Build & renovate
Roof Pitch Calculator
One right triangle answers everything: the pitch in every notation and the rafter length along the slope.
Calculator
Fill in the fields above to see results.
How it works
Pitch = atan(rise / run); US notation expresses the same as inches of rise per 12 of run. Rafter length = sqrt(rise2 + run2) - add the eaves overhang and birdsmouth allowance when ordering.
Frequently asked questions
What is a 30 degree roof in x:12?
tan(30) x 12 = 6.9:12 - call it 7:12. Common UK tiled roofs run 30-45 degrees; low-pitch tiles and membranes go shallower with the right spec.
What roof pitch is walkable?
Up to about 6:12 (26 degrees) most trades will walk with care; beyond that it is boards, roof ladders and scaffold - plan access, not bravery.