How roof pitch is written
In the US, roof pitch is the number of inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof (say "six in twelve") climbs 6 inches over each foot. On plans it's drawn as a small right triangle sitting on the roof line, with 12 on the level leg and the rise on the upright one, like the symbol in the top corner of the plate.
pitch = rise ÷ run × 12
angle = arctan(pitch ÷ 12)
slope % = pitch ÷ 12 × 100
multiplier = √(1 + (pitch ÷ 12)²)
Say you hold a level on a roof and measure 5 1/2″ straight down at the 12″ mark. That's a 5.5/12 pitch: 24.6°, a 45.8% slope, and a rafter multiplier of 1.100. Most roofs are framed to whole or half pitches, so if you measure something odd, measure again in another spot before you trust it.
How to measure pitch with a level and a tape
- Mark 12″ on a level. A 24″ level is best. Put a piece of tape at the 12″ mark.
- Put one end on the roof and hold it level. The end touches the roof on the uphill side; the level sticks out over the downhill side. Watch the bubble.
- Measure straight down at the mark. Keep the tape plumb from the 12″ mark to the roof surface. That number is your pitch.
- Check it twice. Shingle laps and sagging sheathing can throw you off by a quarter inch. Take a second reading a few feet away.
You can do the same thing from inside: hold the level against the bottom edge of a rafter in the attic. Rafter edges are straighter than a shingled surface, so the attic reading is often the better one.
Using the attic measurement
If you can get into the attic with a tape, measure the height from the top of the wall plate to the underside of the rafters at the ridge, and the building span outside wall to outside wall. Pitch is that height over half the span, scaled to 12. A 64″ rise over a 26′ building works out to 4.92/12, a hair under 5/12. Attic readings are rarely exact, so call it 5/12 and check with a level. Measure to the underside, because the depth of the rafter above that line would make the pitch read too high.
Roof pitch chart
Multiply the run by the multiplier to get the rafter 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 |
Pitch ranges and what they mean for the roof
Below 2/12, asphalt shingles aren't allowed under the IRC, and you're into membrane roofing. From 2/12 up to 4/12, shingles need a double layer of underlayment (IRC R905.2.2). Most houses fall between 4/12 and 9/12, which sheds water well. OSHA counts anything above 4/12 as a steep roof, so plan on a harness, and add roof jacks as the pitch climbs.
Got the pitch? The rafter calculator takes it from here: rafter length, plumb and seat cuts, and the birdsmouth.