🏠 Roof Pitch & Rafter Calculator

Roof pitch is the rise in inches for every 12" of run. Enter the pitch and half-span (run), and get the rafter length, angle and roof-area multiplier.

How the maths works

The rafter is the hypotenuse of a right triangle: rafter = run × √(1 + (pitch/12)²). The same √ factor is the roof-area multiplier — multiply your floor area by it to order roofing material for the sloping surface. A 12/12 pitch is 45°; most UK roofs sit between 25° and 45°.