US roof pitch & slope calculator

US roof pitch and ADA slope guidelines

In American construction and roofing, slope is almost universally expressed as roof pitch: the number of inches of vertical rise per 12 inches of horizontal run (e.g. 4/12, 6/12, or 8/12 pitch).

Under the Americans with Disabilities Act (ADA) Standards for Accessible Design, the maximum allowable slope for an accessible ramp is 1:12 (an 8.33% grade or 4.76° angle), with a maximum rise of 30 inches per run.

Standard low-slope roofs have pitches between 2/12 and 4/12, while conventional steep-slope residential roofs have pitches of 4/12 to 9/12.

Worked example: 6/12 roof pitch

For a roof with 6 inches of rise per 12 inches of run:

Slope = 6 ÷ 12 = 0.50 (50.0% grade).

Angle of incline = arctan(0.50) ≈ 26.57°.

Rafter length multiplier = √(6² + 12²) / 12 = 1.118 (add 11.8% to horizontal run for rafter length).

Inputs for the US pitch and slope calculator