How the length is worked out
- The run is half the span less half the ridge board: on a 24 foot span with a 1 1/2 inch ridge board, 12 feet less 3/4 inch.
- The slope factor is how much longer the rafter is than its run: √(pitch² + 12²) ÷ 12. At 6/12 it's 1.118.
- The length to the wall is the run times the factor, and the tail is the overhang times the same factor. Together they're the rafter's length along its top edge, before the plumb cut at the ridge and the cut at the tail.
- The cuts: the plumb cut at the ridge is the pitch's angle, the arctangent of the rise over 12, and the seat cut where the rafter sits on the wall is its complement.
- The count is the building's length divided by the spacing, plus one, for each side of the ridge.
What the code says about rafters
- Size: rafters are sized from the span tables, and the span is measured level, along the rafter's horizontal projection (IRC R802.4.1). A Douglas fir-larch No. 1 2×8 at 12 inches on center, under a 20 psf live load and 10 psf dead load with the ceiling not attached, spans up to 22' 5" (Table R802.4.1(1)).
- Ridge board: at least 1 inch nominal thick and no shallower than the cut end of the rafter (IRC R802.3).
- Framing: rafters are framed opposite each other at the ridge, offset no more than 1 1/2 inches (IRC R802.4.2).
- Bearing: each end rests on at least 1 1/2 inches of wood or metal (IRC R802.6).
Common questions
How long is a rafter on a 24 foot span?
At 6/12 with a 1 foot overhang and a 1 1/2 inch ridge board, 14' 6", cut from 16 foot stock. At 12/12, 18' 4".
How do I figure rafter length without a calculator?
Multiply the run in feet by the pitch's slope factor: 1.118 at 6/12, 1.202 at 8/12, 1.414 at 12/12. Add the overhang times the same factor. The roof pitch calculator gives the factor for any pitch.
What spacing do rafters use?
The code's span tables list 12, 16, 19.2 and 24 inches on center, and closer spacing lets the same rafter span farther. A Douglas fir-larch No. 1 2×8 spans up to 22' 5" at 12 inches, 19' 5" at 16, 17' 9" at 19.2 and 15' 10" at 24, under a 20 psf live load and 10 psf dead load with the ceiling not attached (IRC Table R802.4.1(1)).
Does the length include the birdsmouth?
The length runs along the rafter's top edge from the ridge cut to the tail. The birdsmouth is cut into the bottom edge where the rafter sits on the wall and doesn't change it.