Roof pitch from rise & run
Two measurements in, a pitch angle out — no trig required.
Rise & run
Vertical height gained over the run.
Horizontal distance the rise is measured over.
Roof pitch
— a ratio of 0.40 : 1 (rise : run).
Site measurement, not a design figure. Measure rise and run over the same clean section of roof — check against the actual truss or rafter spec before ordering materials.
Rise and run turn up everywhere on a roof — a rafter spec, a Velux installation guide, a planning drawing — but they're rarely given as the pitch angle you actually need to talk about the job. This tool does the trig for you: put in how much height is gained (the rise) over a given horizontal distance (the run), and get the pitch in degrees straight back, along with the simplified ratio.
Why rise and run, not the pitch itself
Pitch is awkward to measure directly on a sloped surface with a tape measure — a spirit level and a tape gives you rise and run cleanly, which is why most on-site measurements start there. Rest a level along the slope, hold it level, and measure straight down to the roof surface at a known distance along the level: that vertical drop is the rise, and the distance along the level is the run. The conversion from rise/run to degrees is simple trigonometry (the arctangent of rise divided by run), but doing it in your head on a roof isn't something anyone wants to be doing, and it's easy to mistype into a phone calculator halfway up a ladder.
The ratio this tool also gives you — rise-to-run, simplified down to a single decimal against 1 — is useful in its own right, since it's how a lot of older drawings and some roofing suppliers still specify pitch, rather than in degrees.
A worked example
A rafter that rises 400mm over a 1000mm run works out to a pitch of 21.8°, or a ratio of 0.4:1. Steepen that to a 600mm rise over the same 1000mm run and the pitch jumps to 31°. Small changes in rise make a bigger difference to the angle than they might look like on paper — which is exactly why it's worth checking the actual figure rather than eyeballing it.
What this isn't
This assumes rise and run are measured accurately over the same section and along the same line — a measurement taken across an uneven or sagging section of roof will throw the result off. It doesn't account for roof coverings that have their own minimum pitch requirements (some tiles and membranes have a stated minimum), so check the manufacturer's spec once you've got the actual pitch.
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