Roof pitch describes how steep a roof is: the number of inches the roof rises for every 12 inches of horizontal run. A roof that climbs 6” over 12” of run is a 6/12 (“six-in-twelve”) roof, also written 6:12.
Pitch vs. slope vs. angle
In everyday US construction, pitch and slope are used interchangeably for “rise per 12 inches of run,” and that is how building codes and shingle manufacturers express it (for example, “2:12 minimum”).
Traditional carpentry texts define pitch differently — as total rise divided by the full span — so a 6/12 slope was called a “1/4 pitch” and a 12/12 slope a “1/2 pitch.” You will still see that wording on old plans, so check which one is meant.
The angle in degrees is the arctangent of rise ÷ run:
| Pitch | Angle | Rafter multiplier |
|---|---|---|
| 4/12 | 18.43° | 1.054 |
| 6/12 | 26.57° | 1.118 |
| 8/12 | 33.69° | 1.202 |
| 12/12 | 45.00° | 1.414 |
How to measure roof pitch
- Hold a 2-ft level horizontally against the underside of a rafter in the attic (or on the roof surface).
- Mark 12” from the end where the level touches the roof.
- Measure straight up (plumb) from that mark to the rafter or roof surface. That distance in inches is the rise: 7” means a 7/12 pitch.
From the ground, a phone inclinometer or a speed square held against the rake board gets you close; confirm with a level before ordering materials.
The slope factor (rafter multiplier)
The slope factor converts horizontal run to sloped length: √(rise² + 12²) ÷ 12. Multiply a rafter’s run by it to get rafter length, or multiply a roof’s flat footprint area by it to get actual roof area for shingles and underlayment.
Why pitch matters
- Roofing materials. Under IRC R905.2.2, asphalt shingles may be used on slopes of 2:12 or greater; slopes from 2:12 up to 4:12 require two layers of underlayment. Low-slope roofs below that need membrane systems.
- Safety. Most roofers can walk up to about 6/12 without roof jacks; 8/12 and steeper calls for jacks, toe boards and fall protection.
- Snow and rain. Steeper roofs shed water and snow faster, which is why pitches of 8/12–12/12 are common in snow country.
- Attic space. Higher pitches create usable attic or bonus-room height.