Rafter reference · low slope
3/12 Roof Pitch
Everything a framer or DIYer needs to lay out rafters on a 3/12 pitch: angle, slope percent, rafter length per foot of run, and the Speed® Square method Albert Swanson invented for it in 1925.
Angle
14.04°
Slope
25%
Rafter / 12″ run
12.369″
Multiplier
1.0308
What a 3/12 pitch looks like on the jobsite
3/12 is the minimum "walkable" pitch in most jurisdictions and the usual floor for standard shingle installation.
Common use cases:
- Modern and desert-style homes
- Ranch-style porches
- Shed and garage roofs
Rafter length by run
Multiply the common rafter run (half the building width) by 1.0308. That is the rafter length before you add any tail overhang. The table below shows a few common runs:
| Run (ft) | Rafter length (ft) |
|---|---|
| 8 | 8.25 |
| 10 | 10.31 |
| 12 | 12.37 |
| 14 | 14.43 |
| 16 | 16.49 |
| 18 | 18.55 |
| 20 | 20.62 |
Add rafter tail length (typically 12 to 24 inches) if you want the full ridge-to-tail dimension.
Laying it out with a Speed® Square
This is the One Number® method from the Blue Book of Rafter Lengths, the pocket reference Albert Swanson packaged with the first Speed® Square in 1925.
- 1Pivot the square on the top edge of the rafter. Find 3 on the common scale and align it with the top edge. Scribe a plumb line, that is the ridge cut.
- 2From the long point of the plumb, measure run × 1.0308 along the top edge to locate the seat cut.
- 3At the seat mark, scribe a plumb at the same 3 common reference, then a level line equal to the top-plate width. Cut both ends and test-fit.
Tools for the Technique
The tools for this technique
Every SKU below is what a Swanson pro would reach for on this job. Made in Frankfort, Illinois, since 1925.
