Roof Area Calculator
Works out the real surface area of a roof from the building footprint and the pitch. Gives square metres, square feet and roofing squares.
Includes the waste allowance.
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The formula
footprint = (length + 2 × overhang) × (width + 2 × overhang) roof area = footprint × slope factor × (1 + waste)
A roof covers its footprint, but its surface is larger because it is tilted. The slope factor is the ratio between the two, and it depends only on the pitch — not on whether the roof is hipped, gabled or has dormers.
What to watch out for
The slope factor is the whole trick. A roof's footprint — the area it covers on the ground — is always smaller than its actual surface. Multiply by √(1 + (rise ÷ run)²) and you get the real area to cover.
At 6/12 that factor is 1.118, so a 100 m² footprint is 112 m² of roof. At 12/12 it is 1.414, and the difference is 41%. Ordering against the footprint is the most common way to run short of materials on a roof.
Hip and gable roofs over the same footprint have the same area. What differs is waste: a hip roof has four slopes meeting at angles, so more of every sheet or bundle ends up as an offcut. That is why the waste allowance changes and the area does not.
Dormers, valleys and chimneys all add work and waste but little area. Measure the main planes and let the waste percentage carry the rest.
Frequently Asked Questions
How do I calculate roof area?
Multiply the footprint including overhangs by the slope factor for your pitch. A 10 × 8 m building with 300 mm eaves and a 6/12 pitch has a 91.2 m² footprint and about 101.9 m² of roof.
How many squares is my roof?
A roofing square is 100 square feet, or 9.29 m². Divide the roof area by that — a 150 m² roof is about 16 squares.
Is a hip roof bigger than a gable roof?
No, not over the same footprint and pitch — the area is identical. A hip roof wastes more material because of the angled cuts, so allow 15% rather than 10%.