How this calculator works
A roof plane is longer than the building footprint because it rises as it runs. The pitch factor, the square root of one plus rise over run squared, converts the flat footprint into the sloped surface. A 6/12 roof stretches every foot of plan to about 1.118 ft of slope, so a 600 ft² footprint becomes about 671 ft² of roof. Shingles are sold by the square, which is 100 ft² of finished roof, and by the bundle.
Formula
A = footprint × √(1 + (rise ÷ run)²); squares = A(ft²) ÷ 100; bundles = ceil(squares × bundles per square × (1 + waste%)). 1 ft² = 0.09290304 m², so 1 m² is 10.764 ft².
Worked example
A 20 × 30 ft gable at 6/12 with 10% waste: 600 ft² × 1.1180 = 671 ft², or 6.71 squares. At 3 bundles per square and 10% waste that is 23 bundles. The same footprint at 0/12 is 600 ft² and 20 bundles.
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Frequently asked questions
Does this cover the whole roof or one slope?
It covers all slopes. Enter the full building footprint; the pitch factor applies to both planes of a simple gable, which is why the result is not doubled.
What counts as a square?
One square is 100 ft² of finished roof surface. Bundles are packed three or four to a square depending on the product, which is why you enter bundles per square separately.
What is not included?
Starter strips, ridge caps, drip edge, vents and ice and water shield at the eaves. Most Canadian jurisdictions require that shield along the eave, so budget for it as a separate line item.