Roof pitch is written as rise over run. A 6/12 roof climbs 6 inches for every 12 inches it travels sideways. The 12 never changes, so people shorten it and talk about a "six pitch."
Roof pitch chart
The multiplier column is the useful one on site. Multiply the run of a rafter by it to get the length along the slope.
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 1/12 | 4.8° | 8.3% | 1.003 |
| 2/12 | 9.5° | 16.7% | 1.014 |
| 3/12 | 14.0° | 25.0% | 1.031 |
| 4/12 | 18.4° | 33.3% | 1.054 |
| 5/12 | 22.6° | 41.7% | 1.083 |
| 6/12 | 26.6° | 50.0% | 1.118 |
| 7/12 | 30.3° | 58.3% | 1.158 |
| 8/12 | 33.7° | 66.7% | 1.202 |
| 9/12 | 36.9° | 75.0% | 1.250 |
| 10/12 | 39.8° | 83.3% | 1.302 |
| 11/12 | 42.5° | 91.7% | 1.357 |
| 12/12 | 45.0° | 100.0% | 1.414 |
| 13/12 | 47.3° | 108.3% | 1.474 |
| 14/12 | 49.4° | 116.7% | 1.537 |
| 15/12 | 51.3° | 125.0% | 1.601 |
| 16/12 | 53.1° | 133.3% | 1.667 |
| 17/12 | 54.8° | 141.7% | 1.734 |
| 18/12 | 56.3° | 150.0% | 1.803 |
| 19/12 | 57.7° | 158.3% | 1.873 |
| 20/12 | 59.0° | 166.7% | 1.944 |
| 21/12 | 60.3° | 175.0% | 2.016 |
| 22/12 | 61.4° | 183.3% | 2.088 |
| 23/12 | 62.4° | 191.7% | 2.162 |
| 24/12 | 63.4° | 200.0% | 2.236 |
Using the multiplier
Say a garage has a 12-foot run from the wall to the ridge and a 6/12 pitch. The multiplier is 1.118, so the rafter is 12 × 1.118 = 13.42 feet along the slope, or about 13′ 5″. That's the distance from the wall to the ridge. It leaves out the overhang and the thickness of the ridge board, and those change what you cut. The rafter calculator takes both into account and draws the plumb and seat cuts.
The same number works on area. A roof over a 1,500 square foot footprint at 6/12 has about 1,500 × 1.118 = 1,677 square feet of surface. Roofers sell by the square, which is 100 square feet, so that's about 17 squares before you add for waste, valleys and ridge caps.
What each range means for the roof
The limits most homeowners run into are the ones on asphalt shingles. Below 2/12 they aren't allowed under IRC R905.2.2, so you're looking at a membrane or another low-slope system. From 2/12 up to 4/12 they need a double layer of underlayment.
From 4/12 to 9/12 is where most houses sit, and water runs off well. OSHA calls anything above 4/12 a steep roof (29 CFR 1926.500), so don't treat that range as safe to walk. Dew or loose granules can take your feet out on a much lower roof. Wear a harness, add roof jacks as the pitch climbs, and expect a roofer to charge more for the work.
Finding your own pitch
You need a level and a tape. Hold the level flat with one end on the roof, measure 12″ out along it, and measure straight down to the shingles. The number you read is the rise, so 5 1/2″ means a 5.5/12 pitch. A phone level laid on a rafter gives you the angle instead, and you can find the nearest pitch on the chart above.
Take the reading on a rafter in the attic if you can get up there. Rafter edges are straighter than shingles. If you want to check how you took it, the roof pitch calculator shows the measurement drawn out.