Roofing calculator
Measure the footprint of the roof from above, including the overhangs, and pick the pitch. A 40 × 30 ft footprint at 6/12 pitch is about 1,342 sq ft of roof, or 13.4 squares before waste.
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How the calculator works
A sloped roof has more surface than the ground it covers. The pitch multiplier, √(1 + (rise ÷ 12)²), converts the flat footprint into the real roof area. It works for gable and hip roofs alike, as long as every face has the same pitch. One roofing square is 100 sq ft.
Squares, bundles and waste
Roofers measure in squares of 100 sq ft. GAF, one of the largest shingle makers, says most of its asphalt shingles come three bundles to a square, some heavier lines need four or five, and you should add 10–15% for cutting and waste (GAF). Complex roofs with valleys and dormers sit at the top of that range.
This estimate covers the field shingles. Starter strip, ridge cap, underlayment, drip edge and flashing are bought separately and depend on the length of eaves, ridges and valleys.
Pitch multipliers
| Pitch | Multiplier | Angle |
|---|---|---|
| 3/12 | 1.031 | 14.0° |
| 4/12 | 1.054 | 18.4° |
| 5/12 | 1.083 | 22.6° |
| 6/12 | 1.118 | 26.6° |
| 7/12 | 1.158 | 30.3° |
| 8/12 | 1.202 | 33.7° |
| 9/12 | 1.250 | 36.9° |
| 10/12 | 1.302 | 39.8° |
| 12/12 | 1.414 | 45.0° |
Don't know your pitch? Use the roof pitch calculator.