How to use the Roofing Material
- Measure the building footprint length and width.
- Enter the roof pitch as rise over 12 inches of run.
- The tool applies the pitch multiplier to get true surface area.
- Read squares needed and bundle count including waste.
How the calculation works
The pitch multiplier is √(rise² + 12²) ÷ 12. A 6/12 pitch gives √(36+144)/12 = 1.118, so the true roof surface is 11.8% larger than the footprint. A 12/12 pitch multiplies by 1.414. Roof area is footprint area × multiplier, and roofing is sold by the square, which is 100 square feet — three bundles of standard architectural shingles per square.
Waste allowance depends on complexity: 10% for a simple gable, 15% for a hipped roof, and up to 20% where there are multiple valleys, dormers or chimneys, because every hip and valley requires cut shingles. Budget separately for starter strip and ridge cap by linear foot rather than by square, underlayment at one roll per four squares typically, drip edge along all eaves and rakes, and ice-and-water shield in valleys and at eaves in cold climates. Above a 6/12 pitch, labour costs rise because roof jacks and additional fall protection become necessary.
Pitch multiplier = √(rise² + 144)/12 ; squares = footprint area × multiplier / 100 × (1 + waste)Source: Standard roofing geometry: pitch multiplier method and the 100 sq ft roofing square convention.
Worked example
A 40 ft × 28 ft footprint hipped roof at 6/12 pitch, 15% waste.
- Footprint = 1,120 sq ft.
- Multiplier for 6/12 = 1.118, so surface = 1,252 sq ft.
- Squares = 12.52; with 15% waste = 14.4.
- Bundles = 14.4 × 3 = 43.2, round to 44.
About 14.4 squares, or 44 bundles, plus ridge cap and starter strip by the linear foot.
Frequently asked questions
What is a roofing square?+
100 square feet of roof surface — typically three bundles of architectural shingles.
Why is roof area bigger than the footprint?+
Pitch adds slope length. A 6/12 roof is 11.8% larger than its footprint.
How much waste should I allow?+
10% for simple gables, 15% for hips, and up to 20% for complex roofs with valleys and dormers.
Is ridge cap included in the squares figure?+
No. Ridge cap and starter strip are ordered by linear foot separately.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.