What it works out
Enter the roof length and width as seen from above, plus the pitch. The plan area is scaled up by a slope factor to give the true roof area, then you get roofing squares, shingle bundles and sheathing sheets. Waste is added first and the bundles and sheets are rounded up once. It assumes one pitch on every plane and adds nothing for ridge caps, valleys or dormers.
You enter
- Pitch (inches per 12 of run)
- Length
- Width
The formula
- Factor
- √(1 + ratio²)
- Roof area
- plan area × factor
- Squares
- area ÷ 100 ft²
- Bundles
- area ÷ bundle coverage
- Sheets
- area ÷ sheet area
In words: Factor = the square root of 1 plus ratio squared; roof area = plan area times factor; squares = area divided by 100 square feet; bundles = area divided by bundle coverage; sheets = area divided by sheet area
Worked example
A 4 in 12 roof over a 20 ft by 30 ft footprint
- Pitch 4 in 12
- Length 20 ft
- Width 30 ft
- Plan area = 20 × 30 = 600 ft²
- Slope factor = √(1 + (4 ÷ 12)²) = 1.0541
- Roof area = 600 × 1.0541 = 632.5 ft²
- Roofing squares = 632.5 ÷ 100 = 6.32
Bundles and sheets are worked out from the area after waste, then rounded up once.
Good to know
- One pitch on every plane.
- Nothing is added for ridge caps, valleys or dormers.
- Bundle and sheet sizes come from your settings.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
Why is the roof area bigger than the footprint?
The slope adds surface. The steeper the roof, the larger the slope factor.
Is waste included?
Yes, you choose the allowance. It is added before the bundles are rounded up.