Rafter Length Calculator
Calculate rafter length from horizontal run and rise.
Rafter Length Formula
Using a right triangle, rafter length = √(run² + rise²). Add overhang separately when required.
How Rafter Length Is Calculated
Rafter length depends on the horizontal run of the roof and the vertical rise created by the roof pitch. For a simple gable roof, the common rafter is the hypotenuse of a right triangle formed by half the building span and the roof rise.
Rafter Length Example
If the horizontal run is 12 ft and the roof rises 6 ft over that run, the basic rafter line is approximately 13.42 ft before adding overhang, birdsmouth details or ridge adjustments.
Important Roof Framing Factors
Actual cut length can differ from the geometric line because of ridge-board thickness, overhang, fascia details and seat cuts. Use framing plans and local code requirements for construction decisions.
Frequently Asked Questions
Does the calculator include overhang?
Only if the overhang is included in the dimensions you enter.
Is rafter length the same as roof slope length?
For a simple common rafter, they are closely related, but framing cuts can change the final board length.
Can this replace a framing plan?
No. It is a geometry aid, not a structural design service.
Calculation Assumptions and Practical Limits
Basic rafter length follows roof run and rise using right-triangle geometry. Real cut length can also include overhang, ridge-board adjustment, birdsmouth geometry and connection details, so framing dimensions should follow the actual roof design.