How long is a rafter on a 6/12 pitch?
A 6/12 roof gives 13.4164" of common rafter per foot of run — the number printed on a framing square. Rafter lengths, cut angles and hip figures for every common pitch.
A 6/12 pitch gives 13.4164" of common rafter per foot of run.
On a 24' span that is a line length of 13 ft 4 3/16 in, or 14 ft 10 1/16 in with the tail — cut plumb at 26.6° and level at 63.4°.
How that is worked out
- Run is half the span, less half the ridge thickness. Rise is the run times the pitch: 6 in 12 means 6" of rise for every 12" of run.
- The rafter is the hypotenuse: √(12² + 6²) = 13.4164" per foot of run. That is the figure in the first line of a framing square's rafter table, and it has been printed on squares for a century.
- Multiply by the run in feet for the line length, then add the tail. The plumb cut is 26.6° off square and the seat cut is the complement of it.
Per foot of run, and a 24' span with a 16" tail
| Pitch | Common per foot | Hip per foot | Plumb cut | Rafter length (24' span) | Buy |
|---|---|---|---|---|---|
| 3:12 | 12.3693" | 17.2337" | 14.0° | 13 ft 8 1/8 in | 16' |
| 4:12 | 12.6491" | 17.4356" | 18.4° | 13 ft 11 7/8 in | 16' |
| 5:12 | 13.0000" | 17.6918" | 22.6° | 14 ft 4 1/2 in | 16' |
| 6:12 | 13.4164" | 18.0000" | 26.6° | 14 ft 10 1/16 in | 16' |
| 7:12 | 13.8924" | 18.3576" | 30.3° | 15 ft 4 3/8 in | 18' |
| 8:12 | 14.4222" | 18.7617" | 33.7° | 15 ft 11 3/8 in | 18' |
| 9:12 | 15.0000" | 19.2094" | 36.9° | 16 ft 7 1/16 in | 18' |
| 10:12 | 15.6205" | 19.6977" | 39.8° | 17 ft 3 5/16 in | 20' |
| 12:12 | 16.9706" | 20.7846" | 45.0° | 18 ft 9 3/16 in | 20' |
Worth knowing
- The hip figure is per foot of *common* run, not per foot of its own run — that is the convention the framing square uses and the source of most hip arithmetic going wrong.
- Asphalt shingles need at least 2 in 12 of rise (IRC R905.2.2), and under 4 in 12 they need doubled underlayment. 2021 IRC, and defaults rather than law — the building department decides.
- The birdsmouth is limited in how much rafter it can remove. The calculator reports the maximum seat cut and refuses to cut deeper rather than quietly trimming the notch to fit.
- These are geometry, not structure. Whether a 2x8 spans 24 feet at this pitch under your snow load is a span-table question, and the answer is usually no.
Work out your own numbers
- Rafters
Common, hip and jack rafters for any pitch, with the birdsmouth laid out and the cut angles alongside.
- Roofing calculator
Squares, bundles, starter strip, ridge cap and underlayment from a building footprint and a pitch.
Related questions
- What does 6/12 pitch mean?
- Six inches of rise for every twelve inches of horizontal run — a slope of about 26.6°. It is the most common residential roof pitch in North America: steep enough to shed water and snow well, shallow enough to walk on and to frame without staging.
- How do you calculate rafter length?
- Work out the run — half the span, less half the ridge thickness — then multiply it by the per-foot figure for the pitch. At 6/12 that figure is 13.4164" per foot of run. Add the tail for the overhang, measured along the rafter rather than horizontally. The framing square has the per-foot numbers stamped on it for exactly this.
- What angle do you cut a 6/12 rafter?
- 26.6° off square for the plumb cut at the ridge and at the heel, and 63.4° — its complement — for the level seat cut. Set a speed square to 6 on the common scale and it gives you the same line without any arithmetic at all.