Shingles & Squares

Roofing material is sold by the square — 100 square feet of roof surface. Not 100 square feet of ground, and that distinction is where most estimates fail.

A 40 × 30 ft house covers 1,200 ft² on the ground. The roof over it, at an 8/12 pitch, is 1,442 ft². That is two and a half squares of shingle nobody ordered.

The pitch factor is not an estimate

factor = √(1 + (rise ÷ 12)²)

4/12 gives 1.054. 6/12 gives 1.118. 8/12 gives 1.202. 12/12 gives 1.414. It is the hypotenuse of the roof triangle and there is no judgement in it — which makes it the most reliable number in the whole estimate.

Waste is a property of the roof shape

Ten percent on a simple gable, where the only cuts are at the rakes. Fifteen on a hip roof, which cuts along every hip. Twenty on anything with valleys and dormers, where a fifth of what you buy ends up in the dumpster.

Quoting one waste figure for all roofs is how a cut-up roof comes up short.

The two lines that get left off every estimate

Starter course along every eave and rake, and hip and ridge cap along the top. Both are separate products. Cutting up field shingles to use as starter voids most manufacturers' wind warranties, and that is discovered at the claim rather than at the register.

Tear-off is half the job

A single layer of asphalt shingle weighs 200–350 lb per square. A 30-square roof is around four tons of material to strip, carry down and skip — and on a second layer, double it. That line belongs in the budget before the shingle line does.

The one thing to check before anything else

Whether you can stand on it. Above 8/12 most people cannot, and roof jacks or scaffolding change the economics of doing it yourself more than any material price does.