Roof shape
What is a mansard roof?
A roof that is mostly wall: four steep faces, often with dormers, under a top so shallow you cannot see it from the street.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionShapeAlso called: French roof, curb roof
Mansard roof — a roof with two slopes on each of its four sides, the lower slope steep — often nearly vertical and pierced by dormers — and the upper slope so shallow as to be nearly flat, so that a full storey is enclosed within the roof; the NRCA glossary records the term, named for the seventeenth-century French architect François Mansart, and the shape is the four-sided counterpart of the gambrel.
What it is
The mansard was Paris's answer to a tax on storeys: a storey built inside the roof was not a storey. It became the signature of the Second Empire style in the 1850s and 1860s and crossed to America in the decades after, on town houses, courthouses and hotels; it returned in the 1960s and 1970s as a shallow decorative band on commercial buildings and apartment blocks, where the "roof" is a steep skirt of shingles around a flat roof behind.
To a roofer the shape is two jobs. The steep lower slope is roofed like a wall — slate, shingles, standing-seam or shingled metal, sometimes with curved "bellcast" edges — and drains onto a small eave at the bottom. The upper slope is a low-slope roof and needs a membrane or, if it is at least two in twelve, shingles over double underlayment under the IRC's slope rules. The break between them is a change of slope, which the IRC's flashing rule in Section R903.2.1 requires to be flashed.
Why it matters
The upper roof is out of sight, which is where trouble hides. It is where ponding, failed seams and clogged drains go unnoticed, and where water from a failed membrane runs down inside the mansard's framing to appear as a stain two storeys below. The dormers on the lower slope add the usual sidewalls and valleys. And the steep lower slope, being nearly vertical, takes wind pressure like a wall and hail almost edge-on — which is kind to it in hail, hard on it in wind.
A mansard on a historic building is often slate or patterned metal, and repair or replacement on those is conservation work: matching the material, the pattern and the profile is part of the price and, in a historic district, part of the approval.
What it costs and who does it
Two systems, two prices. The steep slopes are priced by the square at steep-work rates, with staging or lifts, and by the material — slate and standing-seam metal cost many times what shingles do; the upper roof is priced as a low-slope replacement with its drains, curbs and edge flashing. Roofing contractors who do both steep and low-slope work, or a pair of specialists, do the job; on decorative commercial mansards the skirt is a shingle job and the roof behind it a membrane job. The directory's guide to flat and low-slope roofing covers the upper roof's options, and its roof repair cost guide how repairs are estimated.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
What is the difference between a mansard and a gambrel roof?
Why do mansard roofs leak?
Can you put shingles on a mansard roof?
Next step
Find a roofer who does this
A mansard needs a contractor at home on both steep slate and flat membrane, or two who will work together. The directory lists roofing contractors by state and city.
Sources
- Roofing glossary — National Roofing Contractors Association (NRCA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)