Roofing Calculator

What this means

A roof's surface area is larger than its footprint because it's sloped. The area scales by 1 ÷ cosine of the pitch — a 30° roof is about 15% bigger than its footprint, and a 45° roof about 41% bigger. Roofing is bought in 'squares' (100 ft² or 9.29 m²), and asphalt shingles come three bundles per square. Add a waste allowance for cuts, ridge caps and valleys, then round up to whole bundles.

Recommendations

• •Waste runs higher on roofs with valleys, hips and penetrations — bump the allowance toward 15%.
• •Order all shingles at once from the same batch to avoid colour variation across the roof.
• •Include ridge caps and starter strips when you order — they're sold separately from field shingles.

Watch out

• •This is a material estimate for a simple gable — hip and mansard roofs need a more detailed take-off.
• •Installation is dangerous; serious roofing should be handled by professionals.