Roof calculator
Mono-pitch, gable, hip, L and T roofs: rafters, tiles or sheets, membrane, battens, gutters
How the calculation works
Choose the roof shape (mono-pitch, gable, hip, L or T with valleys, or manual planes), enter wall dimensions, pitch and overhangs. Set rafter spacing and sections and the available timber lengths, the covering (tiles or profiled sheet) and the layers. The calculator derives planes, roof lines and every rafter length from the geometry, groups the same material before rounding and shows a plan and a section.
Formula and assumptions
True area = plan area ÷ cos α. Hip of a hip roof = W/2 × √(2 + tan² α). Rafter = plan length ÷ cos α. Tiles = area × pcs/m². Membrane rolls = ⌈area × (1 + reserve) ÷ ((width − side overlap) × (length − end overlap))⌉.
Worked example
Gable roof 10 × 8 m, pitch 30°, no overhang: 80 ÷ cos 30° = 92.38 m²; at 12 tiles/m² that is 1109 tiles, and each rafter is 4 ÷ cos 30° = 4.62 m. Hip roof 12 × 8 m, 35°: ridge 12 − 8 = 4 m, each hip 4 × √(2 + tan² 35°) = 6.31 m.
More worked examples
- Mono-pitch 6 × 4 m, 10°: rafters 4 ÷ cos 10° = 4.06 m.
- L-roof 10 and 8 m, width 6 m, 40°: ridges 7 + 5 m, hip and valley 4.93 m each.
Area factor by pitch (true area = plan × factor)
| Pitch (°) | Factor 1/cos α | Roof m² per 100 m² of plan |
|---|---|---|
| 15 | 1.035 | 103.5 |
| 22.5 | 1.082 | 108.2 |
| 30 | 1.155 | 115.5 |
| 35 | 1.221 | 122.1 |
| 45 | 1.414 | 141.4 |
What affects real consumption
- Pitch sets the true area and the rafter length.
- The roof shape sets the ridge, hips and valleys and the flashings.
- Batten gauge, effective sheet width and membrane overlaps set the piece counts.
Common mistakes
- Taking roof area from the plan without the pitch.
- Forgetting eave and verge overhangs.
- Rounding the same material separately for every roof part.
Frequently asked questions
Why is the roof area larger than the plan?
A sloping plane is longer than its projection: with equal pitch on all planes the true area is the plan area divided by the cosine of the pitch.
Does the calculator check the roof structure?
No. You set rafter sections and spacing; snow and wind capacity must be designed by a qualified engineer.