Calculate the roof area in m² based on building dimensions, roof type and slope angle — with eave overhang and a margin for the roofing material.
| Roof type | Formula |
|---|---|
| Shed | rafter = width/cos(angle); area = rafter × length |
| Gable | rafter = (width/2)/cos(angle); area = 2 × rafter × length |
| Hip | rafter = (width/2)/cos(angle); ridge = length − width; area = rafter × (2×ridge + width) |
| Flat | area = width × length |
Dimensions (width/length) are increased in the calculation by the eave overhang on both sides. The hip formula applies when all four slopes have the same pitch and building length > width.
For residential homes, a typical slope is 30–45°. A shallower slope (15–25°) is used for utility buildings, a steeper one (over 45°) — for attic/loft roofs.
Rafter length increases as the angle increases (dividing by cos), so the roofing area grows faster than the building's floor area.
10–15% is standard — covers ridge, valley and hip joints. For complex-shaped roofs (many slopes, windows) — up to 20%.
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