BuildCalcs Rafter Length Calculator
Estimate the geometric common-rafter length from the building span, roof angle and horizontal eaves overhang.
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.
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.
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What the rafter length represents
The calculator uses half of the building span as the horizontal roof run, combines that run with the entered pitch angle and then adds the horizontal eaves overhang. The result is the geometric slope length of a common rafter line. It is useful for checking dimensions and early material planning on a simple symmetrical pitched roof.
Geometric length is not a cutting length
A real rafter normally needs additional setting-out information for the ridge, birdsmouth, wall plate, fascia line and any structural connection details. Timber section size, permissible notches and bearing requirements are structural matters and are not determined here. Do not cut rafters solely from the geometric result.
Measure span and pitch consistently
Use the building span described by your chosen roof setting-out method rather than mixing external wall dimensions with an internal run. The overhang field is horizontal, not measured along the slope. If the roof is asymmetric, hipped, vaulted or contains dormers, calculate each relevant roof plane separately and confirm the final geometry from the project drawings or a competent designer.