How roof area is found
A sloped roof is bigger than the ground under it. Every plane of the roof has the same relationship to its footprint, so the whole roof is the footprint (including overhang) times the pitch multiplier:
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(pitch² + 144) ÷ 12
The multiplier is the length of the rafter per foot of run. A 6/12 roof has a multiplier of 1.118, so a 42 × 32 ft footprint (a 40 × 30 ft house with 1 ft overhangs) becomes 1,344 × 1.118 = 1,503 sq ft of roof, or 16.5 squares with 10% waste.
Gable, hip and shed: what changes
- Gable. Two planes meet at a ridge as long as the building plus the overhang at each end. Each common rafter runs from the ridge to the eave, which is half the width plus the overhang.
- Hip. Four planes. The area is the same as a gable of the same pitch and footprint, but the ridge is only length minus width, and there are four hip rafters. Plan for more waste because of the extra cuts.
- Shed. One plane. The rafter runs the whole width plus overhang, so a 30 ft wide shed roof needs a much longer rafter than a gable on the same building, and the high wall is taller by the full rise.
Roof area from a 1,500 sq ft footprint, by pitch
| Pitch | Multiplier | Roof area | Squares | Extra vs 4/12 |
|---|---|---|---|---|
| 3/12 | 1.031 | 1,546 sq ft | 15.5 | -35 sq ft |
| 4/12 | 1.054 | 1,581 sq ft | 15.8 | +0 sq ft |
| 5/12 | 1.083 | 1,625 sq ft | 16.2 | +44 sq ft |
| 6/12 | 1.118 | 1,677 sq ft | 16.8 | +96 sq ft |
| 8/12 | 1.202 | 1,803 sq ft | 18.0 | +222 sq ft |
| 10/12 | 1.302 | 1,953 sq ft | 19.5 | +371 sq ft |
| 12/12 | 1.414 | 2,121 sq ft | 21.2 | +540 sq ft |
This is the footprint including overhang. Dormers, a second roof plane at a different pitch, or a porch roof are measured on their own and added. Measure the footprint from the outside of the walls. If you already have the footprint and only want squares, the roof squares calculator is the shorter route, and the shingle calculator turns squares into bundles.
Frequently asked questions
How do I calculate roof area from the size of the house?
Add the overhang to each side, multiply length by width, then multiply by the pitch multiplier. For a 6/12 roof the multiplier is 1.118.
Is a hip roof bigger than a gable roof?
No. With the same footprint, overhang and pitch the surface area is the same. A hip roof has more cuts and hip caps, so allow more waste.
How many squares is a 2,000 sq ft roof?
20 squares, because a roofing square is 100 square feet. Add waste for the number you order.