A mansard roof is a roof in which each face is made up of two slopes of different pitch: a steep lower slope and a much shallower upper slope, with a break between them. It takes its name from the French architect François Mansart (1598–1666), who used the form extensively in the seventeenth century.
Why the break?
The gain is usable space. On an ordinary pitched roof the plane runs in from the eaves immediately, so the edges of the loft are too low to use. By setting the lower slope steeply — often around 60 to 70 degrees — the wall of the roof space stands almost upright, producing a storey with near full headroom without giving the building an extra full storey.
Historically, rules and taxes played their part too: where permitted height was measured to the eaves, space under the roof was cheaper than another storey.
Elements and points to watch
- The break. The junction between the two slopes is the vulnerable point of the roof. Two pitches, two water paths and usually a structural node all meet there. Detail the junction carefully and make sure the covering laps correctly through it
- Covering. The steep slope can be tiled, slated or shingled. The shallow upper slope is often below the minimum pitch for tiles and is then finished in felt, EPDM or zinc — so expect to combine two different roofing systems
- Structure. At the break there is usually a purlin or longitudinal member taking the horizontal thrust from the steep slope. Nothing there should simply be cut out during alterations
- Dormers. The steep slope lends itself well to dormers and rooflights, increasing the usable area further
- Insulation. Insulating internally loses less space on the steep slope than on a normal pitched roof, but the break demands a continuous, taped vapour control layer. Do not interrupt it there
- Planning. Adding or altering a mansard, especially on a street elevation or in a conservation area, frequently needs consent. Check before you design
Related terms
- Roof shape
- Roof structure
- Gable roof
- Dormer
- Purlin
- Roof pitch
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