The formula
pitch % = (rise ÷ run) × 100, and the degrees are the arctangent of that ratio
Where it comes from
Percentage and degrees get confused constantly, and they are neither the same nor close. A roof at 100 % is not vertical: it is 45 degrees. The percentage is the tangent of the angle times a hundred, that is, how much it rises for every hundred of horizontal run, and it can comfortably go past 100. Degrees, by contrast, never reach 90 on a real roof. A 30 % pitch — ordinary on a house — is about 16.7 degrees.
How to work it out by hand
- Measure how far the roof rises from eaves to ridge
- Measure how far it runs horizontally over that same stretch
- Divide the rise by the run
- Multiply by a hundred for the percentage, or take the arctangent for the degrees
What is worth knowing
Pitch decides what covering can go on, and it is a question of keeping water out rather than of style: barrel tile wants at least a 26 % pitch, pantile comes down to 20 %, and felt or profiled sheet will take far less. Below about 5 % it stops being a roof and becomes a flat roof, with continuous waterproofing and outlets. The other figure that comes out of this is the rafter factor, which is what you multiply the plan area by to get the true roof area: at a 30 % pitch, every hundred square metres on plan is a hundred and four of roof, and those four missing metres are tiles nobody ordered.