Calculate the number of roof shingle bundles needed for your roof.
Roofing is sold by the square, which is 100 sq ft of roof surface, and the first thing to get right is that roof surface is larger than the footprint it covers. A 6/12 pitch multiplies the plan area by 1.118, a 10/12 by 1.302, so a 1,600 sq ft footprint is 1,789 sq ft of roof at 6/12 — nearly two extra squares. Three bundles of standard three-tab or architectural shingles cover one square. Waste runs about 10% for a simple gable and higher for a cut-up roof with valleys, dormers and hips, because every valley cut throws away part of a shingle. Hip and ridge cap is bought separately and covers roughly 20 linear feet per bundle.
Roof Shingle Count
Roof area = footprint x pitch multiplier; squares = area / 100; bundles = squares x bundles per square x (1 + waste%); ridge cap bundles = ridge length / 20
A material estimate only. Roof work involves fall risk and structural and fire-code requirements including underlayment, ice barrier and ventilation — have re-roofing specified and carried out by a licensed roofer.
Three for most three-tab and architectural shingles. Heavier designer and slate-look shingles run four or five, so check the bundle coverage printed on the wrapper.
Because it slopes. The pitch multiplier is the ratio of slope length to horizontal run, so a 6/12 roof has 11.8% more surface than the area it shelters.