How roof area is calculated from the ground
A sloped roof has more surface than the footprint beneath it. The steeper the pitch, the bigger the difference.
Pitch factor = √(1 + (rise ÷ 12)²)
Roof area = footprint × pitch factor
Squares = roof area × (1 + waste %) ÷ 100
Bundles = squares × bundles per square, rounded upWorked example: a 40 × 30 ft footprint at 6/12 pitch
- Footprint: 40 × 30 = 1,200 sq ft.
- Pitch factor for 6/12: √(1 + 0.25) = 1.118.
- Roof area: 1,200 × 1.118 = 1,341.6 sq ft.
- With 10% waste: 1,475.8 sq ft, or 14.76 squares.
- At 3 bundles per square: 44.3, so buy 45 bundles.
- Synthetic underlayment at 1,000 sq ft per roll: buy 2 rolls.
Pitch factors
| Pitch | Multiply footprint by |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Choosing a waste allowance
A simple gable roof generally needs about 10%. Hip roofs need more, usually around 15%, because each hip needs angled cuts. Valleys, dormers and other complex shapes can take it to 20% or beyond. Published figures differ, so if you have a roofer's measured takeoff, use its waste figure.
What this estimate leaves out
- Starter strip, ridge caps, drip edge, flashing, vents and nails are extra.
- Ice and water shield may be required at the eaves and valleys in cold climates.
- Tear-off, disposal and replacing damaged decking are additional costs.
- Different roof sections with different pitches should each be entered separately.
Measure your footprint from the ground or from a plan where you can. Roof work is dangerous, and this estimate doesn't replace a roofer's measurement or the shingle manufacturer's installation instructions. Local codes may also limit how many layers you can install and which underlayment you can use.