What is a Birdsmouth Cut in Rafter Framing?
A birdsmouth cut is a V-shaped triangular notch cut into the bottom edge of a common roof rafter where it sits atop the exterior wall top plate. The cut consists of two perpendicular incisions:
1. The Seat Cut: The horizontal flat surface that bears down squarely onto the 2x4 or 2x6 top plate.
2. The Heel Cut (or Plum Cut): The vertical cut that runs up flush with the outside face of the exterior wall framing or sheathing.
Alaska Pro-Tip: In Alaska, where design snow loads in Anchorage and Eagle River reach 40 to 60 pounds per square foot (PSF), over-notching a birdsmouth cut can cause rafters to split along the grain under heavy wet snow accumulation.
IRC Code Limitations on Rafter Notches
The International Residential Code (IRC Section R802.7.1) restricts the size of bearing notches. Cutting away too much lumber severely reduces the shear strength of the rafter at its critical reaction support point.
Never notch deeper than one-third (33.3%) of the actual rafter lumber depth. If your pitch is steep (such as 10/12 or 12/12) and a full 3.5" or 5.5" seat cut results in a heel cut that exceeds the 1/3 limit, you must shorten the seat cut or sister the rafter with Simpson Strong-Tie H-clips or hurricane ties.
| Rafter Size | Actual Depth | Max 1/3 Notch Limit | Recommended Wall Bearing |
|---|---|---|---|
| 2x4 Rafter | 3.50 inches | 1.17 inches | 2x4 Wall (Low pitch only) |
| 2x6 Rafter | 5.50 inches | 1.83 inches | 2x4 or 2x6 Wall |
| 2x8 Rafter | 7.25 inches | 2.42 inches | 2x4 or 2x6 Wall |
| 2x10 Rafter | 9.25 inches | 3.08 inches | 2x6 Wall (Heavy snow areas) |
| 2x12 Rafter | 11.25 inches | 3.75 inches | 2x6 Wall |
Frequently Asked Questions
Common questions regarding this calculation and code compliance
According to IRC R802.7.1, a birdsmouth notch can never exceed one-third of the total depth of the rafter (for example, maximum 1.83 inches on a 2x6, or 2.42 inches on a 2x8).