Roofing
Rafter Length Calculator
Calculate common rafter length from span and pitch, including overhang.
Result
This is the theoretical line length including overhang. It does not account for the plumb cut, birdsmouth seat or any bevel, cut one rafter as a pattern and test-fit it before cutting the rest.
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About this calculator
Why deduct half the ridge thickness?
The rafter meets the side of the ridge board, not its centreline. Ignoring it makes every rafter slightly long and pushes the walls out.
Does this work for hip rafters?
No. Hip and valley rafters run diagonally and use a 17-inch run rather than 12. This calculates common rafters only.
The formula
rafter = √(run² + rise²) + overhang × slope multiplier
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