How it works
Start with the footprint — the flat area the roof covers when viewed from directly above, which is usually the same as the building's floor plan. A sloped roof has more actual surface area than its footprint, because the pitch stretches that same footprint over a longer diagonal distance. A pitch multiplier converts footprint into actual roof area, and a waste percentage accounts for cuts and offcuts around edges, valleys and hips.
| Pitch (rise/12) | Multiplier |
|---|---|
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.25 |
| 10/12 | 1.302 |
| 11/12 | 1.357 |
| 12/12 | 1.414 |
In the US, roofing materials are priced and packaged by the square — 100 square feet of roof area — and shingle bundles are sold three to a square, so bundle count is squares × 3, rounded up. Squares themselves are rounded up to the nearest 0.1 so a small remainder still buys enough material. Metric roofing has no equivalent packaging concept, so metric results show only the area.
Worked example
Two examples below — one in US units, one in metric.
- Footprint is 2000 sq ft, pitch 6/12 (multiplier 1.118).
- Multiply by pitch: 2000 × 1.118 = 2236 sq ft.
- Add 10% waste: 2236 × 1.10 = 2459.6 sq ft.
- Squares: 2459.6 ÷ 100 = 24.6 squares.
- Bundles: 24.6 × 3 = 73.8, rounded up to 74 bundles.
Order 24.6 squares, or 74 bundles of shingles.
- Footprint is 200 m², pitch 6/12 (multiplier 1.118).
- Multiply by pitch: 200 × 1.118 = 223.6 m².
- Add 10% waste: 223.6 × 1.10 = 245.96 m².
The roof needs 245.96 m² of covering material. Metric roofing has no squares or bundles concept, so that's the final figure.
Common questions
What's a roofing 'square'?
A square is roofing-trade shorthand for 100 square feet of roof area — it's how shingles and bundles are priced and packaged in the US, which is why it doesn't have a metric equivalent.
Why does roof pitch increase the material needed?
A steeper roof has more actual surface area than its footprint suggests, because the slope stretches that same footprint over a longer diagonal distance. A 6/12 pitch adds about 12% more area than the flat footprint; a 12/12 pitch (45°) adds over 41%.
How much waste allowance should I add?
10% is standard for a straightforward gable roof. Complex rooflines with lots of valleys, hips, or dormers should use 15–20%, since more cuts mean more offcuts that can't be reused.