How roof area is estimated
The building footprint (length × width) is multiplied by a pitch factor that accounts for slope: a 6/12 roof has about 12% more area than the footprint, and steeper roofs add more. The result is then converted to squares — one square equals 100 square feet, the unit roofers use to price and order material.
Shingles are sold in bundles: three-tab shingles usually need 3 bundles per square, and thicker architectural shingles often need 4. Always add 10% waste for hips, valleys, ridge caps and mistakes, and measure any complex roof as several simple rectangles added together. For sloping roofs with multiple planes, an on-site measurement gives the most accurate result.
Roofing units
- 1 square = 100 sq ft of roof area
- 3-tab shingles: ~3 bundles per square
- Architectural shingles: ~4 bundles per square
- Average US home roof: 20-30 squares