Mansard Roof Cost Calculator

How the estimate is worked out

The tool splits the roof at the break, the line where the steep face meets the flat top, and prices each part on its own. Our mansard roof guide recommends exactly that, because membrane and shingles are priced differently.

• Inset = steep face height x 12 / pitch. This is how far the face reaches in from each wall.
• Steep face area = slope length x (2 x (length + width) minus 4 x inset), where slope length is the square root of height squared plus inset squared
• Flat top = (length minus 2 x inset) x (width minus 2 x inset), times a small factor for its 1/12 to 3/12 slope
• Steep face material = area x 1.15 for waste, plus 25 sq ft per dormer
• Break line = the perimeter of the flat top, which gets custom metal curb flashing

The default inputs make a worked example. A 40 x 30 ft house with an 8 ft steep face at 20/12 (about 59 degrees) has a face that reaches 4.8 ft in from each wall. The slope is 9.3 ft long, so the four sides cover 1,127 sq ft. The top is 30.4 x 20.4 ft, or 629 sq ft at 2/12, and the break line runs 102 ft.

With architectural shingles, the steep face costs $2,790 to $4,610 in material and $2,820 to $4,730 in normal labor. The mansard premium adds $2,820 to $5,070. TPO on the top is $4,090 to $6,920, tear-off is $1,760 to $2,630, curb flashing is $1,020 to $3,050 and four dormers add $1,000 to $2,000.

The total is $16,290 to $29,010, or $9.28 to $16.52 per sq ft of roof. The same house in natural slate comes to $29,470 to $63,150.

Why does a mansard roof cost more?

Labor on the steep face is the biggest reason. The lower slope is often steeper than 12/12, and nobody can walk on 20/12, so crews work from scaffolding, roof jacks and harnesses and every course goes on slowly. The mansard guide puts architectural shingles at $4.50 to $7.50 a sq ft on a normal roof and up to $10 to $12 on a mansard, with all of the increase in labor.

The tool turns that into a premium of $2.50 to $4.50 a sq ft ($250 to $450 a square) on the steep face at 12/12 and steeper. That puts architectural shingles at $7 to $12 a sq ft, and the same premium goes on every other face material. Under 12/12 it starts at the main calculator's 15% steep-roof labor factor at 8/12 and rises to the full premium by 12/12, so the price doesn't jump at one pitch.

• You are buying two roofs. The top needs a membrane and the sides need shingles, slate or metal, each with its own crew skills and materials.
• Dormers slow everything down. Each one needs its own flashing and extra cutting. The tool adds $250 to $500 each, plus 25 sq ft of waste, the same as the main roof cost calculator.
• The break has to be flashed by hand. The curb flashing is bent on site to the roof's angle, tucked under the membrane and lapped over the top row of the face. The tool prices it at $10 to $30 a foot, the wall flashing rate in our roof flashing guide, and two to three times that in copper.

Mansard prices by material

The table shows each face material at normal-roof rates (the same numbers as the main calculator) and on a mansard face in this tool. Waste, dormers, tear-off and the top membrane are extra.

Natural slate's $15 to $35 comes from our slate roof guide, and synthetic slate's $9 to $18 from the synthetic vs natural slate comparison. Shingle prices match types of shingles. Steel standing seam runs $10 to $16, per the standing seam guide, and long panels from curb to eave leave very few horizontal seams.

Copper is a floor, not a range. Our metal guides say only that the metal alone often costs more than $15 to $20 a sq ft, so the tool adds standing seam labor to that and labels it a starting price. Copper flashing on a steel roof corrodes the steel, so match the flashing to the panels.

The flat top needs a membrane, priced like a flat roof

The top of a mansard usually sits between 1/12 and 3/12. The International Residential Code does not allow asphalt shingles below 2/12, so the top gets a single-ply membrane. The tool uses the flat roof calculator's prices: EPDM at $4 to $10.50 and TPO at $6.50 to $11 a sq ft, each including about 2 inches of polyiso and the flashings.

White TPO reflects heat off the floor below, and black EPDM absorbs it. For seams, rolls and insulation boards on the top, run its dimensions through the flat roof calculator.

Stories, tear-off and the face height move the total most

The steep face is the expensive part, so its height matters more than the footprint. Every extra foot of face height adds steep area at mansard rates and shrinks the cheaper flat top. A steeper face does the opposite: it covers less area for the same height.

• Tear-off costs $100 to $150 a square per layer on both sections, the main calculator's rate.
• Three stories or more adds $500 to $1,500 for a lift or crane, from our flat roof guide.
• Decorative cornices or custom trim at the break can add several thousand dollars. The tool leaves them out because the cost varies so much.
• Gutters are separate. Water comes off a mansard face fast, so most need 6-inch or 7-inch gutters, which the gutter calculator can size.

When another roof makes more sense

A mansard earns its price by turning attic space into a full top floor. If you don't need that room, a hip or gable roof costs far less to build and to maintain, and the main roof cost calculator will price one from your address.

Converting an existing roof into a mansard is a framing job before it is a roofing job. Get a structural engineer's report and check with the building department first, because some towns limit eave height or materials on steep slopes. On a historic house, the right answer may be repair rather than replacement, especially with slate.