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Roof Pitch & Rafter Calculator
Convert rise and run to pitch in degrees, X-in-12 and ratio, and get the common rafter length and slope factor for working out true roof area.
Pitch in 3 formatsRafter lengthSlope factorPDF reportFree to use
Roof geometry
Enter the vertical rise and horizontal run.
Horizontal distance from wall plate to ridge — half the span for a symmetrical gable.
Horizontal projection past the wall — added to the rafter along the slope.
Footprint area of the roof slope — multiplied by the slope factor for covering quantities.
Roof pitch — Conventional pitch
33.7°
8.0 in 12
1 : 1.50
1 : 1.50
Rafter (over run)
3.606 m
Rafter + overhang
3.966 m
Slope factor
1.202
Roof area
60.1 m²
Reading the result
The rafter length is gross geometry before plumb/seat cuts and ridge deduction. The slope factor converts the footprint of a slope to the real area on the slope — multiply your plan area by it for tile, slate or sheet quantities.Common pitch conversions
| Degrees | X-in-12 | Ratio | Slope factor |
|---|---|---|---|
| 15° | 3.2 in 12 | 1 : 3.73 | 1.035 |
| 22.5° | 5.0 in 12 | 1 : 2.41 | 1.082 |
| 30° | 6.9 in 12 | 1 : 1.73 | 1.155 |
| 35° | 8.4 in 12 | 1 : 1.43 | 1.221 |
| 40° | 10.1 in 12 | 1 : 1.19 | 1.305 |
| 45° | 12.0 in 12 | 1 : 1.00 | 1.414 |
| 60° | 20.8 in 12 | 1 : 0.58 | 2.000 |
Using this correctly
Frequently Asked Questions
Disclaimer: This calculator provides roof geometry for inspection, estimating and snagging. Rafter lengths are gross figures before cuts and deductions, and the tool does not perform structural design or member sizing. Always follow the covering manufacturer minimum pitch and fixing guidance (e.g. BS 5534) and use recognised span tables or a structural engineer for loaded timber sizing.
