Roof Slope / Rise-Run Calculator
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Enter the rise and the run and get the roof angle, the slope percentage, the pitch (in 12) and the rafter length — everything you need to cut rafters and order sheeting.
Roof Slope / Rise-Run Calculator
Angle, slope %, pitch (in 12) and rafter length from the rise and run
Roof section
18.4° · 4 in 12
The run is the horizontal distance from the wall to the ridge — usually half the building width for a symmetrical gable.
Roof angle
18.4°
slope 33.3% · pitch 4 in 12 · rafter 3.16 m
Angle
18.4°
Slope
33.3%
Pitch
4 in 12
Rafter length
3.16 m
Common pitches
2 in 12
9.5°
17% slope
4 in 12
18.4°
33% slope
6 in 12
26.6°
50% slope
12 in 12
45°
100% slope
Sloped roof area is 1.054× the plan area — order sheets, tiles and waterproofing against the rafter length, not the flat span. A 4-in-12 pitch (~18°) is a common low slope; steeper sheds water and snow faster but costs more.
How it works
- Angle = arctan(rise ÷ run)
- Slope % = rise ÷ run × 100
- Pitch = rise-in-12 (rise ÷ run × 12)
- Rafter length = √(run² + rise²)
- Sloped area = rafter ÷ run × plan area — the roof surface is always larger than the plan.
Worked example
A roof with a 3 m run and 1 m rise:
- Angle = arctan(1 ÷ 3) = 18.4°
- Slope = 33.3%, pitch = 4 in 12
- Rafter = √(3² + 1²) = 3.16 m
Common pitches
| Pitch | Angle | Use |
|---|---|---|
| 2 in 12 | 9.5° | Low slope, sheds only |
| 4 in 12 | 18.4° | Common shallow roof |
| 6 in 12 | 26.6° | Standard sloped roof |
| 12 in 12 | 45° | Steep / heavy rainfall |
Frequently asked questions
How do I calculate roof slope? Divide the rise by the run. As a percentage multiply by 100; as an angle take the arctangent; as a pitch multiply by 12. A 1 m rise over a 3 m run is 33.3%, 18.4°, or 4 in 12.
What is the rafter length for a 3 m run and 1 m rise? √(3² + 1²) = √10 = 3.16 m — the sloping length from wall to ridge.
Why is the roof area more than the floor area? Because the roof slopes, its surface is longer than the flat span — for a 4-in-12 roof, about 1.05× the plan area. Order sheets and waterproofing against the rafter length.
What is a good roof slope for heavy rain? A steeper pitch (6 in 12 / 26° or more) sheds water fast and is common in high-rainfall areas; low slopes need better sealing at the laps.
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CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.