Roof Area Calculator
Calculate the actual sloped roof area from building footprint, pitch and overhang - for gable, hip and gambrel roofs.
The roof area calculator returns the actual sloped roof surface from your building footprint and pitch - the number you actually need for ordering shingles, underlayment or roof coatings. The math is simple: footprint x sqrt((rise/12)^2 + 1). A 6/12 pitch multiplies by 1.118; a 12/12 pitch multiplies by 1.414. Eave overhangs add to the footprint before the multiplier - a 12 inch overhang on every side adds 4-8% to the total depending on house size. For hip and gable roofs the total surface area is the same (they only differ in shape), so the calculator gives a single number for any standard roof.
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Frequently asked questions
Multiply the footprint by the pitch multiplier: sqrt((rise/12)^2 + 1). For a 6/12 pitch the multiplier is 1.118, so a 40x30 ft footprint = 1,200 sq ft footprint = 1,342 sq ft of roof surface. Add the eave overhang to the footprint dimensions first before multiplying - a 12 inch overhang adds 2 ft to each side.
No - they have the same total surface area for the same footprint and pitch. A hip roof spreads that area across four slopes instead of two, but the math comes out identical. Hip roofs do have more linear feet of ridge and hip (which adds ridge cap material) and more valleys at intersections with dormers, but the field shingle count matches a gable.
12 inches is the typical residential overhang for protecting walls from rain. 18-24 inches is common in modern designs and energy-efficient homes (deep overhangs shade south-facing windows in summer). Older homes and bungalows often have 6 inch overhangs. The overhang adds to the footprint before the pitch multiplier - a 12 inch overhang on every side of a 40x30 house turns it into 42x32 for roof calcs.
Footprint is the horizontal area the roof covers (what you would measure from a floor plan). Roof area is the actual surface of the roof - always larger than the footprint unless the roof is perfectly flat. For roofing materials you want roof area; for snow load and rainwater capture you also want roof area; only for shading and building permits do you want footprint.
Calculate each roof plane separately and add them up. A typical Cape Cod with two front-facing dormers: calculate the main gable roof, then add each dormer roof (usually small gable or shed roofs themselves). For roofs with many planes, ridges and valleys, add 15-20% waste instead of 10% because cuts compound at every intersection.