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This rafter length calculator works out the length of the common rafters for a gable roof. Enter the building span, roof pitch, ridge board thickness and overhang, and it gives the rafter run, rise, rafter and tail lengths, the plumb cut angle and the lumber length to buy.
How to Use the Rafter Length Calculator
- Enter the span: the distance between the outside faces of the two walls.
- Enter the roof pitch as inches of rise per 12 inches of run.
- Enter the ridge board thickness (1½ in for 2× lumber).
- Enter the horizontal overhang past the wall.
- Optionally enter the building length and rafter spacing to count rafters.
Rafter Length Formula
Run = Span ÷ 2 − Ridge thickness ÷ 2
Rafter length = Run × √(1 + (Pitch ÷ 12)²)
The square-root term is the slope factor. Multiply the overhang by the same factor for the tail length.
Example
A 24 ft span with a 6/12 pitch and a 1½ in ridge has a run of 11 ft 11¼ in. The slope factor is 1.118, so the rafter is 13 ft 4¼ in to the wall, plus 1 ft 1⅜ in for a 12 in overhang: 14 ft 5½ in in total. Buy 16 ft boards.
Slope Factors
| Pitch | Angle | Slope factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Tips
- Cut one pattern rafter, test it in several places, then use it to mark the rest.
- Mark the bird’s-mouth seat cut so it bears fully on the wall plate.
- Check a rafter span table for the lumber size and spacing your snow and roof loads require.
Need the pitch first? Use the roof pitch calculator.
Frequently Asked Questions
How do you calculate rafter length?
Find the run (half the span minus half the ridge thickness) and multiply it by the slope factor √(1 + (pitch ÷ 12)²). Add the overhang times the same factor.
How long is a rafter for a 24 ft span with a 6/12 pitch?
About 13 ft 4 in from ridge to wall, or about 14 ft 6 in with a 12 in overhang.
What is the plumb cut angle for a 6/12 roof?
26.6 degrees from vertical at the ridge, which is the roof angle.
How many rafters do I need?
Divide the building length by the rafter spacing, add one, and double it for both sides of a gable roof.
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