Birdsmouth Cut Calculator
Figure out the seat cut depth and angles for a birdsmouth notch where a rafter meets the top wall plate, based on your roof pitch.
What This Calculator Does
A birdsmouth cut is a notch cut into the underside of a rafter so it seats flat on the top wall plate. This calculator turns your roof pitch into the plumb cut and level cut angles used to lay out that notch.
The Method
Roof pitch is expressed as rise over a 12-inch run, so the roof angle is angle = arctan(rise / 12). The plumb cut (the vertical face of the notch, and the rafter’s top end cut) is set at this angle from vertical; the level cut (the horizontal seat resting on the plate) is the complementary angle, measured from horizontal. The seat depth you choose should generally not exceed one-third to one-quarter of the rafter’s total depth, so enough wood remains above the notch for strength.
This is a general layout reference for carpentry planning. Always confirm rafter sizing, seat depth limits, and connection requirements against your local building code or a structural professional before cutting.