By Zencalcy Editorial TeamLast reviewed About us
The birdsmouth is the notch that lets a rafter sit flat on the wall plate. Cut too deep and the rafter loses strength; too shallow and the roof rides high. The safe rule is a seat cut no deeper than one quarter of the rafter depth, with at least 3½ inches of heel left above the plate.
Seat and plumb cut dimensions for a rafter notch.
seat depth ≤ rafter depth ÷ 4 seat length = seat depth ÷ sin(angle) plumb cut = rafter depth ÷ cos(angle)
The seat cut is horizontal and must cover the plate width; the plumb cut is vertical against the ridge. A deeper notch reduces the rafter's effective depth at the most highly stressed point — the notch is where rafters break.
| Symbol | Meaning |
|---|---|
| D | Rafter depth. |
| θ | Roof angle. |
| h | Heel height above the plate. |
The rafter is 7¼ inches deep, so the seat cut should be no more than 1.81 inches. At 26.57° the seat length is 4.05 inches and the plumb cut is 8.10 inches.
Continue with the Roof Pitch Calculator, Rafter Length Calculator, Roof Truss Calculator, or the Roofing Calculator — or browse more roofing and all construction calculators.
One quarter of the rafter depth, or a third at most. For a 2 × 8 that means about 1¾ inches.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026