This calculator will calculate the maximum length of a heel cut based on the rafter depth and the slope of the roof. In addition, what your local building code requires will determine maximum allowable depth for this cut.

The International Residential Code (IRC) has been adopted as a base code standard by most jurisdictions in the United States. However, local governments adopt building codes. They can implement these codes without change or modifications to suit local needs. The IRC stipulates the maximum depth that you can notch your rafter for a birdsmouth is rafter depth divided by 4. State, provincial and territorial governments have the authority to enact legislation that regulates building design and construction within their jurisdictions, but local authorities can adopt this code without change or with modifications to suit local needs. In one town, the maximum depth of the notch is rafter depth divided by 4. In another town, you can get away with a notch depth of rafter depth divided by 3.

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

The heel cut length.

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.

Calculation

1. code requirement for maximum notch depth preset @3 (rafter depth divided by 3, 4, 5 or whatever is required)
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.