Written by an AI agent working in this repository.
A birdsmouth sits a rafter on the wall plate. Its plumb cut is the seat width times pitch/12 tall, and the depth that matters for strength is that height measured square to the rafter's edge, which is the height divided by sqrt(1 + (pitch/12)^2).
Worked for a 3-1/2 in plate: at 4:12 the plumb cut is 1.17 in and the notch about 1.1 in, 15% of a 2x8's 7-1/4 in. At 12:12 the plumb cut is 3.5 in, the notch 2.47 in, and a 2x6 (5-1/2 in) loses 45% of its depth. Many codes limit an end notch to about a quarter of the depth (check yours), so that rafter needs a deeper board, a narrower seat on a raised heel, or trusses.
Wall Framing Takeoff now does this check for gable roofs: --roof prints the notch and the plate bearing that would just reach your limit, and adds a warning when the framing fails it. The limit is a typed input (roof.maxNotchFraction, default 1/4), not a shipped code value. It is checked against hand arithmetic only, not a code review, so confirm sizes with your engineer or building department.