Why footprint × pitch multiplier
You measure the house on the ground; the roof is steeper and therefore bigger. The pitch multiplier converts the flat footprint to the sloped surface: a 6/12 pitch (roof rises 6 inches per 12 of run) multiplies area by about 1.12, a 12/12 pitch by 1.41. Measure the footprint, pick the pitch, and the calculator produces the sloped area.
Squares and bundles explained
Roofing is sold in 'squares' — 100 square feet each — and asphalt shingles come 3 bundles per square (heavy architectural shingles may be 4). The calculator adds a 10% waste allowance for starter courses, ridge caps, hip cuts and the bundle that always gets rained on.
Complex roofs need extra
Dormers, valleys and hips add cut waste that the simple footprint model under-counts. For cut-up roofs, bump the waste allowance to 15% and add each dormer's footprint separately. Gutters, ridge vents and underlayment are priced separately from the shingle count.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- National Roofing Contractors Association — installation standards for steep- and low-slope roof systems.
- FEMA Building Science — wind and snow load guidance for residential roofs.
- American Society of Civil Engineers — ASCE 7, the source of the ground snow load maps used in roof design.
Figures, formulas and reference tables last verified: 19 September 2026.