birdsmouth

Birdsmouth, also called a bird’s mouth, is a notch cut into a roof rafter to provide bearing on the top of a wall plate in traditional timber-framed roofs. The notch is formed by two cuts: a seat cut that creates a flat bearing surface on the bottom of the rafter so it can sit on the wall plate, and a diagonal or plumb cut at the end of the rafter that shapes the edge so the rafter can meet the ridge line at the correct angle. Together, these cuts allow the rafter to rest securely and align with the roof slope.

Purpose and use: The primary function of a birdsmouth is to transfer the rafter’s load to the wall and to ensure the rafter sits firmly with the proper rake. The seat bears on the top of the wall plate, while the plumb cut provides a clean end that fits against the ridge or adjacent rafters. Properly cut birdsmouth notches help maintain structural integrity while enabling straightforward installation in conventional stick framing.

Construction considerations: The seat width is designed to match the wall plate bearing, and the notch depth should be limited to preserve as much of the rafter’s strength as practical. It is important that the cuts are square and the rafter sits flat on the wall plate. In modern construction, some roofs use alternative hardware or framing methods (such as metal hangers or ties) where notching is not desirable or permitted by code.

See also: Rafter, roof framing, wall plate, ridge beam.