How this calculator works
Subtract ridge thickness before halving the building width. Pitch gives rise and rafter slope. A horizontal seat cut produces a vertical heel depth of seat times tangent of pitch, but the removed depth measured perpendicular to the rafter is seat times sine of pitch. The entered notch fraction compares that perpendicular depth with the actual board depth. The starting 25% is an end-notch reference, not approval for every birdsmouth or cantilever detail. Bearing length, overhang rules and structural loads need separate checks.
Formula
run = (building width - ridge thickness)/2; rafter = (run + horizontal overhang)/cos(angle); heel = seat x tan(angle); perpendicular notch = seat x sin(angle); minimum depth = notch/entered fraction.
Worked example
At 45 degrees, a 100 mm seat produces a 100 mm vertical heel but a 70.71 mm perpendicular notch. An adjusted 4 m run rises 4 m before the overhang.
Advertisement
Frequently asked questions
Why are heel and notch depth different?
Heel is vertical. The notch fraction compares depth perpendicular to the sloping board edge, which is a shorter measurement.
Does the calculation approve a 25% notch?
No. The permitted detail depends on the code, rafter position and any cantilever. Enter a verified limit and have the entire framing detail checked.
Does rafter length include the ridge-board deduction?
Yes. Half the ridge thickness is deducted from each side run. Tail fascia cuts, saw kerf and fitting allowance remain separate.