How the calculator works
Shingles are sold by the square: 100 square feet of roof surface. The job is to find the true surface area of the roof, add a waste allowance, and divide by 100. Everything else (bundles, underlayment rolls, drip edge) follows from that number.
- Footprint. Measure the length and width of the roof from drip edge to drip edge, so overhangs are included. Multiply them to get the plan area.
- Slope factor. A sloped roof has more surface than the ground under it. The factor is the square root of 1 + (rise ÷ 12)². Multiply the plan area by it.
- Waste. Add a percentage for cuts, starter rows and mistakes. Owens Corning suggests 10% to 15% as a rule of thumb; cut-up roofs need more.
- Bundles. Multiply squares by the bundles per square printed on the wrapper (three for most laminate shingles) and round up.
For a hip roof with every plane at the same pitch, the same footprint-times-factor math gives the right area. For roofs with dormers, different pitches or L shapes, switch to each roof plane and enter the sloped length and width of every plane.
| Pitch | Angle | Multiply footprint by | 1,000 sq ft footprint becomes |
|---|---|---|---|
| 3/12 | 14° | 1.031 | 1,031 sq ft |
| 4/12 | 18° | 1.054 | 1,054 sq ft |
| 5/12 | 23° | 1.083 | 1,083 sq ft |
| 6/12 | 27° | 1.118 | 1,118 sq ft |
| 7/12 | 30° | 1.158 | 1,158 sq ft |
| 8/12 | 34° | 1.202 | 1,202 sq ft |
| 9/12 | 37° | 1.250 | 1,250 sq ft |
| 10/12 | 40° | 1.302 | 1,302 sq ft |
| 12/12 | 45° | 1.414 | 1,414 sq ft |
Worked example
A simple gable roof measures 40 by 30 feet including overhangs, with a 6/12 pitch.
- Plan area: 40 × 30 = 1,200 sq ft
- Slope factor at 6/12: √(1 + 0.5²) = 1.118, so roof surface = 1,342 sq ft (13.4 squares)
- With 10% waste: 1,476 sq ft, or 14.8 squares to order
- Shingles at 3 bundles per square: 14.76 × 3 = 44.3, rounded up to 45 bundles
- Synthetic underlayment at 10 squares per roll: 2 rolls
- Drip edge: two 40-foot eaves plus four rakes of 15 × 1.118 = 16.8 feet each, about 147 feet. With 10% extra and 10-foot pieces, that is 17 pieces.
- Ridge: 40 feet of ridge cap
Rules and product details to check
- Bundles per square vary. Owens Corning says laminate shingles generally come three bundles to a square; other shingle types can differ. The wrapper is the authority.
- Low slopes need more underlayment. Owens Corning's Duration instructions treat 2:12 to under 4:12 as low slope, with two layers of synthetic underlayment as one option. The calculator doubles the underlayment in that range.
- Ridge cap and drip edge come in different lengths and coverages. Enter the numbers from the product you plan to buy.
- Tear-off. If old layers come off, budget for disposal. Ask your local building department whether a re-roof needs a permit and whether a new layer can go over the old one.
What to do next
Use the material total to check roofer quotes, which also include labor, flashing, vents and disposal. A new roof is a large maintenance line, so put it in your home maintenance budget and review your insurance coverage. For other areas, the square footage calculator handles floors and lots. In New York City, see when you need a DOB permit.