Roof pitch calculator
Roof pitch is quoted three ways on the same project: x-in-12 on US drawings, degrees on metric sections, and a percentage on falls. All three come from rise over run. The slope factor below is what you multiply plan area by to get true roof area for material take-off.
roof pitch calculator
- Pitch
- 26.57°
- US ratio
- 6 in 12
- Rafter length
- 4,472mm
- True roof area
- 134.16 m² (slope factor 1.118)
Formula
pitch = atan(rise ÷ run) · rafter = √(rise² + run²) · slope factor = rafter ÷ runRun is the horizontal half-span, not the full span. Multiply plan area by the slope factor for tiles, membrane and underlay quantities.
Worked examples
| Input | Result |
|---|---|
| 2000 mm rise over 4000 mm run | 26.57° — 6 in 12 |
| 1000 over 4000 | 14.04° — 3 in 12, minimum for most tiles |
| 120 m² plan at 6 in 12 | 134.16 m² true roof area |
Questions
What does 6/12 pitch mean in degrees?
26.57°. The ratio is rise per 12 units of horizontal run, so 6/12 is atan(6/12).
Why is roof area larger than plan area?
A sloped surface is longer than its horizontal projection. Multiply plan area by the slope factor — 1.118 at 6 in 12 — or you will under-order roofing by more than 10%.