Result
Geometry only. Roofing products have minimum pitches (many asphalt shingles need 2/12 or more, with extra underlayment below 4/12); check the product label and local code.
Pitch diagram
Tools for marking pitch
- Speed squares with a common-rafter scale for plumb lines.
- Pitch gauges and angle finders for reading an existing roof.
- Speed squares at Home Depot.
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How roof pitch works
Pitch is rise over run. US framers say it as inches of rise per 12" of run: a roof that climbs 6" over 12" is a 6/12 (or 6:12) pitch. Slope = rise ÷ run, angle = arctangent(slope), grade = slope × 100%.
The slope factor √(1 + slope²) turns a level distance into a length along the roof. Multiply run by it for rafter length, and a footprint area by it for roof area. The hip/valley factor √(2 + slope²) does the same for a hip or valley rafter, per foot of the common rafter's run, when both roof planes have the same pitch.
Cut angles: the plumb cut is the roof angle measured from square; the seat cut is 90° minus that. On a speed square, hold the pivot on the edge and turn until the pitch number on the common scale lines up, then mark along the square for a plumb line.
Common pitches
| Pitch | Angle | Grade | Factor | Per ft run |
|---|---|---|---|---|
| 3/12 | 14.04° | 25% | 1.0308 | 12-3/8" |
| 4/12 | 18.43° | 33.3% | 1.0541 | 12-5/8" |
| 5/12 | 22.62° | 41.7% | 1.0833 | 13" |
| 6/12 | 26.57° | 50% | 1.1180 | 13-7/16" |
| 7/12 | 30.26° | 58.3% | 1.1577 | 13-7/8" |
| 8/12 | 33.69° | 66.7% | 1.2019 | 14-7/16" |
| 9/12 | 36.87° | 75% | 1.2500 | 15" |
| 10/12 | 39.81° | 83.3% | 1.3017 | 15-5/8" |
| 12/12 | 45.00° | 100% | 1.4142 | 17" |
Questions
What angle is a 6/12 roof pitch?
26.57°. A 6/12 roof rises 6" per 12" of run, a slope of 0.5, so the grade is 50% and each foot of run takes about 13-7/16" of rafter.
How do I measure roof pitch?
Hold a level flat against a rafter or the roof, mark 12" along it, and measure straight up from that mark to the roof. That number is the pitch over 12. From inside the attic, measure on a rafter rather than the sheathing.
What are the plumb and seat cut angles?
The plumb cut (ridge and tail) is at the roof angle; the seat cut (on the wall plate) is 90° minus the roof angle. For 6/12: 26.57° and 63.43°.
Next: work out full lengths with the rafter length calculator, or roof area and bundles with the roofing calculator.
Safety
Pitch is geometry, not a structural check. Rafter size and spacing must meet local code (IRC R802) or an engineer's design. Working on a roof is a fall risk: use fall protection (OSHA guidance) or hire a licensed roofer.