CalcCoverageHow much roofing do I need?
Roofing is sold by the square — 100 square feet of roof surface. The catch is that the roof surface is bigger than the building underneath it: the steeper the pitch, the more roof there is per foot of floor. So the estimate runs footprint × slope factor, where the factor comes straight from the pitch.
Roof area = length × width × √(1 + (pitch ÷ 12)²) · Squares = roof area ÷ 100
Example: a 30 × 24 ft building with a 6/12 pitch: footprint 720 sq ft × slope factor 1.118 = 805 sq ft of roof = 8.05 squares. With 10% waste that is 27 shingle bundles (3 per square) and 3 rolls of underlayment.
What the buying numbers assume
| Item | Assumption used here |
|---|---|
| Shingle bundles | 3 bundles per square (standard for most architectural shingles — check your wrapper) |
| Underlayment | 400 sq ft of coverage per roll (a standard felt roll after laps) |
| Waste | 10% covers cuts, starter courses and ridge work on a simple roof |
The footprint method suits a simple gable roof. Measure the building including the overhang the roof actually covers — a 12-inch eave overhang on both sides adds 2 ft to the width the roof spans. Complex roofs (hips, valleys, dormers) are best measured plane by plane: run each plane's slope length × width through the square footage calculator and add them, then divide by 100.