Roof Area Calculator
True sloped roof area, rafter length and ridge rise from pitch.
Inputs
Result
Formula
| Projected area | A = (L + 2e) × (W + 2e) |
| Slope factor | k = 1 ÷ cos(θ) |
| True roof area | A = projected × k |
| Half span | b = width ÷ 2 for a gable |
| Rafter length | L = b ÷ cos(θ) |
| Ridge rise | h = b × tan(θ) |
A 12 × 9 m plan at 30° with 0.5 m overhangs: 13 × 10 = 130.00 m² projected, slope factor 1.1547, true area 150.11 m², rafters 5.77 m and a 2.89 m ridge rise.
FAQ
Why is roof area larger than floor area?
A pitched roof is a slope, so the same plan projection develops into a larger surface — by exactly 1/cos(pitch). Eaves, parapet inner faces, gutters and upstands add more. At 30° with 0.5 m overhangs, roof area typically runs 30-40% above floor area, so estimating tiles from floor area loses money.
What pitch keeps a roof watertight?
It depends on the covering: plain tiles 30° minimum, shingles 15°, profiled metal 5°, membrane flat roofs 2% with gutters at 1%. Too shallow cannot be fixed by adding layers — water sits under the tiles and rots the battens first. Above 45° add mechanical fixings or the tiles slide.
Is the rafter length the same as the sloped length?
Close but not equal. Rafters also need the eaves overhang, the ridge lap and cutting allowance, so cut lengths run 300-500 mm longer. Timber also depends on purlin positions; for steel, divide the sloped length by the purlin spacing and round before working back, otherwise the last sheet needs cutting on site.
Related calculators
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.