Estimating, Plans, Specifications, and Contracting
Measure roof surface, not footprint
Every quantity in a roofing estimate starts from squares, where one square equals 100 square feet of finished roof surface. On any sloped roof the plan area must be converted with a pitch multiplier: multiplier = √(1 + slope²), with slope expressed as rise over 12. Useful values: 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 7/12 = 1.158, 8/12 = 1.202, 9/12 = 1.25, 12/12 = 1.414. The classic trap is pricing the building footprint and forgetting the multiplier, which under-bids a 7/12 roof by nearly 16 percent.
Waste, accessories, and true cost
Add waste to the roof-surface number, not the footprint: about 10 percent for a simple gable, 15 percent for a hip roof or one with several penetrations, and up to 20 percent for a cut-up roof full of valleys and dormers. Then price accessories as counts and linear feet rather than afterthoughts - drip edge, starter course, ridge and hip cap, valley metal, step flashing, pipe boots, ridge vent, pan flashing. Underlayment is ordered in rolls whose coverage varies (15-pound felt is roughly four squares per roll, 30-pound about two, and synthetic products commonly ten). Labor should carry a burden of roughly 25–40 percent for payroll taxes, insurance, and benefits, and overhead plus profit is priced on top of direct cost.
Worked example
A 30 ft × 40 ft gable at 7/12: footprint 1,200 sq ft × 1.158 = 1,390 sq ft = 13.9 squares. Add 10 percent waste → 15.3 squares → round to 16 squares. At three bundles per square that is 48 bundles, plus starter, cap, underlayment, and flashing measured separately.
Contracting
Scopes must state inclusions and exclusions, decking repair pricing, change orders in writing, payment schedule, and who warrants what - manufacturer material warranty versus contractor workmanship warranty are separate promises.