Rafter Length Chart: Common Rafters by Span and Pitch
Part of the project planner: Planning a roof replacement.
For a 24 ft wide building with a 6/12 roof pitch, a common rafter is 13.42 ft from the ridge to the outside of the wall, and 14 ft 6 7/16 in with a 12 in overhang. The chart shows other spans and pitches.
Common rafter length by building width and pitch
From the center of the ridge to the outside of the wall, with no overhang. Add the overhang along the slope:
| Building width | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| 16 ft | 8 ft 5 3/16 in | 8 ft 11 5/16 in | 9 ft 7 3/8 in | 10 ft 4 15/16 in | 11 ft 3 3/4 in |
| 20 ft | 10 ft 6 1/2 in | 11 ft 2 3/16 in | 12 ft 0 1/4 in | 13 ft 0 3/16 in | 14 ft 1 11/16 in |
| 24 ft | 12 ft 7 13/16 in | 13 ft 5 in | 14 ft 5 1/16 in | 15 ft 7 7/16 in | 16 ft 11 5/8 in |
| 28 ft | 14 ft 9 1/16 in | 15 ft 7 13/16 in | 16 ft 9 15/16 in | 18 ft 2 11/16 in | 19 ft 9 9/16 in |
| 32 ft | 16 ft 10 3/8 in | 17 ft 10 11/16 in | 19 ft 2 3/4 in | 20 ft 9 15/16 in | 22 ft 7 1/2 in |
Pitch, angle and the length factor
The rafter length is the run times the factor:
| Pitch | Roof angle | Length factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
How to calculate it
- The run is half the width of the building, from the outside of the wall to the center of the ridge: 12 ft for a 24 ft wide building.
- The rise is the run times the pitch over 12: for 6/12 that is 6 ft.
- The rafter length is the hypotenuse: 12 × √(1 + 0.5²) = 12 × 1.118 = 13.42 ft.
- The overhang adds its own length along the slope: a 12 in overhang adds 1 × 1.118 = 1.118 ft, for 14.53 ft in all (14 ft 6 7/16 in).
Put your own numbers into the rafter length calculator. Take off half the thickness of the ridge board (measured along the slope) at the top end before you cut.
How many rafters?
You need one at each end and one every 16 or 24 inches between, on each side of the ridge:
| Roof length | Spacing | Rafters per side | Rafters in all |
|---|---|---|---|
| 20 ft | 16 in | 16 | 32 |
| 20 ft | 24 in | 11 | 22 |
| 30 ft | 16 in | 24 | 48 |
| 30 ft | 24 in | 16 | 32 |
| 40 ft | 24 in | 21 | 42 |
Before you cut
- Check the span. The size, grade and spacing of the rafters must suit the span and the roof load. Look at a span table or ask an engineer, and add ties or a ridge beam where the design needs them.
- Buy the next length up. A 14.5 ft rafter needs a 16 ft board.
- Make a pattern rafter. Mark the plumb cut at the ridge, the birdsmouth and the tail on one board, test it on the wall, and trace the rest from it.
- Check permits. Roof framing usually needs a permit and an inspection.
Mistakes that cost people money
- Measuring the plan width of the building and not the actual width of the walls.
- Forgetting to deduct half the ridge board, which makes the rafters too long.
- Cutting all the rafters before testing the first one.
- Ignoring the overhang, or measuring it along the slope when the plan gives it horizontally.
Frequently asked questions
How do you calculate rafter length?
Take half the building width as the run, multiply by the square root of 1 plus the pitch squared over 144, and add the overhang using the same factor. For a 24 ft wide building with a 6/12 pitch and a 12 in overhang that gives 14.53 ft.
What is the rafter length for a 6/12 pitch?
The rafter is 1.118 times the run. A 12 ft run (a 24 ft wide building) gives 13.42 ft before the overhang.
How many rafters do I need?
One every 16 or 24 inches along the length, plus one for the end, on each side of the ridge. A 30 ft roof at 24 in on center takes 16 on each side.
What is the angle of a 6/12 roof?
26.6 degrees. A 4/12 roof is 18.4 degrees, an 8/12 is 33.7 degrees, and a 12/12 is 45 degrees.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.