Donaldson R F Roofing Calculator
Estimate the roof area and the materials you need — shingles, metal panels or tiles — from the building size and roof pitch.
How the estimate is made
The building footprint plus the overhang gives the roof's plan area; multiplying by the pitch factor √(1 + (rise ÷ run)²) turns it into the real sloping surface (a 6/12 pitch gives 1.118, so the roof is about 12% larger than the plan). Gable, hip and shed roofs with one pitch have the same total surface for a given footprint; a flat roof uses a factor of 1. Roofing is sold in “squares” (100 sq ft ≈ 9.29 m²): asphalt shingles come in about 3 bundles per square, tiles are counted per square metre, and metal panels by their covered width. The waste allowance covers cuts, starter courses and mistakes — 10% for a simple gable, 15% or more for hip roofs and complex shapes. Valleys, dormers, chimneys, skylights and multiple roof sections are not included, so this is a planning estimate: have your supplier or roofer measure before you order.