How do you calculate roof area from pitch?
Short answer. To calculate roof area from pitch, multiply the house's footprint, including overhangs, by the pitch multiplier: about 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12, 1.302 for 10/12, and 1.414 for 12/12. Divide by 100 to get roofing squares, then add 10 to 15 percent for waste when ordering material.
Pitch multipliers
- 3/12: 1.031
- 4/12: 1.054
- 5/12: 1.083
- 6/12: 1.118
- 7/12: 1.158
- 8/12: 1.202
- 9/12: 1.250
- 10/12: 1.302
- 11/12: 1.357
- 12/12: 1.414
The multiplier is the square root of 1 plus the pitch fraction squared. It converts flat footprint area into sloped roof surface area.
Worked example
- Footprint of house and garage: 1,800 square feet.
- Add overhangs: with about 1 foot of overhang all around, the covered area might reach roughly 1,980 square feet.
- Multiply by the 7/12 factor of 1.158: about 2,293 square feet of roof.
- Divide by 100: about 22.9 squares.
- Add 10 percent waste for a simple gable roof: about 25 squares to order. Use 15 percent or more for hips, valleys, and dormers.
Mixed pitches
If the main roof is 8/12 and the porch is 3/12, calculate each section separately with its own multiplier and add them together. Applying one pitch to the whole house is a common source of error.
What this does not include
Ridge, hip, valley, and eave lengths are calculated separately, because they drive ridge cap, starter, drip edge, and leak barrier quantities. See how roofers measure a roof for the full method, and roof replacement cost in Georgia for how area turns into a realistic price range.