Roofing
Roof Area Calculator
Calculate true roof surface area from the building footprint and the pitch.
Result
This assumes a simple gable. Hips, valleys, dormers and multiple ridge lines all add area and, more importantly, labour, which is why the complexity allowance matters more on cut-up roofs.
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About this calculator
Why is roof area bigger than the house footprint?
Because the roof is sloped. A 6/12 pitch adds about 12% to the flat area, and a 12/12 adds 41%. Ordering on footprint alone always comes up short.
Should I include the overhang?
Yes. Eaves are roof surface that needs covering, and an 18-inch overhang on a typical house adds a meaningful amount of material.
The formula
roof area = footprint × slope multiplier × complexity factor
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