How to use the rafter length calculator
- Enter the building span from the outside of one wall to the outside of the other.
- Enter the roof pitch (rise per 12 in of run).
- Enter the overhang, measured level from the wall to the end of the rafter tail.
- Enter the ridge board thickness. A 2× ridge is about 1½ in.
The result is the line length along the rafter. Buy stock longer than this to allow for the plumb cut at the ridge and the tail cut. Cut one pattern rafter and test-fit it before cutting the rest; Decoratoradvice recommends this on every roof, however carefully you measured.
Rafter formulas
Run = span ÷ 2 − ridge thickness ÷ 2
Pitch factor = √(1 + (pitch ÷ 12)²)
Rafter to wall = run × pitch factor
Overhang length = overhang × pitch factor
Rafter length = rafter to wall + overhang length
Plumb cut angle = arctan(pitch ÷ 12)
Worked example
A 24 ft span, 6/12 pitch, 1½ in ridge, 12 in overhang:
Run: 144 in − 0.75 in = 143.25 in
Pitch factor: 1.118
To the wall: 143.25 × 1.118 = 160.2 in (13 ft 4⅛ in)
Overhang: 12 × 1.118 = 13.4 in
Rafter: 173.6 in, about 14 ft 5⅝ in
Rise to the ridge: 144 × 0.5 = 72 in (6 ft)
Plumb cut angle: 26.6°
So a 16 ft board leaves room for the cuts.
Choosing rafter size
Length is only half the question. Rafter size and spacing depend on span, roof load, snow load and the lumber grade. Use your plans or the span tables in your local building code. For lumber quantities, see the board foot calculator.
Tips
- Cut one rafter as a pattern, test-fit it at both ends, then use it to mark the rest.
- Check the walls are parallel. If the span varies, measure at each rafter position.
- Find the pitch from an existing roof with the roof pitch calculator.
- Once framed, the roofing calculator gives the area for decking and shingles.
Common mistakes
- Forgetting the ridge board. Without subtracting half its thickness, every rafter is ¾ in too long.
- Measuring overhang along the slope. Enter it level; the calculator converts it.
- Buying stock at the exact length. The cuts at each end need extra.
Frequently asked questions
How long is a rafter for a 24 ft span at 6/12?
About 13 ft 4⅛ in to the wall with a 1½ in ridge, or 14 ft 5⅝ in with a 12 in overhang.
What is rafter run?
Half the span, less half the ridge thickness: the horizontal distance the rafter covers from wall to ridge.
How do I find the plumb cut angle?
It equals the roof angle: arctan(pitch ÷ 12). For 6/12, 26.6°.
Does the birdsmouth change the length?
The birdsmouth notch sits on the wall plate and does not change the line length, but it affects where you mark the seat cut.
Can I use this for a shed roof?
For a single-slope roof, enter twice the horizontal run as the span and 0 for the ridge.
What about metal roof panels?
Panel length is the rafter length with overhang, measured along the slope; see the metal roofing calculator.
Key takeaways
- Run = span ÷ 2 − ridge ÷ 2.
- Rafter length = (run + overhang) × pitch factor.
- The plumb cut angle equals the roof angle.
- Size rafters from span tables, not from length alone.