How the roofing calculator works
A sloped roof has more area than the ground under it. The calculator multiplies the footprint, including the overhang, by a pitch factor.
Footprint = (length + 2 × overhang) × (width + 2 × overhang)
Pitch factor = √(1 + (rise ÷ 12)²)
Roof area = footprint × pitch factor
Bundles = roof area ÷ 100 × (1 + waste) × 3, rounded up
Worked example
A 40 by 30 foot house with a 12 inch overhang has a 42 by 32 foot footprint: 1,344 sq ft. A 6/12 pitch has a factor of 1.118, so the roof is 1,503 sq ft, or 15 squares. With 10% waste for a gable roof that is 16.5 squares, or 50 bundles.
Pitch factors
| Pitch | Factor | Pitch | Factor |
|---|---|---|---|
| 3/12 | 1.031 | 8/12 | 1.202 |
| 4/12 | 1.054 | 9/12 | 1.250 |
| 5/12 | 1.083 | 10/12 | 1.302 |
| 6/12 | 1.118 | 12/12 | 1.414 |
| 7/12 | 1.158 |
Materials list
- Shingles: 3 bundles per square for most three-tab and architectural shingles.
- Nails: about 320 per square (4 per shingle); 480 in high-wind areas.
- Underlayment: covers the whole roof deck. Rolls vary from about 400 to 1,000 sq ft.
- Drip edge: runs around the roof perimeter in 10 foot pieces.
- Starter strip and ridge cap: bought by length; check the coverage on the bundle.
Safety
Roof work causes serious falls every year. Use proper fall protection, never work on a wet or icy roof, and consider hiring a licensed roofer for steep pitches.
Common questions
What is a roofing square?
100 square feet of roof. A 1,500 sq ft roof is 15 squares.
How many bundles of shingles are in a square?
Three bundles per square is standard for three-tab and most architectural shingles. Some heavier designer shingles use four or five; check the wrapper.
How do I find my roof pitch?
Pitch is how many inches the roof rises for every 12 inches across. Hold a level horizontally against the roof underside or rafter, measure 12 inches along it, and measure straight up to the roof. A 6 inch rise is a 6/12 pitch.
How much waste should I add for roofing?
About 10% for a simple gable roof and 15% for hip roofs or roofs with valleys and dormers, which need more cutting.
How many nails per square of shingles?
About 320 for three-tab shingles: 80 shingles per square with 4 nails each. High-wind areas often require 6 nails per shingle (480 per square).