The formulas
Footprint = (length + 2 × overhang) × (width + 2 × overhang) Pitch multiplier = √(1 + (rise ÷ 12)²) Roof area = footprint × pitch multiplier Squares = roof area ÷ 100 Bundles = squares × 3
Example: a 40 × 30 ft house with a 12-inch overhang and a 6/12 roof: 42 × 32 = 1,344 square feet × 1.118 = 1,503 square feet, or 15 squares. With 10% extra you need 50 bundles.
Quick answers for common house sizes
| Footprint with overhang | 4/12 pitch | 6/12 pitch | 8/12 pitch |
|---|---|---|---|
| 1,000 sq ft | 35 | 37 | 40 |
| 1,500 sq ft | 53 | 56 | 60 |
| 2,000 sq ft | 70 | 74 | 80 |
| 2,500 sq ft | 87 | 93 | 100 |
Pitch multipliers
| Pitch | Multiplier |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Good to know
- This works for simple gable and hip roofs. Valleys, dormers and turrets add waste; use 15 percent or more for complex roofs.
- Ridge caps, starter strips, underlayment and flashing are bought separately.
Sources and assumptions
Three bundles per square is the standard coverage of 3-tab and most architectural asphalt shingles; heavier designer shingles can take four or five, so check the wrapper.
Questions
What is a square in roofing?
100 square feet of roof surface. A 1,500 square foot roof is 15 squares.
How many bundles of shingles are in a square?
Three for most asphalt shingles. Some heavier designer shingles take 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 12 inches out from the roof surface and measure straight up to the roof.