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 based 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 together with the eaves overhang establishes the length of the the lower roof slope.

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 + inches
  2. the width of the building feet + 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 + inches
  8. waste factor in a percentage

The Results

  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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