Rafter Length Calculator
The common rafter is the most frequently used rafter type in residential construction. It runs perpendicular to the ridge board and spans from the wall plate to the ridge. Calculating the correct rafter length before cutting lumber is essential to avoid waste and ensure the roof fits properly. The rafter length is the hypotenuse of the right triangle formed by the total rise and the total run. The run is half the building width for a symmetrical gable roof. The overhang (eave projection) adds additional length beyond the wall. This calculator takes the building width, roof pitch (expressed as rise per 12 inches of run), and overhang in inches to give the full rafter length. This is the theoretical centerline length. In practice, deduct half the ridge board thickness at the top and account for the bird's mouth notch at the seat. Consult the American Wood Council's prescriptive residential wood deck construction guide or your local building plans for precise cutting details. Always verify rafter lengths before cutting a full set of lumber.
Rafter length formula
Run = Building width / 2
Rise = Run x (Pitch / 12)
Rafter (no overhang) = sqrt(Run^2 + Rise^2)
Overhang run (ft) = Overhang (in) / 12
Overhang rafter = Overhang run x sqrt(1 + (Pitch/12)^2)
Total rafter = Rafter (no overhang) + Overhang rafter
Worked example
Building: 28 ft wide. Pitch: 6:12. Overhang: 18 in.
- Run = 28 / 2 = 14 ft
- Rise = 14 x (6/12) = 7 ft
- Rafter = sqrt(14^2 + 7^2) = sqrt(245) = 15.65 ft
- Overhang run = 18/12 = 1.5 ft
- Overhang rafter = 1.5 x sqrt(1 + 0.25) = 1.5 x 1.118 = 1.68 ft
- Total = 15.65 + 1.68 = 17.32 ft
Frequently asked questions
How do I calculate rafter length?
Rafter length is the hypotenuse of the rise/run triangle. The run is half the building width (for a symmetric gable roof). Divide the run by 12 and multiply by the rafter length per foot of run (which equals the square root of (rise squared plus 144) divided by 12). Add the overhang run separately, calculated the same way.
What is the overhang in rafter length calculations?
The overhang (or eave projection) is the horizontal distance the roof extends beyond the building wall. It is entered in inches. The rafter must be long enough to span both the building run and the overhang run. This calculator adds the overhang rafter length to the main span rafter length.
What is the bird's mouth cut in a rafter?
The bird's mouth is a notch cut near the bottom end of a rafter where it sits on the top plate of the wall. It consists of a seat cut (horizontal) and a plumb cut (vertical). The depth of the bird's mouth affects the rafter length slightly but is typically accounted for in the structural design, not in the raw length calculation.
How much should I add for rafter seat and ridge board?
At the ridge, you typically deduct half the ridge board thickness from the theoretical rafter length (usually 3/4 inch for a 1.5-inch ridge board). At the seat (wall plate), the bird's mouth depth affects the tail length. This calculator gives the theoretical centerline rafter length; consult your building plans for exact cutting dimensions.
Can I use this calculator for a hip or valley rafter?
No. This calculator is for common rafters only (perpendicular to the ridge). Hip and valley rafters run diagonally and require a different calculation involving the diagonal run. A hip rafter run equals the common rafter run multiplied by the square root of 2 (approximately 1.414).
Sources
- American Wood Council: Prescriptive Residential Wood Deck Construction Guide (DCA6).
- International Residential Code (IRC): rafter span tables and requirements.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Theoretical length only. Consult your building plans for cutting details.