Roofing Calculator

A sloped roof covers more surface than the ground beneath it, and the multiplier is set by pitch. A 6/12 roof has a slope factor of 1.118, so a 40 × 30 ft footprint of 1,200 sq ft is really 1,342 sq ft of roof. Roofers then work in squares — 100 sq ft each — because shingles are sold three bundles to the square.

The footprint is 1,200 sq ft, and the slope factor brings the roof surface to 1,342 sq ft, or 13.4 squares. With 10% waste that is 14.8 squares: about 45 bundles of architectural shingles and 4 rolls of standard underlayment. The waste is not padding — it covers starter course, ridge caps, and every cut at a rake or valley.

Ten percent is right for a simple gable with two clean planes. A hip roof needs 12–15% because every hip is a diagonal cut, and a complex roof with valleys, dormers, and skylights can justify 15–20%. Cut-up roofs also consume far more in the accessories that don't scale with area: ridge cap, valley metal, step flashing, and pipe boots.

A re-roof needs underlayment, drip edge along eaves and rakes, ice-and-water membrane in valleys and at eaves in cold climates, starter strip, ridge cap, ridge or soffit ventilation, and nails — roughly 2.5 lb per square when hand-nailed. Flashing around chimneys and walls is where most leaks begin, and it is the part that gets reused when it shouldn't be.

Asphalt shingles weigh 2–3 lb per square foot, so a 14-square roof is 3,000–6,000 lb — worth knowing before stacking bundles in one spot on the deck. Most codes allow only two layers total, and a tear-off is the only way to inspect and replace bad sheathing. Above about a 6/12 pitch the work needs staging and fall protection; past 8/12 it stops being a reasonable do-it-yourself job.