Roof Pitch Calculator UK
Measure rise (vertical height) and run (horizontal distance) in consistent units to get pitch in degrees, ratio (x:12), percentage, and rafter length.
Results
Angle: °
Ratio (x:12):
Percentage: %
Rafter Length: mm
How to Measure
- Locate roof edge and peak from attic or ladder.
- Run equals half building width from wall to ridge centre.
- Rise is vertical distance from run end to peak height.
- Use spirit level for 12-inch run checks on site.
- Convert inches to mm if mixing units: 1 inch = 25.4 mm.
UK Roof Categories
| Type | Angle Range | Ratio (x:12) | Notes |
|---|---|---|---|
| Flat | 2°-10° | 0.5:12-2:12 | Water runoff essential, EPDM common. |
| Low | 10°-20° | 2:12-4:12 | Special sealants needed. |
| Standard | 20°-40° | 4:12-9:12 | Most homes, slate tiles suit. |
| Steep | 40°+° | 9:12+ | Wind bracing required. |
Frequently Asked Questions
- What pitch suits UK weather? 35°-45° balances rain shed and wind resistance for most regions.
- Minimum for snow areas? 30° (6:12) helps prevent buildup in northern UK.
- Can low pitches use tiles? Below 17.5° (3:12), switch to metal or membranes.
- How does pitch affect costs? Steeper needs more materials; expect 20-30% extra over flat.
- Walkable pitches? Up to 30° (7:12) safe with care; steeper needs harnesses.
Pitch vs Degrees
| Ratio | Degrees | Percent | UK Example |
|---|---|---|---|
| 3:12 | 14° | 25% | Modern bungalow. |
| 6:12 | 27° | 50% | Typical semi-detached. |
| 9:12 | 37° | 75% | Victorian terrace. |
| 12:12 | 45° | 100% | Steep church style. |
Common Measurement Errors
- Mixing run and full span doubles horizontal value.
- Ignoring overhangs shortens rafter estimates.
- Uneven levels skew rise by 10-20% on long runs.
- Forgetting birdsmouth cuts adds 50-100mm error.
- Windy days shift ladder readings on external checks.
References
BS 5534:2014+A2:2018. Slating and tiling for pitched roofs and claddings. British Standards Institution.
NHBC Standards 2025 Chapter 7.3. Pitched roofs. National House Building Council.
Building Regulations Approved Document F. Ventilation. HM Government.