Squares, pitch factors, tear-off, and shingle waste. Choose a project type below for a complete step-by-step guide with formulas, waste factors, productivity benchmarks, and pro tips.
Roofers price by the square — 100 square feet of finished roof surface — not by the building's footprint. The pitch factor turns flat footprint into actual roof area: 1.06 for a 4/12 pitch, 1.20 for 8/12, 1.42 for 12/12. A 30×40 ranch on an 8/12 isn't 1,200 sq ft of shingles, it's 1,440 — twelve squares plus waste. Bundle counts depend on shingle type: standard 3-tab covers a square in 3 bundles, architectural in 3–4, premium dimensional in 4. Waste runs 10% on simple gables and up to 15–20% on cut-up roofs with multiple valleys, hips, and dormers. Tear-off is always a separate line item. On a roof over 20 years old, layering is rarely allowed by code (most jurisdictions cap at two layers) and is almost always a bad idea regardless — you can't inspect the deck through old shingles. Decking replacement is priced per sheet of OSB at the going rate plus labor, and you should always quote it as an allowance because you can't see all the rot until tear-off. The trade differs from carpenters (who frame and re-deck) and gutter installers (typically subbed), but roofers are responsible for flashing at chimneys, walls, and skylights, which is where most leak callbacks originate.
Multiplies flat footprint into actual roof area. A 12/12 has 42% more surface than the floor below it. Walking surface above 8/12 also requires roof brackets or harnesses, slowing labor.
One layer is fast and clean; two layers double tear-off labor and dump fees. Anything thicker is a code issue.
Cut-up roofs need ridge cap, hip cap, and valley material at 30–50% premium per linear foot vs. field shingles. Count linear feet, not just square count.
Rotten or sagging decking is invisible until tear-off. Always quote sheathing replacement as an allowance per sheet of OSB, and document the assumption in the contract.
A 30×40 footprint isn't 1,200 sq ft of roof — at 8/12 pitch it's 1,440. Pitch factor is non-negotiable on every estimate.
Tear-off is its own line and its own crew time. Folding it into the install rate produces a thin margin you can't recover.
When tear-off reveals rot, you either eat the cost or have an awkward change-order conversation. Bake the allowance into the contract.
Shingle replacement is measured in squares (1 square = 100 sq ft). Every component — tear-off, underlayment, ice and water shield, shingles, ridge cap, drip edge — is priced separately per square or per linear foot.
Flat roofing is priced by the square (100 sq ft) like pitched roofing, but the components are entirely different: membrane, insulation board, adhesive, flashings, and drainage. No pitch factor applies.