A birdsmouth is the notch cut into the underside of a rafter where it crosses the wall, so the rafter sits flat on the top plate instead of balancing on its edge. It is made of two cuts that meet at 90°: the seat cut, which is level and rests on the plate, and the heel cut (or plumb cut), which is vertical and bears against the outside edge of the plate. Viewed from the side, the notch looks like an open bird’s beak — hence the name.
Both cuts follow the roof pitch. On a speed square, pivot to the pitch number on the “common” scale to mark the plumb line; the seat line is square to it. The seat is usually as long as the wall is thick — 3-1/2” on a 2x4 wall, 5-1/2” on a 2x6 wall. The IRC (R802.6) requires rafter ends to have at least 1-1/2” of bearing on wood or metal.
How deep can you cut? A widely used framing rule is to leave at least two-thirds of the rafter’s depth above the seat cut, so the tail can still carry the overhang. Mark one pattern rafter, test it at both ends of the building, and keep the height above plate — the distance from the seat cut up to the top edge of the rafter — the same on every rafter so the roof plane stays flat.