For homeowners, buyers, and residential contractors

A mansard is two roofs on one building, and the line between them decides it.

Steep-slope and low-slope on one building · single-family, rowhouse, and small commercial

Two slopes on every side: a near-vertical lower face and a shallow upper deck. Each half is governed by a different set of rules, and the joint where they meet is the part nobody prices.

30-second answer

What is a mansard roof, and why is it harder to re-roof than it looks?

A mansard has two slopes on every side: a near-vertical lower face and a shallow upper deck. That lower face behaves like a wall and the upper one like a flat roof, so the two halves often need different coverings, different fastening, and different detailing — and the line where they meet, called the deck line, is the joint everything depends on.

Stop — immediate hazard

A mansard drops whatever comes loose straight onto the ground people walk on

On a gable or hip roof, a piece that lets go slides down a slope and usually stops in a gutter. On a mansard the lower face is close to vertical and it is directly over the entrance, the sidewalk, and the windows below. There is nothing between a loosened slate and the pavement.

Nothing on this page requires you to go up. A mansard cannot be walked in any case — the lower face is too steep to stand on and the upper deck can only be reached across it — so everything here is assessed from the ground with binoculars, from an upper window of the building across the street, or from a contractor’s report. Read what a real roof inspection covers before you accept anyone’s account of what is happening up there. For a crew, OSHA requires fall protection for residential construction work six feet or more above a lower level, which on a mansard means staging, a lift, or a fall-arrest system rather than a ladder and good intentions.

If slate, tile, shingles, or trim are sliding, hanging, or landing below a mansard face, keep people and vehicles clear of the ground underneath it, keep the doors under it out of use, and call a roofing contractor. If the material is falling onto a public sidewalk or street, tell your municipality as well: many jurisdictions place obligations on a building owner when a facade or roof sheds material onto the public way, and what those are where you live is a question for the municipality and, if it matters, for a lawyer.

Learning paths and saved lessons

At a glance

The short versionSection link

A mansard is defined by its two slopes. Almost everything difficult about it follows from the fact that the two are governed separately.

The shape: Two slopes on all four sides: a steep lower face, a shallow upper deckA gambrel is the same idea on two sides only, closed by gable ends. On a mansard the break line runs continuously around the building and turns four corners.
The transition: The deck lineThe horizontal line where the steep face stops and the shallow roof begins. Zoning codes use the same term for the same line.
The lower face: Near-verticalThe worked example below uses 75°, which is about 44.8 inches of rise per 12 inches of run. ARMA puts 21:12 — about 60.3° — as the maximum slope it considers suitable for normal asphalt shingle application. Whether yours falls above or below that line is the first thing to establish.
The upper deck: Shallow, and frequently low-slopeThe U.S. Department of Energy puts the minimum for asphalt shingles at 2 in 12 and defines low-slope as anything under it, where the assembly has to be a waterproofing membrane rather than overlapping units. Measure it before assuming which side of that line it is on.
Why the shape exists: Usable floor area inside the roofOn the worked geometry below, a mansard gives about 1.9 times the floor area with five feet of headroom that a gable of the same overall height gives on the same footprint.
What interrupts it: Windows and dormers in the steep faceThe shape exists to make the space behind that face habitable, and habitable space needs light. Each opening adds a head, two cheeks, and usually two valleys. The Park Service records slater output falling from two to three squares a day on clear expanses to below one where chimneys, dormers, and valleys are present.

The same decision, answered twice, on one roof. Sources for each row are in the source list at the foot of the page; the angles and pitches are the illustrative geometry used in the worked example below.
What is being decided	The steep lower slope (near-vertical)	The shallow upper slope (shallow enough to need a designed fall)
What can be laid on it	Overlapping units — slate, tile, metal shingles, panels — or asphalt shingles only under a steep-slope application. ARMA calls 21:12 the maximum slope suitable for normal asphalt shingle application.	Below 2:12 this is not shingle territory at all. DOE treats anything under 2:12 as a low-slope assembly clad with a waterproofing membrane: single-ply, modified bitumen, or built-up.
How it is fastened and sealed	More fasteners than usual, plus hand-sealing. ARMA’s stated reason is that very steep slopes reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas.	Seams welded or adhered rather than lapped by gravity, and an attachment pattern that is a wind-uplift question for the specific membrane system on the specific building.
Historic slate headlap	Historically reduced. The Park Service records slate headlap generally cut to 2 inches on mansard roofs and on particularly steep slopes, against a 3-inch standard.	Historically increased. The same source records headlap raised to 4 inches or more on low-pitched roofs — and slate not laid at all below 4:12.
How water leaves it	Fast, and all at once, at the eave — and where the upper deck falls outward across the deck line, this face carries that water too.	Slowly, and only where a designed fall exists. It may fall outward across the deck line, or inward to a drain or scupper. Which one it does changes what the joint has to survive.
What interrupts it	Windows and dormers, wherever the space behind the face is in use — light is what makes that space habitable, and that space is the reason the shape was built.	Usually clear of openings, and therefore the place vents, equipment, and later additions get put where nobody can see them.
How a worker reaches it	Not walkable. Staging, a scaffold, or a lift, and fall protection from the first six feet of exposure.	Walkable once you are on it — but you get onto it by passing the steep face, so its access cost is the steep face’s access cost.
Where a zoning code measures height	Its top edge is the deck line, which is the point two of the municipal codes quoted below measure a building’s height to.	Sits above that measuring point, so under the general rule in each of those two ordinances it does not count toward the height limit there. Both have exceptions and neither is evidence about any other municipality.

Read this table one item at a time

What can be laid on it

The steep lower slope (near-vertical): Overlapping units — slate, tile, metal shingles, panels — or asphalt shingles only under a steep-slope application. ARMA calls 21:12 the maximum slope suitable for normal asphalt shingle application.
The shallow upper slope (shallow enough to need a designed fall): Below 2:12 this is not shingle territory at all. DOE treats anything under 2:12 as a low-slope assembly clad with a waterproofing membrane: single-ply, modified bitumen, or built-up.

How it is fastened and sealed

The steep lower slope (near-vertical): More fasteners than usual, plus hand-sealing. ARMA’s stated reason is that very steep slopes reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas.
The shallow upper slope (shallow enough to need a designed fall): Seams welded or adhered rather than lapped by gravity, and an attachment pattern that is a wind-uplift question for the specific membrane system on the specific building.

Historic slate headlap

The steep lower slope (near-vertical): Historically reduced. The Park Service records slate headlap generally cut to 2 inches on mansard roofs and on particularly steep slopes, against a 3-inch standard.
The shallow upper slope (shallow enough to need a designed fall): Historically increased. The same source records headlap raised to 4 inches or more on low-pitched roofs — and slate not laid at all below 4:12.

How water leaves it

The steep lower slope (near-vertical): Fast, and all at once, at the eave — and where the upper deck falls outward across the deck line, this face carries that water too.
The shallow upper slope (shallow enough to need a designed fall): Slowly, and only where a designed fall exists. It may fall outward across the deck line, or inward to a drain or scupper. Which one it does changes what the joint has to survive.

What interrupts it

The steep lower slope (near-vertical): Windows and dormers, wherever the space behind the face is in use — light is what makes that space habitable, and that space is the reason the shape was built.
The shallow upper slope (shallow enough to need a designed fall): Usually clear of openings, and therefore the place vents, equipment, and later additions get put where nobody can see them.

How a worker reaches it

The steep lower slope (near-vertical): Not walkable. Staging, a scaffold, or a lift, and fall protection from the first six feet of exposure.
The shallow upper slope (shallow enough to need a designed fall): Walkable once you are on it — but you get onto it by passing the steep face, so its access cost is the steep face’s access cost.

Where a zoning code measures height

The steep lower slope (near-vertical): Its top edge is the deck line, which is the point two of the municipal codes quoted below measure a building’s height to.
The shallow upper slope (shallow enough to need a designed fall): Sits above that measuring point, so under the general rule in each of those two ordinances it does not count toward the height limit there. Both have exceptions and neither is evidence about any other municipality.

Row four is drainage behaviour, not a code requirement. Rows one and two are guidance and trade-association recommendation, not law. Row seven quotes two specific municipalities and says nothing about any other.

Tradeoffs

Where treating it as two roofs helps, and where it does notSection link

This page argues that a mansard should be scoped, priced, and detailed as two roofs meeting at a joint, rather than as one roof with a bend in it. That is not always the right frame, and the exceptions are real.

Best when

The upper deck is genuinely low-slope. If it is under 2:12 the argument is settled for you: DOE puts that outside asphalt shingle territory, so the upper deck is a membrane roof whatever the lower face is.
The lower face is steeper than about 60°. Above 21:12, ARMA’s recommendation is that normal asphalt shingle application no longer applies and the shingles have to be hand-sealed with additional fasteners. That is a different scope of work from the deck above it, priced differently.
There are windows or dormers in the steep face. Every one is a head, two cheeks, and usually two valleys, and flashing is where roofs leak. Scoping them separately from the field is the only way a proposal becomes comparable.
The building is in a historic district or a landmarked property. The steep face is visible from the street and the upper deck generally is not, so the two halves may well attract very different levels of design-review interest — but that is a prediction about priorities, and the reviewing body sets its own scope. Ask it rather than assume.
You are comparing bids. Two proposals that both say “re-roof the mansard” can mean wildly different things about the deck. Splitting the roof in the scope is what exposes that.

Think twice if

The upper slope is not actually low-slope. Some mansards carry an upper slope of 4:12 or steeper running to a real ridge. There, one covering across both planes can be entirely appropriate and the transition becomes a change-of-pitch detail rather than a change-of-system one.
It is a false mansard — a type New York City’s code names explicitly. Here the mansard is a decorative band standing off the face of a building whose actual roof is flat and continuous behind it. Almost nothing on this page about drainage applies; what applies is what the band is made of and what it conceals.
The steep face is not that steep. A shallow mansard-like profile at, say, 12:12 is inside normal shingle application and none of the hand-sealing argument applies to it.
You are being sold a full replacement of a deck that is fine. The two halves age independently, and separating them cuts both ways: it is also the argument for replacing one and leaving the other.
Nobody has measured the slopes. Every number on this page keys off two angles that a website cannot know about your building. Establishing them is a job for the ground-based methods on our roof pitch page or for someone with the right access.

What changes the answer

The actual angle of the lower face and the actual pitch of the upper deck. Those two figures decide which rulebook each half sits under, and everything else follows from them.
Whether the mansard is structural or a false mansard applied to a flat-roofed building.
Whether the space behind the steep face is habitable, storage, or a sealed cavity — that decides whether it is inside the building’s thermal envelope and therefore what the assembly behind the covering has to do.
How many dormers and windows the face carries, and whether their flashings are in scope.
Whether the building is in a historic district, is individually landmarked, or is seeking historic tax credits — each is a different review with a different decision-maker.
What your jurisdiction has adopted and amended, and what its authority having jurisdiction requires on a permit for this specific work.
The weight of the covering being proposed, particularly if it is slate or tile, and what the existing framing was built to carry.
Access: what a crew has to erect to reach the face, and whether the sidewalk, driveway, or neighbouring property is available to stage from.

The mechanism

The two slopes are not two parts of one roof. They are two roofs.Section link

The lower face is close enough to vertical that water, wind, fastenings, and in at least one jurisdiction the fire code all start treating it as something other than a roof. The upper deck is shallow enough that it stops shedding and starts having to be sealed. Between them is one line.

Above: a section cut through a mansard-roofed building, with a dormer cut through the right-hand face. Below: the deck line enlarged, showing the joint where the two coverings meet. 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 steep lower slope. The near-vertical face between the eave and the deck line, drawn here at about 75° — roughly 44.8 inches of rise for every 12 inches of run. For comparison, the Asphalt Roofing Manufacturers Association puts 21:12, about 60.3°, as “the maximum slope ARMA considers suitable for normal asphalt shingle application.”
The deck line. The horizontal line where the steep face stops and the shallow roof begins. It is the joint the lower half of the drawing enlarges, and it is the single most consequential dimension on a mansard, because it is where two different systems have to be made continuous.
The shallow upper slope. Drawn here at about 1.8:12. The Department of Energy’s Building America Solution Center states that “asphalt shingles should only be installed on roof slopes of 2 in 12 or greater,” and describes low-slope roofs — “slope less than 2:12” — as assemblies clad with a waterproofing membrane. On this geometry the upper deck is a membrane roof, not a shingle roof.
The eave. Where it all arrives, and it arrives fast. In the arrangement drawn here the upper deck falls outward to the deck line and spills onto the steep face, so the eave takes both slopes’ water off an almost vertical surface with nothing slowing it down. That is a gutter-sizing and gutter-fixing problem more than a roofing one.
The dormer. Cut through here on the right-hand face. A dormer in a near-vertical face is essentially a window box: it has a head, two cheeks, a sill, and its own small roof, and each of those is a flashing line. The Park Service is blunt about where roofs fail: “Flashings are the weakest point in any roof.”
The habitable storey. The shaded room between the eave level and the deck line. This is the entire reason the shape exists, and the arithmetic behind it is worked through in the next section.
The height line. Drawn at the deck line, because that is where several municipal zoning codes measure a building’s height on a mansard. La Mirada, California measures to “the coping of a flat roof, deck line of a mansard roof, or peak of the highest gable of a pitch or hip roof.” South Barrington, Illinois measures “to the highest point of the coping of a flat roof or to the deck line of a mansard roof, or to the mean height level between eaves and ridge for gable, hip or gambrel roofs.” Under those two provisions the roof above the deck line does not count toward the limit — while on a gable in La Mirada the measurement runs all the way to the peak. Read the whole section, though: La Mirada’s next subsection measures sloped lots and buildings with varied floor elevations from the average ground level to “the top-most point of the roof” instead, which puts the deck line back inside the measurement. Which rule your lot falls under is a question for the municipality.
The concealed construction. The framing and the cavity behind the visible covering of the steep face. Whether it is inside or outside the building’s thermal envelope, and whether it is a vented cavity or a sealed assembly, is a building-science decision that the covering choice cannot answer.
Where the upper covering stops. In the enlargement, the membrane or upper roofing runs down the shallow deck and has to be terminated somewhere near the corner.
The lap direction. The one rule that survives every combination of materials: the upper covering finishes over the lower one, never under it. Where the upper deck falls outward across this line, as it does in the drawing, every drop off the deck crosses the joint. Where the deck instead falls inward to a drain or a scupper, the joint is tested every time that outlet runs slow. Either way, a reversed lap here is a leak waiting for weather.
Where the steep-face covering starts. Its top course, and with it the fastening and sealing regime that belongs to the steep face rather than to the deck.
The concealed space behind the corner. Where water goes if the lap at point ten is reversed, and the reason a deck-line leak is usually found a long way from the deck line.

Why the lower face behaves like a wall

Three things change as a roof plane approaches vertical, and all three are documented rather than intuited.

The first is sealing. Modern asphalt shingles rely on a factory-applied sealant strip that bonds each course to the one below under its own weight and the sun’s heat. ARMA states that very steep slopes “reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas,” and its recommendation above 21:12 is to hand-seal each shingle with spots of asphalt roof cement complying with ASTM D4586, adding that manufacturer directions “may call for more than the normal number of fasteners per shingle.” Atlas, for one product line, specifies six nails per shingle on steep roofs and six quarter-sized dollops of cement under each shingle, and warns that “excessive use of roofing cement can cause shingles to blister.” That is a product-specific instruction, not a general rule; the general rule is that on a mansard face the shingle stops being a self-sealing product and becomes a hand-sealed one.

The second is overlap. Water runs off a near-vertical face fast and has little opportunity to travel back up under a lap. Historic slating practice adjusted for this explicitly. The Park Service records a standard headlap of 3 inches, generally reduced to two inches “on Mansard roofs and on particularly steep slopes,” and increased to four inches “or more on low pitched roofs.” On a single mansard those two sentences point in opposite directions, which is the whole argument of this page, stated by a 1992 government publication about stone.

The third is durability, and it favours the steep face. “The steeper the pitch, the longer the slate can be expected to last as water will run off faster and will be less likely to be drawn under the slates by capillary action or driven under by wind forces,” the same brief says, and it observes the result directly: “Spires and the steep slopes of Mansard roofs often retain their original slate long after other portions of the roof have been replaced.” The part of a mansard that is hardest to reach is frequently the part that needed reaching least.

And why it is sometimes legally a wall

There is no nationwide building code for site-built construction in the United States, so there is no single answer to whether a steep roof plane is a roof or a wall. But at least one American jurisdiction answers it with a number. New York City’s Administrative Code § 27-338(f), “Construction of sloping roofs,” provides:

“Roofs having a slope of more than sixty degrees to the horizontal shall be constructed of material having the same fire-resistance rating as required for an exterior non-bearing wall of the building of which it is a part. When the slope is sixty degrees or less to the horizontal, the sloping roof shall be constructed as required for the roof of the building. Where the back of a false mansard is exposed to the outdoors, the back shall be covered with noncombustible material or with roof coverings as required for the roof of the building.”

Read that carefully. It does not say a steep roof is a wall. It says a steep roof has to be built out of material with the fire-resistance rating a wall would need. The plane keeps its name and inherits the wall’s requirement — and the threshold sits at 60°, low enough that a genuinely near-vertical mansard face lands on the far side of it. Whether yours does is a measurement, not an assumption. The same section goes on to treat dormers the same way: “Roofs of dormers shall be of the same type of construction and have roof covering of the same class as required for the roof of the building on which they are located. The walls of dormers shall be constructed of materials having the same fire resistance rating as required for non-bearing exterior walls of the building on which they are located” — followed, in the same sentence, by an exception for certain construction classes that allows combustible framing behind noncombustible sheathing where the dormer walls stay under a stated share of the roof area. Quote the whole subsection, not the first half of it.

That provision is New York City law and nowhere else’s, and it belongs to the 1968 Building Code carried in Title 27 rather than to the Construction Codes that govern new work there today. It is quoted here as a demonstration that the wall-or-roof question is real enough for a legislature to have drawn a line at a specific angle — not as a rule that applies to your building. What applies to your building is what your jurisdiction has adopted and how its authority having jurisdiction reads it.

The arithmetic

What the shape actually buys, worked throughSection link

The mansard exists to turn roof volume into floor area. That claim is normally asserted. Here it is calculated, on one stated geometry, so you can see both what it buys and what it costs.

Take a rectangular building 30 feet by 40 feet, and give it a roof 9 feet tall above the top-floor deck. Build that roof twice: once as a mansard with a lower face at 75° rising 7 feet and a shallow upper slope carrying the remaining 2 feet to a ridge, and once as a plain gable of the same overall height. Then ask how much of the floor below has at least 5 feet of headroom over it, and how much roof surface each version has to be covered with.

Five feet is a benchmark chosen here for the comparison, not a code requirement. Jurisdictions define minimum ceiling heights and how sloped portions of a room are counted in their own ways, and that is a question for the authority having jurisdiction, not for this page.

One illustrative geometry, calculated. A 30 ft × 40 ft footprint with a roof 9 ft tall above the top-floor deck, built as a mansard and as a gable, with no overhangs. Original arithmetic, not a measurement of any building.
Measure	Mansard	Gable of the same overall height
Geometry	Lower face 75° rising 7 ft; upper slope carrying the remaining 2 ft to a ridge	One plane each side, 7.2:12 (31.0°)
Pitch of the upper slope	1.83:12 (8.7°)	Not applicable — one slope
Floor area with at least 5 ft of headroom	1,020 sq ft — 85% of the 1,200 sq ft footprint	533 sq ft — 44% of the same footprint
Roof surface to be covered	1,923 sq ft	1,399 sq ft
Share of that surface steeper than 21:12	About 50% — the steep face, at 75°, above ARMA’s 21:12 (60.3°)	None
Share of that surface shallower than 2:12	About 50% — the upper deck, at 1.83:12, below DOE’s 2:12 minimum for asphalt shingles	None

Read this table one item at a time

Geometry

Mansard: Lower face 75° rising 7 ft; upper slope carrying the remaining 2 ft to a ridge
Gable of the same overall height: One plane each side, 7.2:12 (31.0°)

Pitch of the upper slope

Mansard: 1.83:12 (8.7°)
Gable of the same overall height: Not applicable — one slope

Floor area with at least 5 ft of headroom

Mansard: 1,020 sq ft — 85% of the 1,200 sq ft footprint
Gable of the same overall height: 533 sq ft — 44% of the same footprint

Roof surface to be covered

Mansard: 1,923 sq ft
Gable of the same overall height: 1,399 sq ft

Share of that surface steeper than 21:12

Mansard: About 50% — the steep face, at 75°, above ARMA’s 21:12 (60.3°)
Gable of the same overall height: None

Share of that surface shallower than 2:12

Mansard: About 50% — the upper deck, at 1.83:12, below DOE’s 2:12 minimum for asphalt shingles
Gable of the same overall height: None

Calculated from the stated geometry: horizontal run of the lower face is 7 ÷ tan 75° = 1.88 ft per side, putting the deck line at 26.25 ft × 36.25 ft; the upper slope then rises 2 ft over a 13.12 ft half-span. Headroom areas take the 5 ft contour on each profile: on the mansard the contour is set in 5 ÷ tan 75° = 1.34 ft from all four walls, because a mansard slopes on all four sides; on the gable it is set in from the two eave walls only, because the other two are vertical gable ends. Mansard surface is the sloped band around the perimeter (mean perimeter × slant height) plus the upper slope (plan area × pitch factor); gable surface is two planes of rafter length × 40 ft. Overhangs, fascias, gutters, and dormers are excluded, and a different lower-face angle changes every row. This is arithmetic on an assumed shape, not a determination about any building, and it is not a code, structural, or drainage determination.

The headline is that the mansard yields about 1.9 times the usable floor area of the gable — 1,020 square feet against 533, on the same footprint and at the same overall height. That is the whole historical argument for the shape, and it survives contact with arithmetic.

The two rows underneath it are the point of this page. On this geometry, half the roof surface is too steep for a normal asphalt shingle application and the other half is too shallow for asphalt shingles at all. Neither half of the roof is ordinary. A proposal that treats the whole thing as one shingle job has, on a roof shaped like this one, got both halves wrong.

Change the lower-face angle and the picture moves. A 60° face is just inside ARMA’s normal-application range and the whole argument about hand-sealing softens. An 80° face is more nearly a wall and gives up almost nothing in floor area. This is exactly why the first question to ask a contractor is what the two angles actually are.

Approvals

On a mansard, the roof is the elevationSection link

Most roofs are seen obliquely from a distance, if at all. A mansard face is seen straight on from the pavement — and on a designated building, that visibility is exactly what a design review is about, which is why replacing it can be somebody else’s decision as well as yours.

The National Park Service is direct about it: “During some periods in the history of architecture, the roof imparts much of the architectural character. It defines the style and contributes to the building’s aesthetics,” and it names the shape explicitly — “the Mansard roofs, and the graceful slopes of the Shingle Style and Bungalow designs are examples of the use of roofing as a major design feature.” The face was often patterned as well. Because slate came in different colours it “was an effective material for decorative patterns on many 19th century roofs (Gothic and Mansard styles),” and slate “laid in multicolored decorative patterns, was particularly well suited to the Mansard roofs of the Second Empire style.”

That has three practical consequences, and they arrive in this order.

The two slopes are reviewed differently

The steep face is visible from the street. The upper deck, by the geometry of the shape, is set back above it and is generally not visible from ground level at all. A body reviewing the appearance of a streetscape is likely to have a strong interest in the first and much less in the second — a prediction about priorities, not a rule. Ask rather than assume; but where it holds, the asymmetry is useful, because it separates the conversation about the face from the choice of what goes on the deck.

A substitute material is judged on how it looks, not on what it is

Preservation Brief 4’s test for an alternative material is appearance-based: “if the roof is readily visible, the alternative material should match as closely as possible the scale, texture, and coloration of the historic roofing material,” with the warning that on roofs with a high degree of visibility and patterning or texture “the substitution may seriously alter the architectural character of the building.” On a Second Empire building the mansard face is the most visible roof surface there is, and by those two sources often the most patterned, which sets the bar high. Where the Secretary of the Interior’s Standards apply, Standard 6 asks that a replaced distinctive feature “match the old in design, color, texture, and other visual qualities and, where possible, materials.”

The Standards are codified at 36 CFR Part 67 and are regulatory for the federal Historic Preservation Tax Incentives programme. They are not automatically the law in a local historic district — many commissions adopt or adapt them, many write their own guidelines, and some districts review only what is visible from a public way. Which regime you are in is a question of public record and worth answering before a contractor orders anything.

The approval timetable is not the roofing timetable

A commission meets on its own schedule, and that schedule sits in front of ordering rather than behind it — particularly where a face is being matched and the match has a lead time. How long yours takes is a question for that commission and nobody else. A contractor who proposes to start next week on a landmarked mansard has either done the paperwork or has not read the designation.

One more thing about old buildings, stated once because it matters before any work starts rather than after: the roofing felts, mastics, and cements on a pre-1990 roof may contain asbestos. The EPA’s position is that “the only way to be sure whether a material contains asbestos is to have it tested by a qualified laboratory,” that testing is recommended where material is damaged or where a renovation would disturb it, and that samples “should be taken by a properly trained and accredited asbestos professional.” Age is not determinative and nothing here identifies any material on any building. Tear-off disturbs everything, which is why the question belongs before the contract rather than in the middle of the job.

What drives the number

Why a mansard costs more, without a dollar figureSection link

This page publishes no dollar figure for a mansard, for the same reason no page on this site publishes one for a roof shape: no transparent dataset separates the cost of a shape from the covering, the market, and the building. What can be published is the mechanism that makes a mansard expensive.

Clear expanse: About 2–3 squares per dayAn experienced slater and one helper, on uninterrupted roof.
With chimneys, dormers, and valleys: Below 1 square per dayThe same crew. A mansard face is defined by these interruptions.

Units: Roofing squares (100 sq ft of roof surface) installed per working day
Scope included: Installation labour only, for natural slate, by an experienced slater working with one helper
Not included: Everything else: materials, staging and scaffold erection, tear-off, deck repair, flashing metal, disposal, permits, and the second roofing system the upper deck needs.
Geography: United States, national, and not adjusted for any local labour market
Data as of: September 1992, the publication date of Preservation Brief 29
Confidence: Low as a cost input, high as a directional signal. It is a single figure from a single 1992 government publication about one material — natural slate — and it says nothing about crew size, market, access, or any other covering. It is used here only to show the direction and rough magnitude of the effect that dormers and valleys have on labour.
Method: How this figure is built

A mansard is expensive for four reasons that stack, and only the first is obvious.

More surface. On the worked geometry above, a mansard carries about 37 percent more roof surface than a gable of the same overall height on the same footprint. Every square foot of that is bought, carried, fastened, and disposed of.

Two systems. On that same geometry, half the surface sits above the slope ARMA considers suitable for normal asphalt shingle application and the other half sits below the slope DOE puts as the minimum for asphalt shingles at all. That is two material orders, two sets of instructions, sometimes two trades, and one joint between them.

Interruptions. The figures above are the reason. A mansard face is where the windows are, and the Park Service’s output figures fall by more than half where chimneys, dormers, and valleys are present.

Access. Nothing on a mansard is reached by standing on it. Staging or a lift is not an add-on for difficult buildings; it is the normal condition, and it is erected before the first shingle moves. Ask for it as a separate line, because a bidder who has left it out has not priced the same job as a bidder who has. Our guide to normalising quotes covers how to make two proposals answer the same question.

These are labour-productivity figures for slate work published by the National Park Service in 1992, not prices. They are here because they are the clearest published evidence this page could find of what dormers and valleys cost in time, and because labour time is the part of a mansard quote a buyer can least see. Do not convert them into a price for your building.

Considerations

What changes this on a real buildingSection link

Eight things that change what a mansard means on a specific building.

Slope and drainage

This is the axis the whole page turns on. Establish the two angles before anything else, because they decide which rules each half sits under. Our roof pitch guide sets out how to establish slope from the ground and from drawings without going up. Note that pitch factor rises sharply near vertical: a 75° face has a pitch factor of about 3.86, meaning nearly four square feet of covering for every square foot of plan area it covers — which is also why the steep face contributes so little to a footprint-based estimate and so much to a materials order.

Moisture and ventilation

The point of a mansard is habitable space directly behind the steep face. Where that space is in use, the assembly behind the covering is an insulated sloped-ceiling assembly rather than a vented attic with a floor you can walk on. That changes the problem: there is often no accessible cavity to inspect, no place to add a baffle after the fact, and no soffit-to-ridge path of the kind a gable roof offers. Above the deck line there may be a small attic over the upper deck, or there may be nothing at all.

There is no universal ventilation ratio and this page does not publish one. Required net free area, where it is measured, and whether an assembly must be vented at all depend on the adopted code edition, local amendments, climate zone, and assembly type. Vented and correctly designed unvented assemblies are both legitimate — see the ventilation guide.

Fire

A mansard raises two fire questions an ordinary roof does not. The first is what the steep face is built of: New York City’s § 27-338(f), quoted above, requires a roof steeper than 60° to be constructed of material carrying the fire-resistance rating of an exterior non-bearing wall, and requires the exposed back of a false mansard to be noncombustible or covered as a roof. The second is what is behind it. A false mansard on a commercial building is a band standing off the building face with a continuous concealed space behind it, and concealed spaces are a fire-spread question that belongs to the fire and building codes of the jurisdiction, not to a roofing proposal.

Fire classification — Class A, B, or C — applies to a tested roof assembly, deck and underlayment and covering together, and never to a covering in isolation or to a roof shape. The New York City provision quoted here is that city’s law and is not a model rule for anywhere else.

Wind

Two things on a mansard depend on wind and neither is settled by the shape. Hand-sealing on the steep face exists because the factory seal cannot be relied on there, and until a hand-sealed shingle is sealed it is a loose flap. On the upper deck, a membrane’s attachment pattern is a wind-uplift calculation for that membrane on that building. Neither is a number this page can supply.

Wind performance is site- and building-specific. Basic wind speed, exposure category, building height and geometry, pressure zone, enclosure classification, risk category, attachment, and the tested assembly all enter the answer, and none of them is determined by roof shape. A manufacturer’s mph figure on a wrapper is a product test result, not a code determination for your building. That determination is made by a registered design professional against the standard your jurisdiction has adopted.

Code and jurisdiction

There is no nationwide building code for site-built construction in the United States. Everything on this page that comes from a code comes from one named jurisdiction and is quoted as an example of how the question gets answered, not as a requirement anywhere else.

Two separate regimes reach a mansard, and they are administered by different offices. The building code decides what the assembly has to be. The zoning ordinance decides how tall the building is allowed to be and where the measurement is taken — and on a mansard, in the two municipalities quoted above, that measurement is taken at the deck line. A change to the deck line height in a renovation is therefore capable of being a zoning question as well as a construction one.

Confirm the adopted edition, its amendments, its effective date, and what is actually required on a permit with your authority having jurisdiction. This page quotes no text from the International Building Code or the International Residential Code, because the publisher blocks automated retrieval and the text could not be read and verified here. Where a model-code provision matters to your project, get it from the edition your jurisdiction adopted.

Structural weight

Two structural facts follow from the geometry. The first is that a mansard puts its covering weight high on the building and concentrated near the perimeter rather than spread across a span. The second is that where the building is old enough the face may still carry its original slate — the Park Service notes that “spires and the steep slopes of Mansard roofs often retain their original slate long after other portions of the roof have been replaced” — so a “like for like” replacement can be a heavy covering going back onto framing nobody has looked at in a century. See the natural slate guide for what a change in covering weight involves.

Nothing here is a structural determination for any building. Member sizes, spans, connections, load paths, and the adequacy of existing framing for any covering are decided by a licensed design professional looking at the building, working to the code the local authority has adopted.

Access and site conditions

A near-vertical face is not a surface anyone stands on, and on a mansard that face is the way to everything above it. The crew works from staging, a scaffold, or a lift, and where the face rises directly off a pavement or a property line the staging itself may need a permit, a sidewalk shed, or a neighbour’s permission. OSHA requires fall protection for residential construction work six feet or more above a lower level, which on a mansard means from the first course. Price the access before pricing the roof, and compare it first when two bids differ.

Maintenance

Because reaching it costs more than most of the work done up there, maintenance on a mansard tends to happen when something forces it rather than on a schedule, and small problems become large ones between visits. The two places worth watching from the ground are the gutter line at the eave, which receives everything the steep face sheds all at once, and the deck line itself — a stain, a bulge, or a run of failed sealant along that line is the roof telling you the joint has moved.

Warranty and repair

Two coverings, and probably two warranties that do not meetSection link

A mansard sits awkwardly across the warranty system, because a system warranty normally assumes one system.

The joint is the thing to ask about in writing: If the steep face is shingles and the upper deck is a membrane, there are ordinarily two manufacturer warranties covering two different products — and the joint between them is the place to check, because what each document covers is set by the document itself and this page cannot tell you what yours says. Ask in writing which document, if either, covers the deck line, and what it says about a transition to another manufacturer’s system.
Steep-slope application is usually a warranty condition: Where a manufacturer publishes a steep-slope or mansard application — additional fasteners, hand-sealing with a specified cement in a specified pattern — that is ordinarily a condition of the product warranty rather than a suggestion. It is also invisible after installation. A photographic record of the hand-sealing, taken before the next course goes on, is worth more in ten years than the warranty certificate is.
Over-sealing is a workmanship failure, not a product one: Roofing cement is the standard tool for hand-sealing and also a standard cause of trouble: Atlas warns that excessive use “can cause shingles to blister.” A face that has been over-cemented looks fine on the day and fails as a pattern later, and it is a workmanship question rather than a product one. Read how roofing warranties actually work before assuming either document will answer for it.
A warranty is not an approval: On a historic building, the covering that satisfies a manufacturer’s warranty and the covering that satisfies a historic district commission are not automatically the same covering, and neither body is bound by the other’s decision. Establish what the commission will accept before signing anything that specifies a product.

Repairability

The steep face repairs well in one sense and badly in another. Well, because it is a plane of discrete units in courses, and a single damaged unit can be replaced without touching its neighbours — this is the ordinary case with slate, tile, and metal shingles. Badly, because reaching that one unit costs the same staging as re-roofing the whole face. The economics of a mansard repair are dominated by access, which is why a small repair up there is never a small job and why putting it off is so tempting.

The upper deck is the opposite. Membrane repairs are cheap and routine once you are on the deck, and the deck is the easy part to stand on — but you still have to get past the steep face to reach it. A mansard has no cheap repair, only cheap materials.

Matching is the third problem. A steep face that has kept its original slate for a century cannot be patched from a builders’ merchant, and the Park Service notes that artificial mineral fibre slate “is not recommended for restoration work since its rigid appearance is that of a manmade material and not one of nature.” Flashing is where roofs actually leak, and on a mansard most of the flashing is in the face you cannot reach.

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 about your building and about the joint between its two roofs. A contractor who answers them with two numbers and a lap direction has looked; one who answers “it’s a mansard, we do those” has not.

What is the angle of the lower face and the pitch of the upper deck, and how did you establish them?
These two figures decide everything else: which coverings are appropriate, whether the shingles need hand-sealing, whether the deck is a membrane roof, and how much material there is. A good answer gives two numbers and says how they were measured.
Is this a structural mansard or a false mansard applied to a building with a flat roof behind it?
It changes the job completely. On a false mansard the actual roof is the deck behind the band, the band is a cladding question, and what is in the concealed space behind it may be a fire and code question rather than a roofing one.
What exactly happens at the deck line, and which covering laps over which?
The only universal rule at that joint is that the upper covering finishes over the lower one. Ask what the flashing is, what it is made of, how it is fastened, how the two coverings terminate into it, and how it turns the corners of the building.
Is the upper deck below 2 in 12, and if so what membrane system are you proposing on it?
DOE puts 2 in 12 as the minimum slope for asphalt shingles and treats anything below it as a low-slope assembly needing a waterproofing membrane. If a proposal runs shingles over a deck shallower than that, ask what instruction permits it.
Above 21:12, what is your hand-sealing method, how many fasteners per shingle, and which cement?
ARMA’s recommendation above 21:12 is hand-sealing with spots of asphalt roof cement complying with ASTM D4586, and notes that manufacturer directions may call for more than the normal number of fasteners. A contractor who names the cement standard and the spot pattern is reading the instructions.
How are you reaching the face, what does that staging cost, and is it a separate line?
Access is not an incidental on a mansard, it is a major cost centre, and it is easily buried inside a lump sum. If the face is over a sidewalk, ask who obtains the permit and the shed.
How many dormers and windows are in the face, and are their flashings in this scope or excluded?
Each one is a head, two cheeks, a sill, and usually two valleys, and the Park Service’s output figures fall from two to three squares a day to below one where dormers and valleys are present. If they are excluded, the price is not comparable to a bid that includes them.
Is the building in a historic district or individually landmarked, and who has confirmed that?
A mansard is a character-defining feature almost by definition, and approval processes run on their own timetable. This is a question of public record that a contractor can check before ordering anything, and the answer changes what may be specified.

Require these in writing

The measured angle of the lower face and the measured pitch of the upper deck, stated separately.
The covering, underlayment, and fastening schedule for the steep face, stated separately from the covering and attachment for the upper deck.
For asphalt shingles on the steep face: the number of fasteners per shingle, the cement standard, and the number and placement of hand-sealing spots.
The deck-line detail: the flashing metal, its gauge or weight, its dimensions, how it is fastened, how each covering terminates into it, and how it is made continuous at the corners of the building.
For each dormer and window in the face: the head, cheek, sill, and valley flashings, itemised rather than allowed for.
The gutter and drainage work at the eave, including whether the existing gutter and its fixings are being retained.
Access and staging as its own line, including erection, hire period, dismantling, and any sidewalk shed or permit.
A written statement of what was not inspected and why — the deck behind the face, the framing, the concealed space above the ceiling.
Where the covering is being reused or matched: what is being salvaged, what is being sourced, and what happens if the match is not available.

What goes wrong

Misconceptions and failure modesSection link

Common misconceptions

Common belief
A mansard is just a fancy gambrel.
What is actually true
They share the two-slopes-per-side idea, but a gambrel has it on two sides only and closes the other two with gable walls, so its break line stops at each end. A mansard carries the break line continuously around all four sides, which means it also turns four corners — and a corner where two different coverings and a transition flashing all change direction at once is the hardest detail on the building.
Common belief
The steep part is basically a wall, so treat it like siding.
What is actually true
Not quite, and the exception is instructive. New York City’s § 27-338(f) does not reclassify a steep roof as a wall; it requires a roof steeper than 60° to be built of material carrying the fire-resistance rating a non-bearing exterior wall would need, while remaining a roof for other purposes. The plane inherits some of a wall’s requirements and keeps its own. Which of those apply to your building is a jurisdictional question, not a geometric one.
Common belief
You can run the same shingle over the whole mansard.
What is actually true
Sometimes. But on the geometry worked through above, the steep face is above the 21:12 slope ARMA calls the maximum for normal asphalt shingle application, and the upper deck is below the 2 in 12 minimum DOE gives for asphalt shingles at all. Same product, two different rulebooks, and on that geometry neither half qualifies for the standard application. Check your own two angles before assuming either way.
Common belief
The steep face never leaks — water runs straight off it.
What is actually true
The field of the steep face is genuinely durable, and the Park Service records exactly that: mansard slopes often retain their original slate long after other parts of the roof have been replaced. But the field is not where roofs fail. The steep face is where every window and dormer on the building is, and the same source is unambiguous that “flashings are the weakest point in any roof.”
Common belief
A mansard is a cheap way to add a floor.
What is actually true
The floor area is cheap; the roof is not. On the worked geometry above, a mansard yields about 1.9 times the usable floor area of a gable of the same height — and about 37 percent more roof surface, split between two systems, none of it walkable, with the windows that make the floor worth having cut through the part that is hardest to reach.
Common belief
If the building is historic, the roof has to be slate.
What is actually true
Not automatically. What a historic district commission reviews is the visible character of the building, and the Park Service’s guidance for substitute materials is that “if the roof is readily visible, the alternative material should match as closely as possible the scale, texture, and coloration of the historic roofing material.” That is a standard about appearance, applied case by case by a body that has seen the building. It is also worth knowing that the same agency does not recommend artificial mineral fibre slate for restoration work, and that the Secretary of the Interior’s Standards ask that a replaced distinctive feature “match the old in design, color, texture, and other visual qualities and, where possible, materials.”

How it actually fails

The lap at the deck line is reversed: The upper covering was terminated under the steep face’s top course rather than over the flashing that turns down onto it. It sheds ordinary rain perfectly well. It fails when water backs up on the shallow deck — under snow, under debris, under a blocked drain — which is precisely what a shallow deck does.What you can see: A stain that appears only after prolonged rain or a thaw, not during ordinary showers, and that shows up well below the deck line or on the ceiling of the room behind the face rather than at the joint itself.
The shingles on the steep face never sealed: Shingles were laid on the steep face with the normal number of fasteners and left to seal themselves. ARMA’s stated reason this does not work is that very steep slopes reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas — and a mansard face has a shaded side.What you can see: From the ground: a course line that is not straight, tabs standing proud in a run, or a face that flutters visibly in wind. Often worse on the north or shaded elevation than on the sunny one, which is the diagnostic tell.
The hand-sealed face blisters: Roofing cement was applied generously rather than as specified. Atlas states plainly that “excessive use of roofing cement can cause shingles to blister.” Where a whole face was sealed the same way on the same day, the failure tends to show up across the whole face rather than in one spot.What you can see: Regular raised blisters in a repeating pattern that tracks the sealing spots rather than the courses; granule loss concentrated on the raised areas.
The upper deck holds water: Nobody gave the upper deck a fall, or the fall was there and whatever it fell to was not maintained. Which failure you have depends on how the deck was built: one that falls outward to the deck line sheds onto the steep face, while one that falls inward to a drain or a scupper behind a raised edge is a tray, and a tray with a blocked outlet holds water. See why a flat roof is never flat.What you can see: Tide marks, silt lines, or vegetation visible from an upper window of a taller neighbouring building; a deck that is still wet days after rain; interior staining on a top-floor ceiling away from any wall.
Water enters at a dormer head or cheek: A dormer is a head, two cheeks, a sill, and usually two valleys, all crowded into a face that is close to vertical, and the geometry gives a detailer very little length to work with. It packs as much flashing as anywhere on the building into the least run.What you can see: Staining in the reveal or on the ceiling immediately inside a dormer window; peeling paint on the dormer cheeks; sealant that has been applied over a flashing line rather than under it.
Slate slips off the face after a repair: Historic slate is hung on its nails rather than clamped by them. The Park Service warns that “slate nails should not be driven tight as is the case with asphalt and wood shingles” and that “nails driven too far will crack the slate and those left projecting will puncture the overlying slate.” On a near-vertical face a cracked slate does not sit in place; it goes.What you can see: Individual dark gaps appearing in an otherwise sound face, clustered around a previous repair; pieces of slate on the ground below. This is a stop-and-clear condition — see the safety callout at the top of this page.
The roof is not maintained because reaching it is the expensive part: Access cost more than the repair, so the repair was postponed; and the next time anyone looked, the scope had grown to justify the staging on its own. This is not a material failure. It is the predictable economics of a shape nobody can stand on.What you can see: A repair history that consists of two events twenty years apart; gutters that were last cleared when scaffolding was up for something else; a deck line nobody has photographed.

Sources and further readingSection link

Understanding Roofing / Published August 27, 2026

Scope and limitations

It cannot tell you the angle of your lower face or the pitch of your upper deck, and those two numbers decide almost everything else on this page.
They are facts about your building, established by measurement or from drawings.
It publishes no dollar figure.
Roof shape affects price through surface area, system count, staging, and the number of interruptions in the face, but no transparent dataset isolates the shape from the covering, the market, and the building.
The only quantified cost input here is a 1992 slater-productivity figure, used for direction and magnitude only.
It quotes no text from the International Building Code or the International Residential Code.
The publisher blocks automated retrieval, so that text could not be read and confirmed here, and this page does not assert its contents.
The one code provision quoted in full is New York City law and is New York City’s alone.
It cannot tell you whether your steep face is treated as a roof or as a wall for any particular purpose.
That is set by the code your jurisdiction adopted, as that jurisdiction amended it, and read by its authority having jurisdiction.
It cannot make a wind or structural determination for your building.
Uplift, attachment density, framing adequacy, and the effect of a change in covering weight are design calculations performed by a registered design professional against an adopted standard.
It publishes no ventilation ratio and no required net free area for the assembly behind the steep face.
Those depend on the adopted code edition, local amendments, climate zone, and whether the assembly is vented at all.
It cannot tell you what a historic district commission or a landmarks body will approve.
Those bodies decide case by case, on the building in front of them, and the federal Standards quoted here are regulatory only for the federal tax-incentive programme.
It says nothing reliable about how common mansards are, in the United States or anywhere else.
Roof shape is one of the most frequently missing attributes in public building databases and no federal inventory counts it.
Where a mansard is on an older building, pre-1990 roofing felts, mastics, and cements may contain asbestos.
Age is not determinative, this page does not claim any age band is typical of the shape, and it makes no claim about any specific material.

Preservation Brief 29: The Repair, Replacement and Maintenance of Historic Slate Roofs
National Park Service, Technical Preservation Services (author Jeffrey S. Levine) / September 1992
That slate was typically laid with a standard headlap of 3 inches; that headlap “was generally reduced to 2" (5 cm) on Mansard roofs and on particularly steep slopes” and “was increased to 4" (10 cm) or more on low pitched roofs”; that the minimum roof slope for a slate roof was 4:12; that “the steeper the pitch, the longer the slate can be expected to last as water will run off faster and will be less likely to be drawn under the slates by capillary action or driven under by wind forces”; that “spires and the steep slopes of Mansard roofs often retain their original slate long after other portions of the roof have been replaced”; that slate laid in multicoloured decorative patterns “was particularly well suited to the Mansard roofs of the Second Empire style”; that “flashings are the weakest point in any roof”; that “clear roof expanses can be covered by an experienced slater and one helper at the rate of about two to three squares per day” while “more complex roofs and the presence of chimneys, dormers, and valleys can bring this rate down to below one square per day”; that “slate nails should not be driven tight as is the case with asphalt and wood shingles” and that “nails driven too far will crack the slate and those left projecting will puncture the overlying slate”; and that artificial mineral fibre slate “is not recommended for restoration work since its rigid appearance is that of a manmade material and not one of nature”.
Written in 1992 about historic slate. Its headlap figures describe historic slating practice, not a current code requirement anywhere; our natural slate guide is where this site sets out what the model residential code says about headlap by slope. The output figures are for slate laid by a two-person crew, say nothing about any other covering, crew size, access arrangement, or market, and are more than thirty years old.
Preservation Brief 4: Roofing for Historic Buildings
National Park Service, Technical Preservation Services (author Sarah M. Sweetser) / February 1978
That “during some periods in the history of architecture, the roof imparts much of the architectural character” and “defines the style and contributes to the building’s aesthetics”; that “the Mansard roofs, and the graceful slopes of the Shingle Style and Bungalow designs are examples of the use of roofing as a major design feature”; that because slate was available in different colours it “was an effective material for decorative patterns on many 19th century roofs (Gothic and Mansard styles)”; and that “if the roof is readily visible, the alternative material should match as closely as possible the scale, texture, and coloration of the historic roofing material”, with the warning that on roofs with a high degree of visibility and patterning or texture “the substitution may seriously alter the architectural character of the building”.
Preservation guidance for historic buildings, not a code determination and not a rule about any modern building. It does not decide what any particular historic district commission will accept. It is forty-eight years old and its examples are slate, tile, wood and sheet metal rather than modern coverings. The copy read here is the Federal Depository Library Program's archived mirror of the National Park Service page; the same brief is cited elsewhere on this site from the National Park Service's own PDF, and the two are the same 1978 document.
The Secretary of the Interior’s Standards for Rehabilitation
National Park Service
Standard 5, that “distinctive features, finishes, and construction techniques or examples of craftsmanship that characterize a historic property shall be preserved”, and Standard 6, that “deteriorated historic features shall be repaired rather than replaced” and that where replacement is required “the new feature shall match the old in design, color, texture, and other visual qualities and, where possible, materials”.
The page itself states that the Standards are codified at 36 CFR Part 67 and are regulatory for the Historic Preservation Tax Incentives programme. They are not automatically the law in any local historic district; many commissions adopt or adapt them, and many do not. Whether they apply to your project is a question for the reviewing body.
New York City Administrative Code § 27-338, Roof structures (1968 Building Code, Title 27)
The City of New York (published by American Legal Publishing, the City’s codifier; version shown as August 2026, current)
§ 27-338(f) Construction of sloping roofs, in full: that “roofs having a slope of more than sixty degrees to the horizontal shall be constructed of material having the same fire-resistance rating as required for an exterior non-bearing wall of the building of which it is a part”, that “when the slope is sixty degrees or less to the horizontal, the sloping roof shall be constructed as required for the roof of the building”, and that “where the back of a false mansard is exposed to the outdoors, the back shall be covered with noncombustible material or with roof coverings as required for the roof of the building”; and § 27-338(g) Dormers, that “roofs of dormers shall be of the same type of construction and have roof covering of the same class as required for the roof of the building on which they are located” and that “the walls of dormers shall be constructed of materials having the same fire resistance rating as required for non-bearing exterior walls of the building on which they are located” — a sentence that continues with an express exception for construction classes II-A, II-B, II-C and II-D, which this page notes rather than quotes in full.
This is New York City law and nothing else. It belongs to the 1968 Building Code carried in Title 27 of the Administrative Code; New York City’s current Construction Codes sit in Title 28 and govern new construction and most alterations there today, and that text was not read for this page because its publisher blocks automated retrieval. The version read was the one shown as current by the City’s codifier, American Legal Publishing, which carries its own notice that its documents “may not reflect the most current legislation adopted by the Municipality” and “should not be relied upon as the definitive authority for local legislation”. It is quoted here as evidence that the wall-or-roof question is answered by legislatures at a specific angle, not as a requirement anywhere.
Application of Asphalt Shingles on Slopes Greater Than 21:12 (steep slope and mansard construction)
Asphalt Roofing Manufacturers Association / October 30, 2024
That “the maximum slope ARMA considers suitable for normal asphalt shingle application is 21:12, or 21" per foot”; that “such very steep slopes reduce the effectiveness of factory-applied self-sealing adhesives, especially in colder climates and shaded areas”; the instruction to apply a manufacturer-recommended asphalt roof cement complying with ASTM D4586 under all shingle tabs in spots equivalent to the size of a quarter, four spots under each laminated shingle and two under each tab of a three-tab shingle; and that manufacturer directions “may call for more than the normal number of fasteners per shingle”.
A trade-association recommendation from an association of asphalt roofing manufacturers, not adopted law and not a neutral source on whether asphalt shingles are the right covering for a mansard in the first place. It does not state a fastener count; it defers that to the individual manufacturer’s instructions, which are the governing document on any specific product.
Maneuvering Mansard: shingling the steep slopes of mansard-style roofs
Atlas Roofing Corporation
That “if the slope of a roof exceeds 60 degrees (21:12 slope pitch), asphalt roof cement that complies with ASTM D4586 should be used in addition to fasteners”; that six nails per shingle are specified when fastening shingles to steep roofs; that six quarter-sized dollops of asphalt plastic cement are placed under each shingle, about 1 inch up from the bottom edge with the outer spots 1 inch in from each edge; and that “excessive use of roofing cement can cause shingles to blister”.
Product-specific instruction from one manufacturer. It is cited here as an example of what a steep-slope application actually specifies, not as a rule for any other product. The governing document for any shingle is that shingle’s own published installation instructions.
Asphalt Shingle Roofs
U.S. Department of Energy, Building America Solution Center (PNNL)
That “asphalt shingles should only be installed on roof slopes of 2 in 12 or greater”, and that “for roof slopes from 2 in 12 up to 4 in 12, a double underlayment application is required”.
Best-practice guidance for builders, not adopted law. Its code references are to the 2018 International Residential Code, an edition many jurisdictions have since superseded, and it does not establish what is required at any address.
Low-Slope (Flat) Roofs
U.S. Department of Energy, Building America Solution Center (PNNL)
That low-slope (“flat”) roof assemblies are those with a slope less than 2:12, clad with a waterproofing membrane, and that the systems used there are single-ply thermoplastic and thermoset membranes (TPO, PVC, KEE, EPDM), modified bitumen, and built-up systems.
Guidance, not code. Its code references are to the 2012, 2015, and 2018 International Residential Code and to ASCE 7-10 and ASCE 7-16, and are model provisions rather than the law in any jurisdiction. It does not state a minimum designed slope to drain.
La Mirada Municipal Code § 21.64.020, Height measurement
City of La Mirada, California (published by American Legal Publishing) / Ord. 612 Exhibit A (part), 2008; codified version 2025 S-38
That in this municipality the highest point for measuring structure height “shall be the coping of a flat roof, deck line of a mansard roof, or peak of the highest gable of a pitch or hip roof, exclusive of vents, air conditioners, chimneys, and the like” — subsection (a). Subsection (b) is read alongside it: “for sloped lots or buildings with varied floor elevations, the height shall be measured as the vertical distance from the average level of the ground under the building to the top-most point of the roof”.
Adopted law in the City of La Mirada, California, and nowhere else. It is a zoning provision about how tall a building may be, not a construction requirement, and it says nothing about how a mansard must be built. The deck-line rule is subsection (a); subsection (b) measures sloped lots and buildings with varied floor elevations to the top-most point of the roof instead, so the deck-line rule is not the whole of the section. American Legal Publishing’s own notice applies: its documents may not reflect the most current legislation adopted by the municipality.
South Barrington Village Code § 10-15-8, Building height
Village of South Barrington, Illinois (published by American Legal Publishing) / Ord. 2025-1446, adopted 9 October 2025; codified version 2026 S-17
That in this municipality “building height is measured as the vertical distance from grade to the highest point of the coping of a flat roof or to the deck line of a mansard roof, or to the mean height level between eaves and ridge for gable, hip or gambrel roofs”.
Adopted law in the Village of South Barrington, Illinois, and nowhere else, and a zoning rather than a construction provision. American Legal Publishing’s own notice applies.
Residential Fall Protection — Guidance
U.S. Occupational Safety and Health Administration
That “under 29 CFR 1926.501(b)(13), workers engaged in residential construction six (6) feet or more above lower levels must be protected by conventional fall protection”.
An OSHA guidance page about employer obligations to workers. It is not a rule for a homeowner and it does not make any roof safe to climb. It states a height threshold, not a slope threshold.
How do I know if I have asbestos in my home (floor tile, ceiling tile, shingles, siding, etc.)?
U.S. Environmental Protection Agency
That “the only way to be sure whether a material contains asbestos is to have it tested by a qualified laboratory”, that EPA “only recommends testing suspect materials if they are damaged (fraying, crumbling) or if you are planning a renovation that would disturb the suspect material”, and that “samples should be taken by a properly trained and accredited asbestos professional (inspector)”.
General homeowner guidance. It does not say that any particular age of building contains asbestos, and nothing on this page identifies any material as containing it.

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