Roof area is larger than the footprint of the house because a sloped roof covers more surface than the ground beneath it. The steeper the pitch, the bigger the difference. This guide shows the formula, a worked example, and how to turn the result into roofing squares and bundles.
What to measure
- The length and width of the roof footprint, in feet, including overhangs.
- The pitch, written as rise over run (for example 6/12 means 6 inches of rise for every 12 inches of horizontal run).
The formula
Pitch multiplier = square root of (1 + (rise ÷ 12)²). Roof area = footprint area × pitch multiplier.
Worked example
A 40 ft by 30 ft footprint has an area of 1,200 sq ft. At a 6/12 pitch, the multiplier is the square root of (1 + 0.5²) = 1.118. Roof area = 1,200 × 1.118 = about 1,342 sq ft.
Squares and bundles
Roofing is sold by the square, which is 100 sq ft. 1,342 sq ft is about 13.4 squares. Most asphalt shingles come three bundles to the square, so that is about 41 bundles before waste.
Adding an allowance for waste
Hips, valleys and cut edges create waste. A common allowance is 10 percent for a simple gable roof and more for complex roofs. At 10 percent, 13.4 squares becomes about 14.8 squares, or about 45 bundles.
Common mistakes
- Measuring the footprint without the overhangs.
- Using the pitch angle in degrees in a formula built for rise over run.
- Forgetting to add waste.
- Ordering by square footage instead of by squares or bundles.
Use the calculators
Try the roofing calculator, or browse all construction and home improvement calculators. Results are estimates; see the disclaimer.
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