Pitch is rise over run
In the US, roof pitch is written as how many inches the roof rises for every 12 inches it runs horizontally. A roof that rises 6 inches in 12 is a 6/12 pitch (said "6 in 12").
To measure it, hold a level horizontally against the roof or a rafter, mark 12 inches along the level, and measure straight up from that mark to the roof surface.
Converting pitch to an angle or percentage
- Angle: arctan(rise ÷ 12). A 6/12 pitch is arctan(0.5) = 26.6°.
- Percentage: rise ÷ run × 100. A 6/12 pitch is 50%.
Roof area from the footprint
A sloped roof has more area than the ground it covers. Multiply the footprint by the pitch factor, √(12² + rise²) ÷ 12. For 6/12 that is √180 ÷ 12 = 1.118, so a 1,500 sq ft footprint has about 1,677 sq ft of roof. Measure the footprint including the overhangs, and add extra for waste when buying shingles.
Rafter length
The same factor gives the rafter length: horizontal run × 1.118 for a 6/12 roof. A rafter spanning 12 ft horizontally is about 13.4 ft long, before the overhang and the cuts at each end.
The Roof Pitch Calculator converts between pitch, angle and percentage and works out the roof area and rafter length.