Birdsmouth Cut Calculator for Common Rafters

This calculator determines the seat cut length and heel height for a birdsmouth notch on a common rafter. By inputting the roof pitch (rise per 12 units), rafter width, and top plate width, users can calculate the exact dimensions needed for a stable and structurally sound birdsmouth joint.

The calculator considers local building codes, which may determine the maximum allowable depth for this cut. The International Residential Code (IRC), adopted as a base standard by most U.S. jurisdictions, specifies that the maximum notch depth for a birdsmouth should not exceed one-quarter of the rafter depth. However, local governments can adopt these codes with or without modifications to suit their specific needs. For example, one town might allow a notch depth of rafter depth divided by 4, while another might permit a notch depth divided by 3.

Calculator

1. enter the roof pitch as a rise-over-run ratio in the format X: 12 (e.g., 6 for a 6:12 pitch)
2. the maximum notch depth allowed for the rafter in inches, representing the deepest cut permitted
3. the exact total rafter depth as a mixed number in the format integer fraction (e.g., 11 3/8)
4. specify the precision level for the fractional output, such as 16 for sixteenths of an inch, to keep it usable

Result

1. the output represents the maximum allowable length of the heel cut on the rafter, ensuring it fits properly over the wall plate without compromising structural integrity. It accounts for the roof's slope and local building code requirements.

Example

a 2 x 10 rafter. Acutual rafter depth = 9 1/4 inch
Maximum notch depth for the birdsmouth;
Depth/3 = 9 1/4 ÷ 3 = 3 1/16 inch
Depth/4 = 9 1/4 ÷ 4 = 2 5/16 inch
Depth/5 = 9 1/4 ÷ 5 = 1 7/8 inch

As the slope of the roof increases, so can the length of the heel cut get longer while staying within the maximum allowable notch area.