From footprint to roofing squares
Plan area = (length + 2 × overhang) × (width + 2 × overhang). The sloped area is larger than the plan by the slope factor √(1 + (rise ÷ 12)²) for a pitch of rise/12, which equals 1 ÷ cos(angle). Roof area = plan area × slope factor.
Roofing is sold by the square, which is 100 ft² of roof. Squares to order = roof area ÷ 100 × (1 + waste %). Bundles = squares × bundles per square, rounded up.
| Pitch | Angle | Slope factor |
|---|---|---|
| 3/12 | 14.04° | 1.0308 |
| 4/12 | 18.43° | 1.0541 |
| 6/12 | 26.57° | 1.1180 |
| 8/12 | 33.69° | 1.2019 |
| 12/12 | 45.00° | 1.4142 |
Worked example
A 40 × 30 ft house with a 1 ft overhang all round has a plan of 42 × 32 = 1,344 ft². At a 6/12 pitch the slope factor is √1.25 = 1.1180, so the roof is 1,344 × 1.1180 = 1,502.6 ft², or 15.03 squares.
Adding 10% waste gives 16.53 squares. At 3 bundles per square that is 49.6 bundles, rounded up to 50.
Assumptions: gable, hip and waste
When every roof plane has the same pitch, the sloped area is plan area × slope factor whatever the shape, so a gable and a hip roof over the same footprint and overhang have the same area. Hips, valleys and dormers mainly add cutting waste and need hip and ridge caps.
The 3 bundles per square default suits many 3-tab and architectural shingles, but some products differ, so check the wrapper and adjust the field. Waste of about 10% suits simple gables; 15% or more is common for cut-up roofs. Starter strip, ridge caps, underlayment, drip edge and flashing are not included. Metric inputs are converted with 1 ft = 0.3048 m.