CSLB General Building (B)Framing and CarpentryMedium
A framer cuts a birdsmouth (seat cut) into a 2x8 rafter to seat it on the top plate. The rafter is 7.25 inches deep. Per standard framing practice, what is the maximum depth the seat cut should be to avoid excessively weakening the rafter?
- AAbout 5.4 inches (three-fourths of the rafter depth)
- BThe full depth of the rafter, since the plate carries the load
- CExactly 1 inch regardless of rafter size
- DAbout 2.4 inches (one-third of the rafter depth)
Show answer & explanationAnswer & explanation
Correct answer: D. About 2.4 inches (one-third of the rafter depth)
The seat cut of a birdsmouth notch should not exceed roughly one-third of the rafter's depth (about 2.4 inches for a 7.25-inch-deep rafter) so enough solid wood remains above the cut to maintain bending strength. Cutting deeper removes too much of the rafter's cross-section at a critical bearing point, risking a shear/bending failure near the plate.
Why the other options are wrong
- A. Removing three-fourths of the depth would leave almost no wood to resist bending stress.
- B. A full-depth cut would sever the rafter entirely.
- C. A fixed 1-inch rule ignores rafter size and is not the governing method.
Birdsmouth Seat Cut Limit
The notch cut into a rafter to seat it on the top plate should generally not exceed about one-third of the rafter's depth to preserve structural capacity.
- Seat cut ≤ ~1/3 rafter depth
- Plumb cut should be vertical for full bearing
- Excessive notching weakens rafter at a high-stress location
Memory trick: Cut a third, keep the girth.