Birdsmouth
A birdsmouth is the notch cut in a rafter where it bears on the wall plate. AS 1684.2 limits the depth to preserve the rafter's structural section.
Ask Chalkline about this →A birdsmouth is a triangular notch cut into the underside of a rafter at the point where it bears on the wall plate. The notch has two faces: a plumb cut (vertical, matching the rafter pitch angle) and a seat cut (horizontal, sitting level on the plate). Together they create a flat bearing surface that stops the rafter from sliding off the plate.
AS 1684.2:2021 limits the depth of the birdsmouth seat cut. Cutting too deep reduces the net rafter section remaining above the notch, which reduces the rafter’s bending strength. As a guide, the notch should not exceed one-third of the rafter depth, but the exact limit depends on the rafter size, stress grade, and span; the applicable span table input takes this into account. Cutting beyond the standard limit can cause splitting at the plate line under load and is a defect at frame inspection.
Also known as: Bird’s mouth, birds mouth notch, rafter seat cut.
Category: Roof framing / Carpentry.
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Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency.