Place a 12-inch level horizontally against a rafter, then measure the vertical distance from the level's free end down to the rafter underside. That measurement in inches is your rise over a 12-inch run, expressed as X/12.

Roofs up to 6/12 (26.6°) are generally considered walkable for most workers. From 7/12 to 9/12 (30.3°-36.9°) requires extra caution and roof jacks. Anything 10/12 (39.8°) or steeper is classified as a steep-slope roof and typically requires fall arrest harnesses under OSHA rules.

The International Residential Code allows asphalt shingles down to 2/12 (about 9.5°) with a double underlayment, and standard installation from 4/12 upward. Metal panel and membrane systems can go lower; always check the manufacturer's listed minimum for your specific product.

Steeper pitches shed water and debris faster, reducing the time moisture sits on the surface and seeps under fasteners. Low slopes below 3/12 need sealed underlayments because capillary action and wind-driven rain can push water uphill under shingle laps.

Pitch ratio (X/12) is rise per 12 units of run. Angle is the actual degree measurement: arctan(rise/run). Percent slope is (rise/run) × 100. For example, 6/12 = 26.57° = 50% slope. Roofers use the ratio; engineers often use degrees or percent.

ASCE 7 allows a slope reduction factor (Cs) for snow on roofs steeper than 30° on warm roofs, because snow slides off rather than accumulating. Below about 4/12, the full ground snow load typically applies. Steep pitches can still see drifting on the leeward side and at valleys.

Yes. 4/12 = 18.43°, 5/12 = 22.62°, 6/12 = 26.57°, 7/12 = 30.26°, 8/12 = 33.69°, 9/12 = 36.87°, 10/12 = 39.81°, 12/12 = 45°. The slope factor (rafter length per foot of run) ranges from 1.054 at 4/12 to 1.414 at 12/12.