Roofing material is priced and sold in squares, but most people measure their house as a footprint. Bridging the two — via the roof pitch — is the entire calculation.
The footprint-to-surface gap
A sloped roof always has more surface than the floor plan beneath it. The multiplier is 1 ÷ cosine of the pitch: gentle pitches add a few percent, steep pitches can add over half. Getting this right is where most roofing estimates go wrong.
Squares and bundles
Roofing is measured in squares of 100 ft² and shingles are packaged three bundles to a square. Converting your surface area into these units is what makes the number orderable — and comparable between quotes.
Where waste hides
Valleys, hips, ridges and vents all force cuts that discard part of a shingle. Adding 10–15% accounts for them, and buying from a single batch avoids the dye-lot mismatch that appears once the roof is on.
Frequently asked questions
Divide the footprint (length × width) by the cosine of the roof pitch. A 10×8 m footprint at 0° is 80 m²; at 30° it's about 92 m².
A square is 100 square feet (9.29 m²) of roof — the standard unit suppliers quote. Shingles come three bundles per square.
About 10%, rising to 15% for roofs with valleys, hips and multiple penetrations where shingles are cut heavily.