VolunaroRoofing calculator
How the calculation works
- Footprint: we add up every shape you enter and subtract the ones marked as cut-outs. Measure the flat area the roof covers from above, eaves included.
- Roof surface: the footprint times a slope factor. The pitch is the rise in inches for every 12 in of run, and the factor is the square root of 1 plus (pitch ÷ 12) squared. A 6/12 roof has a factor of 1.118.
- Surface to order: the roof surface plus the waste margin for cuts, eaves and valleys.
- Bundles: surface to order divided by one square (100 sq ft), times the bundles per square, rounded up because you cannot buy part of a bundle.
Worked example
A 40 × 30 ft house footprint with a 6/12 roof pitch, the default 10 % margin and 3 bundles per square.
- Roof surface
- 1,341.6 sq ft
- Surface to order (with margin)
- 1,475.8 sq ft
- Bundles of shingles
- 45
A 24 × 20 ft outbuilding with a 4/12 roof pitch and the default margin.
- Roof surface
- 506.0 sq ft
- Surface to order (with margin)
- 556.6 sq ft
- Bundles of shingles
- 17
Default values and sources
| Value | Default | Type | Sources |
|---|---|---|---|
| Waste margin for roofing | 10 % | Our default (editable) | — |
| Bundles of asphalt shingles per square (100 sq ft) | 3 | Typical published value | Owens CorningGAF |
Limitations
- This is an estimate for a simple pitched roof measured from its footprint. It does not draw hips, valleys, dormers or skylights. A roof with many of them needs a larger margin or a measurement of each plane.
- Starter strips, hip and ridge caps, underlayment, flashing and nails are not included. Ask your supplier how they are packaged for your shingles.
- Bundles per square depend on the product: many asphalt shingles come three to a square, but some heavier or specialty shingles use more. Use the number on your package.
- The pitch is rise per 12 in of run and must be at least 1. Flat roofs are not covered. Check your local code and the shingle manufacturer's instructions for minimum slopes.
Results are estimates for planning and ordering, not professional, engineering or legal advice. Confirm quantities, sizes and local building requirements with your supplier or contractor before you buy or build.