How it's calculated
Roofs are measured in squares. Owens Corning defines a roofing square as 100 square feet of roof, and works its own example by adding up each roof plane, dividing by 100 and multiplying by the bundles in a square Owens Corning. The calculator does the same, starting either from a measured roof area or from the house footprint and roof pitch.
From the footprint, the sloped area is the footprint times a pitch factor. A roof that rises 6 in for every 12 in of horizontal run (a 6/12 pitch) has rafters √(6² + 12²) ÷ 12 times as long as the run, so its surface is that much larger than the ground it covers. This works for gable and hip roofs where every plane has the same pitch, as long as the footprint includes the overhangs.
Shingles come in bundles. Owens Corning's data sheet for TruDefinition Duration laminate shingles lists 3 bundles per square with a coverage per square of 98.4 sq ft Owens Corning, and its guide specification lists heavier lines at 4 bundles per square, also covering 98.4 sq ft Owens Corning. The calculator counts bundles from that 98.4 sq ft coverage, which gives slightly more than dividing by an even 100.
Owens Corning advises adding 10–15% to all material totals as a trim allowance (waste factor), more for complicated roofs; the default is 10%. Underlayment is estimated from the same squares: synthetic underlayment comes in 10-square rolls and #15 felt in 4-square rolls, according to the same Owens Corning page.
The formula
- Pitch factor = √(rise² + 12²) ÷ 12, with the rise in inches per 12 in of run.
- Roof area = footprint length × width × pitch factor (or the roof area you measured).
- Squares = roof area ÷ 100.
- Bundles = roof area × (1 + waste %) ÷ (98.4 sq ft ÷ bundles per square), rounded up.
- Underlayment rolls = squares × (1 + waste %) ÷ squares per roll (10 synthetic, 4 felt), rounded up.
- Metric: meters and square meters are converted with NIST's exact factors (1 ft = 0.3048 m; 1 sq ft = 0.09290304 m²) NIST.
Worked example
Owens Corning's example gable roof has two 40 × 30 ft planes. Entered as a roof area with no waste, that is 24 squares, matching their figure, and 6 rolls of #15 felt. Owens Corning multiplies 24 squares by 3 to get 72 bundles; counting from the data sheet's 98.4 sq ft per square, the calculator gives 74. With their suggested 10% waste it becomes 81 bundles.
Now a house with a 50 × 30 ft footprint including overhangs and a 6/12 pitch:
| Line | Amount |
|---|---|
| 57 bundles × $40.00 | $2,280 |
That roof is 1,677 sq ft, or 16.77 squares. It takes 57 bundles and 2 rolls of synthetic underlayment.
Assumptions and limitations
- One pitch. Footprint mode assumes every plane shares the same pitch. For mixed pitches, dormers or complex roofs, measure each plane and use roof area mode.
- Field shingles only. Starter strips, hip and ridge caps, drip edge, ice and water barrier, vents and flashing are separate products.
- Waste depends on the roof. Hips, valleys, dormers and short runs mean more cut shingles; Owens Corning's 10–15% is a rule of thumb.
- Check the wrapper. Bundles per square and coverage vary by product; use the numbers on the package you buy.
- Safety. Measure from the ground or plans where you can. Steep roofs are dangerous to walk.
- Prices are examples. Enter your own.
For the area of individual planes, the square footage calculator adds up rectangles and triangles.