Footprint and pitch
Pitch
Waste factor
Default 10% covers hips, valleys, starter, and a ridge cap pad on a simple rectangle. 15% for a cut-up roof. 0% is raw surface only. Not a contractor quote.
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- Order
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- Bundles rule of thumb
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- After waste
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- Before waste
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Roof surface — · footprint — · factor —
Enter length, width, and pitch.
This is an estimate, not a contractor quote. It does not count ice-and-water, underlayment rolls, drip edge, ridge vent, or how a crew starts a valley. 3 bundles per square is a common asphalt rule of thumb, not a brand spec. Confirm with the yard. Nothing on this page leaves the browser.
How the squares are counted
Footprint is length × width in feet. That is how pitch stretches footprint: multiply by
√(1 + (rise/run)²). Divide roof surface by 100 —
one square is 100 sq ft of roof surface, not the plan.
Multiply by (1 + waste), then round up. You cannot buy a fraction of a square.
factor = sqrt(1 + (rise/run)^2) roof_sqft = length_ft × width_ft × factor squares_before = roof_sqft / 100 squares_after = squares_before × (1 + waste) order = ceil(squares_after) bundles ≈ order × 3 (asphalt rule of thumb)
Example: 40 ft × 30 ft at 6/12. Factor ≈ 1.118. Roof surface ≈ 1,342 sq ft → 13.42 squares before waste. 10% waste → ~14.76 → 15 squares. Rule of thumb: 45 bundles.
FAQ
What is a roofing square?
A roofing square is 100 sq ft of roof surface, not footprint. Pitch stretches footprint: the shingles cover the sloped face, not the plan of the house. Stores sell squares (and bundles), not a raw length × width.
How does pitch change the square count?
Area factor = √(1 + (rise/run)²). Pitch stretches footprint by that multiplier. Presets: 4/12 ≈ 1.054, 6/12 ≈ 1.118, 8/12 ≈ 1.202, 12/12 (steep) ≈ 1.414. Flat 0/12 is 1.0 — same as the footprint.
How many squares for a 40×30 roof at 6/12?
40 ft × 30 ft footprint is 1,200 sq ft. 6/12 pitch, factor ≈ 1.118, roof surface ≈ 1,342 sq ft. Divide by 100: 13.42 squares before waste. With 10% waste → ~14.76. Ceil: order 15 squares. That is roof surface, not the 12-square footprint.
Why 10% waste?
10% is a common pad for hips, valleys, starter, and ridges on a simple rectangle. Use 15% on a cut-up roof. 0% is the raw surface only. This is not a contractor quote and does not count ice-and-water or underlayment rolls.
How many bundles per square?
3 bundles per square is a common asphalt rule of thumb, not a brand spec. Architectural and three-tab packs can differ — read the wrapper. Fixture: 15 squares → 45 bundles. The page labels it as a rule of thumb.
Is this a contractor quote or an underlayment takeoff?
No. Quantity math only: footprint × pitch factor, divide by 100 sq ft, add waste, round up. Not a contractor quote. Not ice-and-water. Not underlayment rolls. A floor is footprint; a roof is not — see the flooring calculator for rooms. Confirm with the yard. Whoever roofs the house has the last word.