Mansard Roof Calculator

Last reviewed September 27, 2026 · How we calculate

A mansard roof has two slopes on every side: a steep lower slope that is almost a wall, and a shallow or flat top. It gives a full upper story under the roof, and it makes the roof harder to measure than most. This works out the area of both parts, the slope lengths and the height.

How the calculation works

On plan, the steep slopes form a ring around the building, as deep as the lower slopes reach in from the walls. The ring’s plan area is the whole footprint less the rectangle left in the middle. Multiply it by the pitch multiplier for the lower pitch and you have the steep roof. The top is the inner rectangle, times the multiplier for its own pitch, which is simply 1 when it is flat.

A worked example

A building 40 ft by 30 ft with lower slopes reaching 4 ft in from the walls, at 24 in 12, and a flat top: the ring covers 496 sq ft on plan, and at a multiplier of 2.236 that is about 1,109 sq ft of steep roof. The flat top is 32 ft by 22 ft, or 704 sq ft. Together the roof is about 1,813 sq ft. Each lower slope is 8.94 ft long and rises 8 ft, at about 63 degrees.

Two roofs in one

A mansard has to be treated as two different roofs. The top is usually too shallow for shingles and needs a low-slope membrane, which the flat roof membrane calculator sizes. The lower slopes are steep enough that shingles need the manufacturer steep-slope method, and slate or metal are common alternatives. Where the two meet, the flashing at the break is the detail most likely to leak.