Birdsmouth Cut Calculator

A birdsmouth cut is a notch cut into a rafter where it sits on the top plate of a wall. It consists of a horizontal seat cut (that rests on the wall plate) and a vertical plumb cut (that fits against the outside of the wall). The cut is called a birdsmouth because its shape resembles a bird's open beak. It provides a stable, secure connection between the rafter and the wall, preventing the rafter from sliding off under load. Building codes require the seat cut to be no deeper than ⅓ of the rafter depth to maintain structural integrity.

Building codes (IRC R802.10.2) limit the birdsmouth cut depth to a maximum of ⅓ (33%) of the rafter depth. For a 2×6 (5.5 in actual): max cut = 1.83 in. For a 2×8 (7.25 in): max = 2.42 in. For a 2×10 (9.25 in): max = 3.08 in. For a 2×12 (11.25 in): max = 3.75 in. Exceeding this limit significantly weakens the rafter. If you need a deeper seat, consider using a rafter tie or hurricane strap instead of deepening the birdsmouth.

The roof pitch determines the angle of the plumb cut, which equals the roof angle. A 6/12 pitch has a 26.6° angle, while a 12/12 pitch has a 45° angle. The seat cut is always horizontal. The birdsmouth length (horizontal portion) increases with steeper pitches for the same seat depth. The heel height (remaining rafter depth above the seat cut) is critical - it must be sufficient for bearing. A minimum heel height of 3.5 inches is recommended for adequate nailing to the wall plate.

The birdsmouth cut has two parts: the seat cut (horizontal cut that bears on the wall plate, typically 3.5-5.5 inches long to match standard 2× wall plates) and the plumb cut (vertical cut against the outside wall face). Together they form the birdsmouth notch. Some rafters are cut with just a seat cut (bearing directly on the plate) without the plumb cut, but this is less common. A full birdsmouth with both cuts provides better load transfer and a more professional installation.