Sixteenth
Roofing Calculator
Enter the building footprint, the pitch and the overhang. You get the actual roof area, the number of squares, shingle bundles and 4×8 sheathing sheets.
Measure the footprint of the walls; the overhang is added on every side. Pitch as 6, 6/12 or 6:12.
How roof area is calculated
A sloped roof is bigger than the footprint it covers. Every square foot of plan becomes the slope factor's worth of roof, whether the roof is a gable or a hip, as long as all sides share one pitch.
plan = (length + 2 × overhang) × (width + 2 × overhang)
roof area = plan × √(144 + pitch²) ÷ 12 · squares = roof area ÷ 100
Slope factors
| Pitch | 3/12 | 4/12 | 5/12 | 6/12 | 7/12 | 8/12 | 9/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|---|---|---|---|
| Factor | 1.031 | 1.054 | 1.083 | 1.118 | 1.158 | 1.202 | 1.250 | 1.302 | 1.414 |
Questions
How do I calculate roof area from the footprint?
Add the overhang to each side of the footprint to get the plan area, then multiply by the slope factor √(12² + pitch²) ÷ 12. This works for gable and hip roofs with one pitch, because the slope stretches every part of the roof by the same amount.
What is a roofing square?
A square is 100 ft² of roof. Most architectural and 3-tab shingles come 3 bundles to the square; some heavy shingles need 4 or 5, so check the wrapper.
How much waste should I add for shingles?
About 10% for a simple gable roof and 15% or more for hips, valleys and dormers. Starter strip and ridge cap are extra.