Measurement tool
Roof Square Footage Calculator
Enter the length and width of the house and the roof pitch. The calculator returns true roof surface area, not the ground footprint. On a sloped roof the two are never the same.
Roof square footage equals length × width × the pitch multiplier. A 50 × 30 ft house with a 6/12 pitch has a 1,500 sq ft footprint but 1,677 sq ft of actual roof, which is 16.8 roofing squares.
Calculate roof square footage
Measure the outside of the house at ground level, including any overhang you want covered.
Roof area
1,677 sq ft
true surface area
Footprint is not roof area
This is the single most common estimating mistake. The number you get from walking around the house with a tape measure is the footprint, meaning the ground the roof covers. The roof itself is longer, because it slopes.
The conversion is one multiplication. Pitch multiplier = √(1 + (rise/12)²). At 6/12 that is 1.118, so a 1,500 sq ft footprint is 1,677 sq ft of roof. At 12/12 it is 1.414, and the same house needs 2,121 sq ft of material. Order from the footprint and you will be roughly two squares short on a typical home.
Do not forget the overhang
Eaves and rakes extend past the walls, usually 12 to 24 inches. On a 50 × 30 ft house a 1-foot overhang adds 164 sq ft of roof, which is more than a square and a half. Measure to the drip edge, not the siding.
For complex roofs, break the shape into rectangles, calculate each separately, and add them. Dormers, wings and porch roofs each get their own rectangle. Hips and valleys do not change total area appreciably; they change waste, which is why the waste factor exists.