From footprint to roof area
A sloped roof has more surface than the ground it covers. Multiply the horizontal footprint (including overhangs) by the pitch multiplier √(1 + (rise/12)²). For a 6/12 pitch the multiplier is 1.118; for 12/12 it is 1.414.
Example: 1,500 sq ft footprint at 6/12: 1,500 × 1.118 = 1,677 sq ft. Plus 10% waste = 1,845 sq ft = 18.45 squares → 56 bundles at 3 bundles per square.
What is a “square”?
Roofers measure in squares of 100 sq ft. Most architectural asphalt shingles come 3 bundles per square; some heavier designer lines are 4 or 5—check the wrapper.
Don't forget
- Starter strip along eaves and rakes
- Ridge cap shingles (measure ridges and hips in linear feet)
- Underlayment, drip edge and ice-and-water shield where local code requires