birdsmouth
A notch cut into the underside of a rafter to ensure that it does not move when resting on the wall plate running across the top of a wall; a similar notch in other timber components.
Birdsmouth: notch that locks a rafter to its seat
A birdsmouth is a notch cut into the underside of a rafter where it bears on the wall plate. The notch consists of two cuts meeting at an angle, shaped roughly like an open beak, that create both a vertical face (the seat cut) and a horizontal face (the heel cut). This geometry allows the rafter to rest flat and square on the plate while a mechanical lock prevents the rafter from sliding downslope under gravity and the thrust forces created by roof load.
The seat cut runs perpendicular to the rafter's length and bears directly on the wall plate; the heel cut (or backing cut) is angled to match the rafter's slope and faces down into empty space or into the plate itself, depending on design. Typical wall plates are 2×4 or 2×6 timber, and the notch removes enough material that the resulting bearing area is roughly 1.5 to 2 inches deep along the plate. The depth and angle of the heel cut must be calculated so that the seat cut does not remove more than one-third of the rafter's cross-sectional depth, or structural capacity is compromised.
Carpenters lay out a birdsmouth using the roof pitch angle and the rafter's slope. For a 4:12 pitch rafter, the heel cut is made at roughly 18.4 degrees from horizontal. The notch is cut with a circular saw or by hand saw, working from the bottom edge of the rafter upward. Poor layout or overcuts that go deeper than intended weaken the rafter, as do notches made in the top (compression) side of the rafter, which is structurally wrong.
Why the name and where it sits
The notch resembles the open mouth of a small bird in profile, hence the name. It appears in gable roof framing on every rafter that meets a wall plate, making it one of the first joints a new carpenter must master. The birdsmouth is not used on collar ties, purlins, or rafters in truss construction, where load transfer and member geometry differ.
The birdsmouth exists because rafters push outward and downward on the walls they rest on, and a simple flat seat offers no resistance to downslope slip. Metal framing anchors and hurricane ties now often work alongside or instead of birdsmouths in regions with high wind or seismic risk, but the birdsmouth remains the standard detail in residential framing because it is fast to cut, requires no hardware, and provides positive mechanical engagement when properly executed.