Rafter Length Calculator
Estimate the geometric common-rafter length from the building span, roof angle and horizontal eaves overhang.
Wall-to-wall span for a symmetrical duo-pitch roof.
Enter your measurements and press Calculate rafter length.
How rafter length is calculated
For a symmetrical roof, half the building span gives the horizontal run to the ridge. The horizontal eaves overhang is added to that run, then the roof angle is used to calculate the corresponding slope length.
What the result does not include
Actual setting-out lengths depend on ridge thickness, wall-plate position, birdsmouth geometry, fascia details and the chosen construction method.
This is a roof-geometry calculator only, not a structural design tool. Timber section sizes, spans, loads, restraints and connections require appropriate structural design information.
Frequently asked questions
Is the overhang measured horizontally?
Yes. Enter the horizontal projection beyond the supporting wall, not the sloping length.
How can I find the roof angle?
If you know the roof rise and horizontal run, use the Roof Pitch Calculator first.