Roofing Calculator
Roof area equals the flat footprint multiplied by the pitch factor. A 6:12 pitch has a factor of 1.118. A 40 x 30 ft house with 1.5 ft overhang on a 6:12 roof gives about 15.0 squares and 45 bundles.
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Formula
- Adjusted length = Length + 2 x overhang
- Adjusted width = Width + 2 x overhang
- Flat area = Adjusted length x Adjusted width
- Roof area = Flat area x pitch multiplier
- Purchase area = Roof area x (1 + wastage%)
- Squares = Purchase area / 100
- Bundles = Squares x 3
Worked Example
| Input | Value |
|---|---|
| Building | 40 x 30 ft |
| Overhang | 1.5 ft each side |
| Flat area | 1,089 sq ft |
| Pitch | 6:12, factor 1.118 |
| Roof area | 1,217 sq ft |
| Wastage | 10% |
| Squares | 13.39 |
| Bundles | 41 |
Frequently Asked Questions
What is a roofing square?
One roofing square is 100 square feet of roof surface. Shingles are sold by the square, usually three bundles per square.
What is the pitch multiplier for a 6:12 roof?
1.118. Other common values are 4:12 = 1.054, 8:12 = 1.202, 10:12 = 1.302 and 12:12 = 1.414.
How do I measure roof pitch?
Pitch is the vertical rise over 12 inches of horizontal run. A roof rising 6 inches over 12 inches of run is a 6:12 pitch.
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Estimates only. Quantities follow recognised standards (ACI 318, CRSI, IRC 2021, IS-1077, PBQS); actual site consumption varies with workmanship, wastage and detailing. Confirm with a licensed engineer before ordering.