoff uses the sloped area, and the drainage uses the flat one. The difference between them is the pitch factor.

Step two: turn inches per hour into gallons per minute

One inch of rain per hour, falling on one square foot, is one twelfth of a cubic foot per hour. A cubic foot is 1,728 cubic inches and a US gallon is 231 cubic inches, so a cubic foot is 1,728 ÷ 231 = 7.4805 gallons. That gives:

(1 ÷ 12) × 7.4805 = 0.6234 gallons per hour, and 0.6234 ÷ 60 = 0.01039 gallons per minute, per square foot of plan area, per inch per hour of rainfall.

So the flow arriving at the bottom of a roof is:
gallons per minute = plan area (sq ft) × rainfall intensity (in/hr) × 0.01039

Step three: apply it to one building, two ways

Take a rectangular building 30 feet by 40 feet — a plan area of 1,200 square feet. Roof it two ways:

As a shed roof sloping across the 30-foot dimension: all 1,200 square feet drain to one 40-foot eave. That is 30 square feet of roof per linear foot of gutter.
As a gable with the ridge running along the 40-foot dimension: 600 square feet to each of two 40-foot eaves. That is 15 square feet per linear foot, on each of two gutters.

Same building, same roof area, same covering. The shed roof asks one gutter to do what the gable asks of two, and asks each foot of it to carry twice as much.

Peak flow arriving at the bottom of a 30 by 40 ft building (1,200 sq ft of plan area), roofed as a single shed plane and as a two-sided gable. Rainfall intensities are the mean 100-year, 60-minute partial-duration estimates from NOAA Atlas 14 at the coordinates in the footnote; flows are computed by the arithmetic above.
Location	NOAA Atlas 14 volume	100-yr, 60-min intensity (in/hr)	Shed roof: flow at the one gutter (gpm)	Gable: flow at each of two gutters (gpm)
Phoenix, Arizona	Volume 1, Version 5 (Southwest)	2.00	24.9	12.5
Portland, Maine	Volume 10, Version 3 (Northeastern States)	2.35	29.3	14.7
Chicago, Illinois	Volume 2, Version 3 (Ohio River Basin)	3.08	38.4	19.2
Raleigh, North Carolina	Volume 2, Version 3 (Ohio River Basin)	3.22	40.1	20.1
Houston, Texas	Volume 11, Version 2 (Texas)	4.85	60.5	30.2

Read this table one item at a time

Phoenix, Arizona

NOAA Atlas 14 volume: Volume 1, Version 5 (Southwest)
100-yr, 60-min intensity (in/hr): 2.00
Shed roof: flow at the one gutter (gpm): 24.9
Gable: flow at each of two gutters (gpm): 12.5

Portland, Maine

NOAA Atlas 14 volume: Volume 10, Version 3 (Northeastern States)
100-yr, 60-min intensity (in/hr): 2.35
Shed roof: flow at the one gutter (gpm): 29.3
Gable: flow at each of two gutters (gpm): 14.7

Chicago, Illinois

NOAA Atlas 14 volume: Volume 2, Version 3 (Ohio River Basin)
100-yr, 60-min intensity (in/hr): 3.08
Shed roof: flow at the one gutter (gpm): 38.4
Gable: flow at each of two gutters (gpm): 19.2

Raleigh, North Carolina

NOAA Atlas 14 volume: Volume 2, Version 3 (Ohio River Basin)
100-yr, 60-min intensity (in/hr): 3.22
Shed roof: flow at the one gutter (gpm): 40.1
Gable: flow at each of two gutters (gpm): 20.1

Houston, Texas

NOAA Atlas 14 volume: Volume 11, Version 2 (Texas)
100-yr, 60-min intensity (in/hr): 4.85
Shed roof: flow at the one gutter (gpm): 60.5
Gable: flow at each of two gutters (gpm): 30.2

Coordinates queried, in order: 33.4484 / −112.0740; 43.6591 / −70.2568; 41.8781 / −87.6298; 35.7796 / −78.6382; 29.7604 / −95.3698. One hour at the hundred-year interval is used here because it is long enough for a gutter to reach steady state and severe enough to be worth designing for — it is not necessarily the duration or return interval your jurisdiction requires, and the plumbing code it has adopted names the rate that governs. These are flows, not gutter sizes: converting a flow into a gutter profile, a downspout size, and a downspout count is done against the adopted code’s tables by whoever is designing the system, and this page publishes no capacity figure because no current adopted table could be read for it.

Two things fall out of the table that are worth saying plainly. First, the ratio never changes: the shed figure is twice the gable figure in every row — the odd tenth is rounding — because the geometry is the whole of it. Second, the absolute numbers change by a factor of two and a half across five ordinary American cities. A drainage assembly that is generous in Phoenix is marginal in Houston. Neither the roof nor the gutter knows which one it is on — the person specifying it has to.

The adopted residential code quoted on this page does not size gutters at all. Its concern is roof drains, and even there it hands the arithmetic off: the Washington amendment on secondary drainage requires that the installation and sizing of overflow drains, leaders and conductors comply with named sections of the Uniform Plumbing Code. Gutter and downspout sizing lives in the same place — in the plumbing code the jurisdiction has adopted, against a design rainfall rate it names. Ask which table was used and which rate went into it. If the answer is a habit rather than a table, you have found the problem before it found you.

The part below the downspout

Every one of those gallons has to go somewhere after it leaves the leader, and on a shed roof they all go to the same narrow strip of ground along one side of the building. Sixty gallons a minute delivered repeatedly to the same few square feet beside a foundation is a soil and foundation problem, not a roofing one, which is exactly why it falls between scopes and gets missed. Extensions, splash blocks, a buried leader run to daylight, and the grade itself are all functionally part of this roof.

The constraint

A shed roof on an addition does not choose its slope — the existing house chooses itSection link

This is the single most useful piece of arithmetic on the page, and it explains why so many additions end up with a roof nobody planned for.

On a new house a designer picks the slope. On an addition, the slope is what is left over. The roof has to start below something — a second-floor window sill, an existing eave, a band of trim, a balcony — and it has to end at a height that gives the addition a usable ceiling. Whatever vertical distance survives that is the rise, and the depth of the addition is the run.

The conversion is unusually clean. Slope is conventionally written as rise per 12 units of run. If the rise is R inches and the depth is D feet, the run is 12D inches, so:

slope = 12 × R ÷ (12D) = R ÷ D, in units per 12.

Inches of usable rise divided by feet of depth. Twenty-four inches across a twelve-foot addition is 2:12 exactly. The same twenty-four inches across sixteen feet is 1.5:12, and at 1.5:12 no lapped shingle of any kind is permitted under the adopted code quoted here.

One warning about R. It is the usable rise, not the tape measurement from the sill to the top of the new wall. Out of that raw figure come the thickness of the covering, the height the base flashing has to turn up the wall, the clearance the counterflashing needs above it, and the depth of the roof framing at the high end. Three or four inches disappearing there is ordinary, and on a deep addition three inches of lost rise is a quarter of a unit of slope.

Resulting roof slope for a shed-roofed addition, by usable rise at the high edge and depth of the addition, with the coverings that slope permits under the 2021 Seattle Residential Code. Slope in units per 12 is rise in inches divided by depth in feet.
Depth of addition	18 in usable rise	24 in usable rise	30 in usable rise	36 in usable rise
8 ft	2.25:12 — asphalt shingle only, double underlayment	3.00:12 — asphalt, tile, wood, metal shingle	3.75:12 — asphalt, tile, wood, metal shingle	4.50:12 — every common covering, slate included
10 ft	1.80:12 — no lapped shingle of any kind	2.40:12 — asphalt shingle only, double underlayment	3.00:12 — asphalt, tile, wood, metal shingle	3.60:12 — asphalt, tile, wood, metal shingle
12 ft	1.50:12 — no lapped shingle of any kind	2.00:12 — asphalt shingle only, double underlayment	2.50:12 — asphalt or tile only, double underlayment	3.00:12 — asphalt, tile, wood, metal shingle
14 ft	1.29:12 — no lapped shingle of any kind	1.71:12 — no lapped shingle of any kind	2.14:12 — asphalt shingle only, double underlayment	2.57:12 — asphalt or tile only, double underlayment
16 ft	1.13:12 — no lapped shingle of any kind	1.50:12 — no lapped shingle of any kind	1.88:12 — no lapped shingle of any kind	2.25:12 — asphalt shingle only, double underlayment
20 ft	0.90:12 — no lapped shingle, and below roll roofing too	1.20:12 — no lapped shingle of any kind	1.50:12 — no lapped shingle of any kind	1.80:12 — no lapped shingle of any kind

Read this table one item at a time

8 ft

18 in usable rise: 2.25:12 — asphalt shingle only, double underlayment
24 in usable rise: 3.00:12 — asphalt, tile, wood, metal shingle
30 in usable rise: 3.75:12 — asphalt, tile, wood, metal shingle
36 in usable rise: 4.50:12 — every common covering, slate included

10 ft

18 in usable rise: 1.80:12 — no lapped shingle of any kind
24 in usable rise: 2.40:12 — asphalt shingle only, double underlayment
30 in usable rise: 3.00:12 — asphalt, tile, wood, metal shingle
36 in usable rise: 3.60:12 — asphalt, tile, wood, metal shingle

12 ft

18 in usable rise: 1.50:12 — no lapped shingle of any kind
24 in usable rise: 2.00:12 — asphalt shingle only, double underlayment
30 in usable rise: 2.50:12 — asphalt or tile only, double underlayment
36 in usable rise: 3.00:12 — asphalt, tile, wood, metal shingle

14 ft

18 in usable rise: 1.29:12 — no lapped shingle of any kind
24 in usable rise: 1.71:12 — no lapped shingle of any kind
30 in usable rise: 2.14:12 — asphalt shingle only, double underlayment
36 in usable rise: 2.57:12 — asphalt or tile only, double underlayment

16 ft

18 in usable rise: 1.13:12 — no lapped shingle of any kind
24 in usable rise: 1.50:12 — no lapped shingle of any kind
30 in usable rise: 1.88:12 — no lapped shingle of any kind
36 in usable rise: 2.25:12 — asphalt shingle only, double underlayment

20 ft

18 in usable rise: 0.90:12 — no lapped shingle, and below roll roofing too
24 in usable rise: 1.20:12 — no lapped shingle of any kind
30 in usable rise: 1.50:12 — no lapped shingle of any kind
36 in usable rise: 1.80:12 — no lapped shingle of any kind

Slopes are arithmetic. Each cell names only the lapped shingle coverings the slope permits; other assemblies are covered by the sentence below it. The covering names are the deck-slope minimums of the 2021 Seattle Residential Code, read verbatim: asphalt shingles 2:12 with double underlayment up to 4:12 (R905.2.2), clay and concrete tile 2-1/2:12 with double underlayment up to 4:12 (R905.3.2), metal roof shingles 3:12 (R905.4.2), wood shingles and shakes 3:12 (R905.7.2, R905.8.2), mineral-surfaced roll roofing 1:12 (R905.5.2), slate 4:12 (R905.6.2). “No lapped shingle” does not mean no roof: mineral-surfaced roll roofing runs down to 1:12, standing-seam metal panels are permitted down to 1/4:12 and lapped metal panels with applied lap sealant down to 1/2:12 (R905.10.2), and single-ply, modified bitumen, built-up, sprayed foam and liquid-applied roofs all carry a 1/4:12 design-slope minimum. That is the law in Seattle and nowhere else; it is used here because it is checkable. Confirm your own adopted edition, amendments and effective date with your authority having jurisdiction, and check the covering manufacturer’s own minimum, which is often steeper than the code’s.

Read the table by row and the pattern is obvious: depth is the enemy of slope. A deep addition with a modest rise is not a shingle roof that needs care — it is a membrane roof, or a standing-seam roof, and it should have been priced and detailed as one from the beginning. A quote that shows architectural shingles on a twenty-foot-deep shed addition has either found rise nobody else could see or has not done this arithmetic.

The honest way to use the table is backwards. Decide what the roof has to be — because of appearance, because of what the house already wears, because of what is available locally — then read off the depth and rise that can support it, and design the addition inside that. Slope is cheap to fix on paper and expensive to fix in framing.

Where the answer lands below the lapped-covering thresholds, the low-slope page covers what changes, and standing-seam metal is worth a serious look — a single unbroken plane with no valleys and no cross-cuts is the geometry that suits it best.

The other edge

A roof with no ridge still has to exhaust somewhereSection link

On a gable, soffit-to-ridge airflow comes free with the shape. A shed roof gets none of that for free, and the strategy has to be named in the design rather than assumed.

The requirement is not shape-specific, which is exactly the problem. Seattle’s adopted residential code requires that enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters “shall have cross ventilation for each separate space.” On a shed roof with a cathedral ceiling, each rafter bay is a separate space — a sealed tube from the low edge to the high one, blocked at both ends by framing. Every one of them needs a low opening and a high one.

Where the reduced 1/300 ratio is used, the same code adds a placement rule: not less than 40 percent and not more than 50 percent of the required ventilating area must come from ventilators “located in the upper portion of the attic or rafter space,” and those upper ventilators must be “located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically.” On a gable the highest point is a ridge, and a ridge vent satisfies that sentence without anyone thinking about it. On a shed roof the highest point is a wall, or a fascia, and somebody has to decide what goes there.

And the cavity itself has a floor on it. Where eave or cornice vents are installed, that code requires that blocking, bridging and insulation not block the free flow of air, and that “not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.” The Department of Energy’s Building America assembly for a vented cathedral ceiling makes the same point and then recommends more: one inch is what code requires, two inches is recommended where ice damming could be an issue or where rafter spans are long. A shed roof over an addition is very often a long span with nothing interrupting it.

None of this is an argument that a shed roof must be vented. A correctly designed unvented assembly is legitimate, is explicitly permitted by the same code under stated conditions, and on a shallow single-slope roof with limited framing depth it is frequently the better engineering. What is not legitimate is leaving it undecided.

Five ways a single-slope roof actually handles the exhaust half of the problem, what each one depends on, and how each one fails. This is a description of approaches in use, not a recommendation of one; which is permitted and on what terms is set by the adopted code and the assembly design.
Strategy	How the air actually moves	What it depends on	How it goes wrong
Exhaust vent set into the plane near the high edge	Intake at the low eave soffit, up each rafter bay, out through a vent cut into the roof plane close to the top.	Enough clear cavity above the insulation in every bay; a vent placed within whatever distance of the highest point the adopted code allows; a product listed for the slope.	The vent is placed for convenience rather than height, or is fitted to some bays and not others, leaving the rest of the roof with intake and no exhaust.
Off-wall vent set back from the high wall	The same path, but the exhaust is a continuous vent set into the plane a short distance down-slope of the wall, so the wall cladding and its flashing are never opened.	A manufactured vent made for this condition. DCI's SmartVent literature describes exactly this off-wall shed application and requires continuous intake below it with an open air path to the soffit.	Installed without the matching intake, which turns a balanced path into a suction point drawing conditioned air out of the rooms below.
Flat ceiling with a wedge-shaped attic above	The ceiling is hung level while the roof slopes, creating a triangular attic that is genuinely one connected space with a low side and a high side, vented like any attic.	Enough headroom to lose to the wedge, and framing that allows the ceiling to be independent of the rafters.	The wedge is too shallow at the low end to keep the intake open once insulation is blown in, so the space vents only at the top.
Unvented compact assembly	It does not move. The insulation and air control are arranged so the sheathing never gets cold enough or wet enough to matter, and the roof has no ventilation cavity at all.	Meeting the adopted code's conditions for unvented assemblies in full — insulation type and placement, vapour retarder class, and the thermal envelope location — as a design, not as an omission.	Treated as the fallback when venting proved awkward, with air-permeable insulation against the sheathing and no continuous air barrier. That is not an unvented assembly; it is an unventilated one.
Vented over-roof above an unvented assembly	A second, ventilated air space is built above the sheathing of an otherwise unvented roof, so the outer surface stays cold independently of the insulated assembly below it.	Deep-snow country. The Department of Energy's guidance presents this as ice-dam control and states that cathedral ceilings where the ground snow load exceeds 50 lb/sq ft require venting over the thermal control layer.	Built in a climate that never needed it, adding cost, height and two more edges to detail for no benefit.

Read this table one item at a time

Exhaust vent set into the plane near the high edge

How the air actually moves: Intake at the low eave soffit, up each rafter bay, out through a vent cut into the roof plane close to the top.
What it depends on: Enough clear cavity above the insulation in every bay; a vent placed within whatever distance of the highest point the adopted code allows; a product listed for the slope.
How it goes wrong: The vent is placed for convenience rather than height, or is fitted to some bays and not others, leaving the rest of the roof with intake and no exhaust.

Off-wall vent set back from the high wall

How the air actually moves: The same path, but the exhaust is a continuous vent set into the plane a short distance down-slope of the wall, so the wall cladding and its flashing are never opened.
What it depends on: A manufactured vent made for this condition. DCI's SmartVent literature describes exactly this off-wall shed application and requires continuous intake below it with an open air path to the soffit.
How it goes wrong: Installed without the matching intake, which turns a balanced path into a suction point drawing conditioned air out of the rooms below.

Flat ceiling with a wedge-shaped attic above

How the air actually moves: The ceiling is hung level while the roof slopes, creating a triangular attic that is genuinely one connected space with a low side and a high side, vented like any attic.
What it depends on: Enough headroom to lose to the wedge, and framing that allows the ceiling to be independent of the rafters.
How it goes wrong: The wedge is too shallow at the low end to keep the intake open once insulation is blown in, so the space vents only at the top.

Unvented compact assembly

How the air actually moves: It does not move. The insulation and air control are arranged so the sheathing never gets cold enough or wet enough to matter, and the roof has no ventilation cavity at all.
What it depends on: Meeting the adopted code's conditions for unvented assemblies in full — insulation type and placement, vapour retarder class, and the thermal envelope location — as a design, not as an omission.
How it goes wrong: Treated as the fallback when venting proved awkward, with air-permeable insulation against the sheathing and no continuous air barrier. That is not an unvented assembly; it is an unventilated one.

Vented over-roof above an unvented assembly

How the air actually moves: A second, ventilated air space is built above the sheathing of an otherwise unvented roof, so the outer surface stays cold independently of the insulated assembly below it.
What it depends on: Deep-snow country. The Department of Energy's guidance presents this as ice-dam control and states that cathedral ceilings where the ground snow load exceeds 50 lb/sq ft require venting over the thermal control layer.
How it goes wrong: Built in a climate that never needed it, adding cost, height and two more edges to detail for no benefit.

Rows describing manufacturer literature are product-specific and are cited as evidence that the detail exists and is manufactured, not as a recommendation. Net free area, listing, and permitted use belong to the specific product and to the authority having jurisdiction. There is no universal ventilation ratio, and nothing in this table establishes what any building requires.

One further thing about the intake. On a shed roof the intake is almost always in the soffit under the low eave — which is the same edge carrying the gutter, the downspouts, and any snow that releases. Everything is at the bottom. A gutter set too high, a fascia detail that closes the soffit, or an insulation job that drifts into the eave will shut the intake for the whole roof at once, and the symptom will appear at the far end, a storey away, months later. A baffle in every bay is cheap when the roof is open and impossible afterwards.

The arithmetic behind net free area, and the reason this page will not publish a ratio as a general rule, is on the ventilation guide.

The junction

When the shed roof is attached to something that was already thereSection link

An addition, a porch, or a dormer off an existing wall is the case this section is about: the high edge dies into a building nobody designed for it, and that junction is where the roofing scope, the siding scope, and the structural scope all meet.

Stop — immediate hazard

A low roof against a taller wall is where snow collects, and a sagging ceiling under it is not a maintenance item

Wind moves snow off high roofs and drops it against whatever interrupts the flow, so a low roof beside a tall wall collects a drift far heavier than the snow that fell on it. That surcharge lands in the strip closest to the wall — which is also where the addition is hanging off the existing building.

How much load that is, and whether the framing and the connection can carry it, is a determination made by a registered design professional for the specific building against the standard the jurisdiction has adopted. It is not a number this or any website can supply.

If the ceiling under a shed roof is sagging or bulging, if doors or windows in the addition have suddenly stopped closing, or if you can hear cracking, leave the addition, keep people out of the rooms below and beside it, and call a licensed contractor or a structural engineer. If wiring is involved or a line is down, call the utility and emergency services first. Do not go onto the roof to clear it.

The high edge, in the order water travels

Water running down the existing wall arrives at the top of the new roof, and everything at that junction exists to get it out onto the roof surface rather than behind it. The adopted code requires flashing “at wall and roof intersections, wherever there is a change in roof slope or direction,” and for a sidewall it requires base flashing not less than 4 inches high and 4 inches wide, directing water away from the wall onto the roof or into the gutter, with the vertical leg run continuous under the siding.

That last clause is the one that gets skipped, because honouring it means removing and reinstalling cladding. A counterflashing sealed onto the face of the siding instead is a repair detail, not a construction detail, and it has a service life measured by the sealant. If a proposal for an addition does not mention taking siding off, ask how the flashing is getting behind it.

The low edge, where it dies into a wall

An addition narrower than the house produces a second junction: the low eave runs into a wall at one or both ends. This is where kickout flashing belongs, and it is the most consequential small piece of metal on the building. The Department of Energy’s Building America guidance is blunt about what happens without it: rainwater can leak into the wall and cause serious water damage to sheathing, framing and insulation and mould inside the wall cavities — and newer claddings like vinyl and fibre cement can conceal that damage for years. The same guidance notes that the end of the gutter should not touch the intersecting wall; a small gap is left so water drains freely.

On a shed roof this matters more than on a gable, because the water arriving at that corner is the runoff of the entire plane rather than half of it. See why flashing causes more leaks than field material does.

Before anything is opened up on an older house

Tying a new roof into an existing wall means cutting into siding and whatever is behind it. On houses built before about 1990, siding, felts, mastics and pipe wrap may contain asbestos. EPA’s guidance is that asbestos-containing material in good condition and left undisturbed will generally not release fibres, but that it may release fibres when disturbed, damaged, removed improperly, repaired, cut, torn, sanded, sawed, drilled or scraped — and that if changes are being made that might disturb it, repair or removal by a trained and accredited asbestos professional is needed. Age is a reason to test, never a finding. Only testing the actual material settles it.

What holds the roof up

The high end of an addition’s shed roof is usually carried on a ledger bolted to the existing structure, which means the existing framing is now carrying a load it was not designed for, through a connection that has to be both structurally adequate and watertight at the same point. Those two requirements are not naturally compatible, and reconciling them is design work.

This page explains the concept and stops there. Whether your existing framing can carry a new roof, what the ledger connection has to be, and what drifted snow adds to it are determinations for a registered design professional and for the authority having jurisdiction on the permit.

Money

What the shape does to the price, without inventing a numberSection link

This page publishes no dollar figure for a shed roof, because no honest one exists at the level of a shape. What can be said is which mechanisms move the number, and in which direction.

A roofing price is built from area, covering, tear-off, disposal, access, complexity, and market. Shape enters through complexity, and on a shed roof it does so in an unusually legible way: it takes cost out of the middle of the roof and puts some of it back at the edges.

Cut waste falls. No hips and no valleys means no angled cuts through the field and no offcuts from them. Ordering is close to the sloped area plus a normal waste factor, rather than area plus waste plus geometry.
Accessory runs fall. No valley metal, no hip-and-ridge, and on a roof vented some other way, no ridge vent either. Those are priced by the lineal foot and a shed roof has none of them.
Labour per square falls. Full-length courses, one staging position, one plane to walk. This is the shape a crew moves fastest across.
Framing is simpler. One set of rafters, all the same length, no hip or valley members, no ridge assembly.
Drainage rises. More downspouts, possibly a larger gutter profile, and more of whatever gets the water away from the building at the bottom. This is where the savings above partly go.
The covering may change category. The largest single cost effect of this shape is not a percentage — it is the possibility that the slope arithmetic pushes the roof out of shingles and into a membrane or a standing-seam panel, which is a different price, a different crew, and a different maintenance profile.
Repair costs over the life of the roof fall. Rarely quoted, and on this shape it runs in the owner’s favour. A single plane with no valleys, reachable from one side, is the cheapest common roof to repair for as long as it stands.

If you want an area figure to reason with, start with pitch and the pitch factor and then use the calculators. If you want the money conversation itself, it lives at roof cost, and the basis behind every figure on this site is published at the cost methodology.

Considerations

What changes this on a real buildingSection link

The axes below are the ones that genuinely move on a single-plane roof. Slope and drainage are the same conversation here in a way they are not on a gable.

Slope and drainage: A shed roof has exactly one slope and no way to hide a shallow one. Where a gable can be steep at the front and let a rear addition be shallow, a shed roof is a single decision applied to the whole plane. The arithmetic below turns available headroom into a slope figure, and the slope figure decides the covering. Below the minimum for lapped coverings the roof is not a shingle roof with a problem; it is a different kind of roof, covered at flat and low-slope roofs.
Minimum slopes are set by the code your jurisdiction has adopted and amended, not by a national rule. The figures used on this page are from the 2021 Seattle Residential Code and are the law in Seattle and nowhere else.
Moisture and ventilation: Single-slope roofs are frequently built as cathedral assemblies with no attic. That removes the buffer volume where moisture problems normally announce themselves slowly, and it means the coldest sheathing on the roof — the high end, farthest from the intake — is directly above the insulation. Condensation there is routinely reported as a leak, because it appears in wet weather and stops in dry weather, which is exactly what a leak does.
There is no universal ventilation ratio and no universal answer. A vented cathedral bay and a correctly designed unvented compact assembly are both legitimate; which is required, and on what terms, is set by the adopted code edition, the climate zone, and the assembly.
Structural weight: Two structural questions belong to this shape specifically. A shed roof on an addition usually hangs its high end on a ledger fixed to the existing building, so the existing framing is now carrying a load it was not built for. And a low roof against a taller wall collects drifted snow against that wall, which concentrates load exactly where the connection is. Both are engineering questions.
This page publishes no structural, snow-load, or connection determination and cannot. Ledger attachment and snow drift surcharge are answered by a registered design professional for the specific building, and by the authority having jurisdiction on the permit.
Wind: A single large plane presents one uninterrupted surface to the wind, and a monoslope roof develops different pressure patterns from a gable of the same footprint — including at the tall fascia or parapet along the high edge, which stands up in the airflow. What those pressures are at a particular address is a design calculation.
Wind performance is site- and building-specific. A covering marketed with an mph figure has passed a laboratory test on a described assembly; it is not a determination about your building, your exposure, or your roof geometry.
Climate: In a snow climate the single plane is both an advantage and a risk: nothing interrupts sliding snow, so it does not build up unevenly around dormers and valleys, and nothing interrupts it, so it can release in one event onto whatever is below. In a heavy-rain climate the concentration at the low edge is the dominant issue. In a hot, dry climate with intense burst rainfall — the Southwest monsoon pattern — the roof spends most of the year irrelevant and then has to handle an hour that matters.
Code and jurisdiction: Every code figure on this page is drawn from one named adopted code — the 2021 Seattle Residential Code, effective 15 November 2024 — because a code figure with a jurisdiction, an edition, and an effective date can be checked and one without them cannot. Seattle’s is the 2021 International Residential Code as amended by Washington State and by Seattle, so it is neither model text nor transferable.
There is no nationwide building code for site-built construction. Confirm the adopted edition, the local amendments, and the effective date with your own authority having jurisdiction before relying on any number here.
Maintenance: The maintenance list for a shed roof is short and almost entirely at the bottom. One gutter run to keep clear, one set of downspouts to keep flowing, one strip of ground to keep draining away from the building. The compensation is that a single plane is the cheapest common roof to reach, stage, and repair, and the only one you can meaningfully assess from a single spot on the ground.

Warranty and repair

What is covered, and what can actually be repairedSection link

Nothing about the shape changes what a warranty document says. What the shape changes is which exclusions you are likely to trip.

The slope exclusion: Covering warranties are written against an installation that followed the manufacturer’s instructions, and those instructions carry a minimum slope and, below a second threshold, additional underlayment requirements. A shed roof is the shape most likely to end up just under one of those lines, because its slope is set by whatever headroom was available rather than by choice. Get the as-built slope written down.
Drainage is usually somebody else's line item: Gutters, downspouts, and leaders are frequently supplied by a different trade under a different line of the contract, and a roofing workmanship warranty may not reach them at all. On a shed roof, that is the component under the most stress. Ask explicitly who warrants the drainage and for how long.
The tie-in belongs to two scopes: Where the high edge meets an existing wall, the flashing is roofing work and the cladding above it is siding work. Water that gets in there will be attributed to whichever trade is not in the room. A single scope of work covering the whole intersection is worth more than either warranty.

Repairability

This is where the shape wins outright. A shed roof is the most repairable common geometry in residential construction: one plane, continuous courses, no valleys to cut into, no hips to rebuild, and — on a single-storey addition — often reachable without staging the whole building. Replacing the whole covering means replacing one rectangle.

The exception is a shallow shed roof carrying a membrane. There the repair is a membrane repair, which needs the right material, the right seam method, and someone who does that work rather than someone who does shingles. See membranes at residential scale.

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

Six questions that are specific to this shape. The first two are the ones that separate a proposal that has thought about a shed roof from one that has priced a rectangle.

What is the finished slope going to be, in rise over run, and how did you arrive at it?
On a shed roof this is not a style choice; it is a consequence of the depth and the available rise. A contractor who has measured it can tell you the number. One who has not will describe the slope in adjectives. The number decides which coverings are permitted, whether double underlayment is required, and whether this is a shingle job at all.
How many downspouts are going on that gutter, and what rainfall rate did you size them for?
The honest answers are a number and a source. A shed roof puts roughly twice the flow per foot of gutter that a same-footprint gable does, and sizing is done against the adopted plumbing code’s tables and the local rainfall rate. “We always put two on” is the answer that produces the characteristic failure.
Where does the water go after it leaves the downspouts?
Everything the roof catches is being delivered to one strip of ground on one side of the building. Extensions, splash blocks, a buried leader to daylight, and the grade itself are all part of the roof on this shape even though none of them are roofing. An answer that stops at the bottom of the downspout is not an answer.
How is this roof exhausting, and where is the intake coming from?
There is no ridge. A competent answer names a specific exhaust — a vent set into the plane near the high edge, an off-wall vent, a small ventilated attic above a flat ceiling — or says plainly that the assembly is unvented and describes how the insulation and vapour control are arranged. An answer that says “ridge vent” has not looked at the roof.
At the high edge, what is the flashing sequence and does the counterflashing get let into the wall or surface-mounted?
This is one of the details most likely to leak on an addition, and it is buried behind cladding forever afterwards. A contractor who can describe the lap order without hesitating has done it. Get the answer before the siding goes back.
What is directly under the low edge, and what happens to it in February?
If the answer is a door, a walk, a deck, or a parking space, the conversation about snow guards, gutter position, and heat cable belongs in the design, not in the second winter. On an uninterrupted plane there is nothing to slow a release down.

Require these in writing

The finished roof slope, in rise over run, stated as a number
Roof area in squares, separately from the plan area of the building
The covering, with the manufacturer's minimum slope for it and whether double underlayment applies
Gutter profile and size, downspout size, and the count of downspouts on the low edge
Where each downspout discharges, and any extension, splash block, or buried leader
The high-edge detail: base flashing, counterflashing, and whether cladding is being removed and reinstalled
The ventilation strategy in words — vented with a named exhaust and named intake, or unvented with the assembly described
Who is responsible for the gutters and downspouts, and under whose warranty

What goes wrong

Misconceptions and failure modesSection link

Common misconceptions

Common belief
A shed roof is basically a flat roof.
What is actually true
Not usually, and the distinction is the whole point. A shed roof is a pitched roof with one plane; a low-slope roof is a roof shallow enough that lapped coverings stop working. A 6:12 shed roof is a steep-slope roof by any definition. What is true is that a shed roof is the shape most likely to become a low-slope roof by accident, because its slope is set by the headroom available rather than chosen.
Common belief
One plane means half the roof, so half the gutter work.
What is actually true
Backwards. The roof area is whatever it is — one plane covering the same footprint has roughly the same area as two planes covering it. What halves is the number of edges available to drain it. Per linear foot of gutter, the load doubles.
Common belief
There is no ridge, so a shed roof cannot be ventilated.
What is actually true
It can, and there are at least five ways to do it, set out in the table below. What is true is that none of them is the default that comes free with a gable, so on a shed roof the ventilation strategy has to be named in the design. Skipping it does not produce an unvented assembly — a correctly designed unvented assembly is a deliberate thing with its own requirements. Skipping it produces a badly vented one.
Common belief
A shed roof is cheaper.
What is actually true
Cheaper to frame, cheaper to cut, cheaper to stage, cheaper to repair. Not cheaper by area, not cheaper by covering, and not cheaper at the low edge, where it needs more drainage capacity than the roof it is being compared with. This page publishes no cost figure for a roof shape, for reasons set out below.
Common belief
Shallower is safer, because less snow slides off.
What is actually true
Shallower means snow stays on longer, meltwater moves more slowly, and the covering options narrow. It also moves the roof toward the slope range where an ice dam has more opportunity to form and to back water up under laps. Slope is not a safety dial.

How it actually fails

The gutter overtops in a heavy burst: The gutter and its downspouts were sized as though the roof had two edges. Water leaves over the front lip in sheets during the heaviest part of a storm, which is the part nobody is outside watching.What you can see: A line of splash staining along the ground or the wall directly below the gutter; soil eroded in a strip; a gutter that is clean, level, and still overflows.
The far end of the run stays full: Enough downspout capacity in total, but all of it at one end of a long single gutter. The far end fills before the water reaches an outlet, so the overflow happens at a predictable spot.What you can see: Overflow always at the same end; fascia paint failing in one localised patch rather than along the run.
The wall below the low eave rots from the inside: Where the low eave dies into a wall — common on an addition narrower than the house — the last few feet of gutter discharge against the cladding instead of into the gutter, because no kickout flashing diverts it. The Department of Energy’s Building America guidance is explicit that without it, water can leak into the wall and damage sheathing, framing, insulation, and cause mould inside the wall cavities, and that vinyl and fibre-cement claddings can conceal that for years.What you can see: Often nothing, for a long time. Then paint or trim failure at the base of that wall, a musty smell in the room behind it, or staining that appears at the bottom of the wall rather than the top. The BASC guidance also notes that the end of the gutter should not touch the intersecting wall.
Condensation at the high end of the rafter bays: A cathedral assembly with intake at the eave and no working exhaust at the high edge, or with the required clear air space closed off by insulation. Moist indoor air reaches the coldest sheathing on the roof and condenses there.What you can see: Damp or staining that appears in cold weather and in a band along the top of the ceiling near the high wall, rather than during rain; fasteners rusting through from the deck side.
The headwall lets water in behind the cladding: The base flashing at the high edge was not carried far enough up the wall, or the counterflashing above it was surface-sealed rather than integrated with the drainage plane. Water running down the wall gets behind the roof rather than onto it.What you can see: Staining on the ceiling in a line parallel to the high wall; sealant beads visible along the top of the flashing, which is usually a repair of this failure rather than a detail.
The whole snow load releases at once: An unbroken plane with a slick covering and nothing — no ridge, no valley, no dormer, no penetration — to hold it. Solar gain releases the bond at the covering and the sheet goes.What you can see: Bent gutters and gutter hangers on the low edge, damaged planting or paving in a strip below it, and a roof that is mysteriously bare after a storm that left the neighbours covered.
A covering used below its minimum slope: The shape drifted below the threshold during design or during a later addition, and the covering was chosen by matching the house rather than by checking the slope. Laps that rely on gravity stop shedding and start holding.What you can see: Water marks along the horizontal lines of the courses; persistent damp at the low third of the plane; a covering that fails uniformly across the field rather than at a detail.

Sources and further readingSection link

Understanding Roofing / Published August 27, 2026

Scope and limitations

It cannot size the drainage on your roof.
The worked example converts area and rainfall into a flow rate, which is one input.
Gutter profile, downspout size and count, and outlet placement are sized against the plumbing code your jurisdiction has adopted and the design rainfall rate it names, by someone who has measured the roof.
It publishes no gutter or downspout capacity table.
The adopted residential code quoted here delegates the sizing of drains, leaders and conductors to the plumbing code, and no current adopted plumbing-code table could be opened and read for this page.
A capacity figure taken from a secondary source would look like evidence and would not be evidence.
It cannot tell you whether your existing framing can carry a new shed roof, or what drifted snow will do at the high wall.
Ledger attachment, load path, and drift surcharge are determinations made by a registered design professional for the specific building.
It publishes no ventilation ratio as a general rule and no required net free area.
The 1/150 and 1/300 figures discussed here are the text of one jurisdiction's adopted code, and the placement rules attached to them apply to that jurisdiction.
Whether the assembly must be vented at all depends on the adopted edition, the climate zone, and the design.
It makes no wind determination.
A single large plane behaves differently from a gable of the same footprint, but the pressures on any real roof depend on basic wind speed, exposure, height, geometry, enclosure classification, and risk category at that address.
It publishes no cost figure.
Roof shape moves a price through framing labour, cut waste, staging, accessory runs, and drainage capacity, but no transparent national dataset isolates the shape from the covering, the market, and the building.
It publishes no figure for how common shed roofs are.
Roof shape is one of the most frequently missing attributes in public building databases, and no federal inventory counts it.

2021 Seattle Residential Code, Chapter 9 — Roof Assemblies (adopted law, City of Seattle)
Seattle Department of Construction and Inspections — the adopting jurisdiction / 2021 edition, in effect 15 November 2024
Read verbatim for this page: R903.2.1, that flashings shall be installed at wall and roof intersections, wherever there is a change in roof slope or direction and around roof openings, that a flashing shall be installed to divert water away where the eave of a sloped roof intersects a vertical sidewall, and that metal flashing shall be corrosion resistant and not less than 0.019 inch (No. 26 galvanized sheet); R905.2.8.3, that base flashing against a vertical sidewall shall be continuous or step flashing, not less than 4 inches in height and 4 inches in width, and shall direct water away from the sidewall onto the roof or into the gutter, with the vertical leg continuous under the siding; R903.4, that unless roofs are sloped to drain over roof edges, roof drains shall be installed at each low point; R903.4.1, a Washington State amendment, which requires the installation and sizing of overflow drains, leaders and conductors to comply with Sections 1101 and 1103 of the Uniform Plumbing Code; and the deck-slope minimums used in the slope table — R905.2.2 asphalt shingles at 2:12 or greater with double underlayment from 2:12 to 4:12, R905.3.2 clay and concrete tile at 2-1/2:12 with double underlayment to 4:12, R905.4.2 metal roof shingles at 3:12, R905.5.2 mineral-surfaced roll roofing at 1:12, R905.6.2 slate at 4:12, R905.7.2 wood shingles and R905.8.2 wood shakes at 3:12, R905.10.2 metal roof panels at 3:12 lapped without sealant, 1/2:12 lapped with applied lap sealant and 1/4:12 standing seam, and the 1/4:12 design slope for built-up (R905.9.1), modified bitumen (R905.11.1), thermoset single-ply (R905.12.1), thermoplastic single-ply (R905.13.1), sprayed polyurethane foam (R905.14.1) and liquid-applied roofing (R905.15.1).
This is the law in the City of Seattle and nowhere else. It is the 2021 International Residential Code as amended by Washington State (sections marked [W]) and by Seattle, so it is neither pure model text nor transferable to another address. It is cited here because a code figure with a named jurisdiction, edition and effective date can be checked. Confirm your own adopted edition, amendments and effective date with your authority having jurisdiction.
2021 Seattle Residential Code, Chapter 8 — Roof-Ceiling Construction, Section R806 Roof Ventilation (adopted law, City of Seattle)
Seattle Department of Construction and Inspections — the adopting jurisdiction / 2021 edition, in effect 15 November 2024
Read verbatim for this page: R806.1, that enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow, with openings of 1/16 inch minimum and 1/4 inch maximum least dimension, opening directly to the outside air; R806.2, that the minimum net free ventilating area shall be 1/150 of the area of the vented space, with the 1/300 exception conditioned on both a Class I or II vapor retarder on the warm-in-winter side of the ceiling in Climate Zones 6, 7 and 8 and on not less than 40 percent and not more than 50 percent of the required ventilating area being provided by ventilators located in the upper portion of the attic or rafter space, not more than 3 feet below the ridge or highest point of the space measured vertically, with the balance in the bottom one-third, and permission to install upper ventilators more than 3 feet below the highest point where framing conflicts; R806.3, that where eave or cornice vents are installed, blocking, bridging and insulation shall not block the free flow of air, and that not less than a 1-inch space shall be provided between the insulation and the roof sheathing and at the location of the vent; and R806.5, the Washington-amended conditions under which unvented attics and unvented enclosed rafter assemblies are permitted.
Adopted law in Seattle only, amended from the 2021 IRC by Washington State and by Seattle. The ratios and placement rules quoted are that jurisdiction's requirements and are not a universal rule; they are used on this page to show what the requirement looks like when it meets a roof with no ridge, not to state what applies elsewhere.
The Seattle construction codes are being updated (notice of the 15 November 2024 effective date)
Seattle Department of Construction and Inspections / 5 September 2024
That the Seattle construction codes were updated to the 2021 editions — building, residential, existing building, energy, mechanical, plumbing, fuel gas and fire — with an effective date of 15 November 2024, and that applicants could use either the 2018 or 2021 Seattle codes until then.
An official announcement of an effective date, not the code text itself, and specific to Seattle.
NOAA Atlas 14 Point Precipitation Frequency Estimates (Precipitation Frequency Data Server)
NOAA National Weather Service, Hydrometeorological Design Studies Center
Every rainfall intensity in the drainage table: the mean 100-year, 60-minute partial-duration precipitation intensity in inches per hour at five coordinates — Phoenix AZ 2.00 (Volume 1 Version 5, Southwest), Portland ME 2.35 (Volume 10 Version 3, Northeastern States), Chicago IL 3.08 and Raleigh NC 3.22 (Volume 2 Version 3, Ohio River Basin), and Houston TX 4.85 (Volume 11 Version 2, Texas).
These are statistical estimates of precipitation frequency at a point, not a forecast and not a design requirement. Each is the mean of a confidence interval the server also publishes, and the volume and version differ by region. Which return interval and duration a roof must be designed for is set by the plumbing code the jurisdiction has adopted, not by this dataset. Seattle is outside NOAA Atlas 14 coverage, which is why it does not appear in the table even though the code figures on this page are Seattle's. NOAA Atlas 15 is in publication and is intended to supersede Atlas 14 as the national standard; Atlas 14 is the published estimate for these five locations as of the access date above, and the figures here should be re-read against Atlas 15 once it covers them.
Step and Kick-Out Flashing at Roof-Wall Intersections
U.S. Department of Energy, Building America Solution Center (PNNL)
The instruction to install step and kick-out flashing at all roof-wall intersections to protect walls from water intrusion; that step flashing should extend at least 4 inches up the wall from the roof deck and at least 4 inches out along the roof deck, and that metal flashing should be galvanized steel at least 0.019 inches thick; that where kick-out flashing is absent or inadequate rainwater can leak into the wall and cause serious water damage to wall sheathing, framing and insulation and mould inside the wall cavities, and that vinyl and fibre-cement claddings can conceal that damage for years; and that the end of the gutter should not touch the intersecting wall, with a small gap left to allow free drainage.
Best-practice guidance for builders, not adopted law. Its dimensional and material citations are to the 2015 IRC and its section references to the 2009, 2012, 2015 and 2018 editions, several of which have been superseded. It does not establish what is required at any address.
Vented Cathedral Ceiling, 2x6 Wall, Interior Insulated Basement (building assembly)
U.S. Department of Energy, Building America Solution Center (PNNL)
That a 2-inch ventilation space is used above the insulation in this assembly, and the statement that only 1 inch of ventilation space above the cavity insulation is required by code but 2 inches is recommended in areas where ice damming could be an issue or where rafter spans are long.
One illustrated assembly from a programme guidance library, not a code requirement and not a design for any particular building. Its code references are to the 2021 IRC and 2021 IECC as model documents. It describes a rafter-vented cathedral ceiling generally and says nothing specific about single-slope geometry.
Vented Over-Roof, Unvented Cathedral Ceiling, 2x6 Wall, Interior Insulated Basement (building assembly)
U.S. Department of Energy, Building America Solution Center (PNNL)
That the function of the vented over-roof is to control ice damming; that cathedral ceilings in locations with a ground snow load greater than 50 lb/sq ft require venting over the thermal control layer; and that the thermal resistance of deep snow can be high enough to allow the lower level of snow in contact with the roof to melt and cause ice damming unless the roof is vented.
Programme guidance presented for IECC Climate Zone 6A, not adopted law and not a determination for any building. Ground snow load at a specific site is established by the adopted standard and the authority having jurisdiction, not by this page.
SmartVent Off Wall and Shed Roof / Roof-to-Wall Application
DCI Products
That a manufactured roof-plane exhaust vent is made specifically for the shed-roof and roof-to-wall condition; that it is installed as close to the shed wall as possible in the standard application and set back into the plane in the off-wall application, which the manufacturer offers as a way of ventilating a shed roof without removing existing flashing and siding from the wall; and that the instructions require proper continuous intake ventilation below the roof-to-wall ventilation and an open air path from the vent to the soffit area.
Manufacturer literature for one product, cited as evidence that this exhaust condition is a recognised and manufactured detail — not as a recommendation of the product and not as a statement of required vent area. Net free area, listing, and permitted use are product-specific and are established by the product's own listing and instructions and by the authority having jurisdiction.
Protect Your Family from Exposures to Asbestos
U.S. Environmental Protection Agency
That asbestos-containing material in good condition and not disturbed by remodeling will generally not release fibres; that asbestos-containing materials may release fibres when disturbed, damaged, removed improperly, repaired, cut, torn, sanded, sawed, drilled or scraped; and that if the material is more than slightly damaged, or if changes are being made that might disturb it, repair or removal by a trained and accredited asbestos professional is needed.
Consumer guidance, not a regulation and not a determination about any material. The page carries no statement tying asbestos content to a construction year; the pre-1990 caution on this page is this site's own screening convention for when to test, not an EPA finding, and age is never determinative. Only laboratory testing of the actual material establishes whether asbestos is present.
Automatic Roof Type Classification Through Machine Learning for Regional Wind Risk Assessment (arXiv:2305.17315)
Shuochuan Meng, Mohammad Hesam Soleimani-Babakamali and Ertugrul Taciroglu, University of California, Los Angeles — preprint submitted to Advanced Engineering Informatics / Preprint dated 27 May 2023
That roof type is, in the authors' words, the most frequently missing building feature from publicly available databases, and that their study classified roof type for 161,772 single-family houses across New Hanover County, North Carolina and Miami-Dade County, Florida, finding high variance in the dominant roof type among census tracts.
Academic research used here only to support the statement that no public inventory reliably records roof shape. It is not a national statistic; both study areas are coastal, hurricane-prone counties, and roof types were assigned by a machine-learning classifier from imagery, which could not assign a type at all for 17 percent of houses in New Hanover and 11 percent in Miami-Dade. Only the abstract was read for this page; the same study's county-level classification figures are quoted, from the full text, on the gable and combination roof pages.

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