Enter any two of rise, run or rafter length — we work out the pitch in degrees, the gradient ratio and the missing side.
| ° | rise/run |
|---|---|
| 15° | 3:12 |
| 20° | 4:12 |
| 25° | 6:12 |
| 30° | 7:12 |
| 35° | 8:12 |
| 40° | 10:12 |
| 45° | 12:12 |
| 60° | 21:12 |
Common pitches: 15° low, 30° standard, 45° steep, 60° very steep. Run = half the building width for a symmetric roof.
Pitch in X-in-12 = tan(angle) × 12. So 30° ≈ 6.9-in-12, 45° = 12-in-12. The calculator does both — enter what you have, read the other.
Most interlocking concrete tiles need 22.5° (about 5-in-12). Slate needs 20°, thatch 45°+. Below the manufacturer minimum, wind-driven rain gets under the laps.
Rafter length = √(run² + rise²). For a 4 m run at 30° pitch, rise = 2.31 m, so rafter = √(16 + 5.34) = 4.62 m. Add 300–400 mm for the eaves overhang.
For a 4 m clear span at 400 mm centres, C24 timber rafters need to be 47 × 175 mm minimum per Eurocode 5 span tables. Wider centres or longer spans push you to 47 × 200 mm.
Estimates only. Check the paint tin / adhesive bag for the exact coverage of your product.