Spike's Calculators

Roof Framing

Roof Sheathing Mansard Roof (Imperial)

Estimate the number of sheets of plywood (or OSB) needed for sheathing a Mansard-style roof. The amount of plywood or OSB needed is 4' x 8' sheets, 32 square feet per sheet.

Roof Sheathing Calculator

Note: # 6 in the calculation can be a bit of a tongue twister. This measurement and the eaves overhang establish the length of the lower roof slope.

Use the calculator below to determine the number of sheets required, including waste percentage:

Roof Sheathing, Mansard Roof

Building Length ft inch
Building Width ft inch
Eaves Overhang inch
Upper Roof Pitch /12
Lower Roof Pitch /12
Lower Roof Length ft inch
Waste Percentage %


Lower Roof Area ft²
Upper Roof Area ft²
Total Roof Area ft²
Estimated Plywood Needed #
Waste Factor adds #
Plywood Needed Including Waste #
Sheets needed split in upper and lower sections
Lower Slope Sheathing #
Upper Roof Sheathing #


  1. enter the length of the building in feet and inches
  2. the width of the building feet and inches
  3. eaves overhang in inches
  4. pitch of the upper slope
  5. pitch of the lower slope
  6. Roof Sheathing Calculator
  7. length from outside wall to edge of upper roof feet and inches
  8. waste factor in a percentage


  1. lower roof area square footage
  2. upper area square footage
  3. total roof area in square feet
  4. number of sheets needed based on exact square footage of the roof
  5. the number of sheets the waste percentage will add
  6. number of sheets needed including the waste
  7. And just in case you are using different materials for the lower and upper slope:
  8. # of sheets for the lower slope (includes waste percentage)
  9. # of sheets for the upper roof (includes waste percentage)

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