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About this tool
Convert roof pitch between rise-in-12, degrees, percent and ratio, with slope factor, rafter length and material suitability.
Roof pitch is a single quantity — the tangent of the slope, rise divided by run — written four different ways, and this calculator converts between all of them: rise per 12 of run, degrees (atan of rise/run), percentage grade and 1-in-n ratio. It also returns the slope factor sqrt(1 + (rise/run)²) that turns plan area into roof area, the hip and valley factor sqrt(2 + (rise/run)²), and the rafter lengths for a given span. Useful when a drawing gives degrees, the roofer talks in twelfths and the sheet datasheet quotes a minimum in percent.
Open Roof Pitch Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Enter degrees, percent, ratio, rise-in-12 or a tape measurement and get the rest instantly.
The two multipliers a take-off actually needs, given to four decimals.
Flags the materials whose minimum pitch your slope fails to reach.
26.57 degrees, because the angle is the arctangent of 6 divided by 12, which is arctan(0.5). It is also a 50% grade, a 1-in-2 ratio, and it makes the roof surface 11.8% larger than the plan area it covers.
Divide the rise by the run and take the arctangent: angle = atan(rise / run). For a pitch written as X-in-12 that is atan(X / 12), so 4-in-12 is 18.43 degrees, 9-in-12 is 36.87 degrees and 12-in-12 is exactly 45 degrees.
Concrete interlocking tiles are generally laid at 4-in-12 (about 18.4 degrees) or steeper, and clay pantiles including Mangalore tiles want roughly 5-in-12 (about 22.5 degrees) upward. Below the manufacturer's stated minimum the tile relies on the underlay rather than the lap, which is where wind-driven monsoon rain gets in.
In everyday use they are the same thing, but in traditional carpentry 'pitch' sometimes means rise divided by the FULL span rather than by the half-span run — so a roof of rise 6 over a 24 ft span is a 1/4 pitch and a 6-in-12 slope. This calculator uses rise over run throughout, which is what modern drawings and datasheets mean.
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