A structural joint consisting of a horizontal seat cut and a vertical plumb cut made in a rafter. This notch allows the angled rafter to bear flat on the top plate of a wall, transferring the roof load straight down. Building codes dictate the maximum depth of the cut to prevent weakening the rafter overhang.
Heard on the job
Lay out the birdsmouth on the pattern rafter and test the fit before you cut the rest of the stack.