The Heel Cut of a Birdsmouth

This calculator will calculate the maximum length of a heel cut based on rafter size, the slope of the roof and the International Residential Code (IRC). The maximum notch depth is the area your birdsmouth cut has to stay within.

The heel cut has to stay within a quarter of the width of the rafter.
As the slope of the roof increases, so will the length of the heel cut.

Example

Enter the actual width of your rafter and use 0 for the roof pitch to get the maximum depth your notch will have to stay within;
A 2 x 10 rafter has an actual depth of 9 1/4 inches. Divide this by four, and you will get the maximum area your notch will have to stay within.
9 1/4 ÷ 4 = 2 5/16
For a 2 x 10 rafter that is 2 5/16 inches.

Calculation

1. code requirement for maximum notch depth
2. enter the roof pitch, rise in 12 inches
3. the actual size of your rafter in inches
4. fraction precision, preset at 16.
   Changing this number to a higher amount will increase the accuracy of the cut length. For the fraction to be helpful, use 8, 16, or 64 in this field.

Results

1. The length of your heel cut within the maximum allowable notch area of the rafter.