Roofing calculator

Multiply the footprint (including overhangs) by the pitch factor: at 4/12 the factor is 1.054, at 6/12 it is 1.118, at 9/12 it is 1.25 and at 12/12 it is 1.414. A gable and a hip roof of the same pitch over the same rectangle have almost the same surface area — the shape mainly changes ridge and hip lengths, not the area.

Three bundles of standard 3-tab or architectural shingles cover one square (100 sq ft). Add 10–15% waste for a simple gable and 15–20% for hips, valleys and dormers, because every cut edge wastes part of a shingle.

Cover the full roof area plus 10–15% for the horizontal overlaps (2–4 in) and vertical laps. A typical synthetic roll covers about 4 squares (400 sq ft); #30 felt covers 2 squares per roll.

Asphalt shingles need at least 2/12 with special underlayment and 4/12 for standard installation. Exposed-fastener metal panels work down to 3/12, standing seam to 1/12. Below that you need a membrane roof, not shingles or panels.