How roof area is calculated
A sloped roof has a larger surface than the floor area beneath it. The footprint is multiplied by a pitch multiplier:
Roof area = length × width × √(1 + (pitch ÷ 12)²)
| Pitch | Angle | Multiplier |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Squares and bundles
Roofing is sold by the square, which covers 100 sq ft. Standard three-tab and most architectural shingles come three bundles to a square. Add 10% waste for a simple gable roof and 15% for hip roofs or roofs with valleys and dormers.