Spike's Calculators

Birdsmouth - Calculate Seat and Heel Cut Lengths

Our Dynamic Birdsmouth Calculator provides an essential tool for carpenters and builders focusing on precise rafter installations. This dual-function calculator offers two functionalities: one for calculating the seat cut based on the heel cut, rise, and rafter depth, and another for determining the heel cut from a known seat cut length and the rise and rafter depth measurements.

Birdsmouth: Heel Cut, Seat Cut Calculators

Birdsmouth Maximums and Minimums

If you are cutting a birdsmouth in your rafter, your local building code will limit the depth of the notch into the rafter. It can be rafter size divided by 3, 4 or whatever the local requirement is.

The limiting factor can be the minimum size of the seat cut that sits on the wall. The International Residential Code stipulates 1 1/2 inches of minimum bearing on the top plate, and here in our location, the rafter requires a minimum of 3 inches bearing on the top plate.

Internatonal Residential Code (IRC)

Birdsmouth Maximums and Minimums

Birdsmouth Calculator for Seat Cut

Rise :12
Heel Cut in
Rafter Depth in

Results:

Seat Cut in
Notch Size in
Remaining Depth in

Calculator

  1. enter the vertical rise of the roof per 12 inches of horizontal run
  2. the length of the heel cut in inches (e.g. 3 1/2)
  3. the size of the rafter in inches (e.g. 9 3/8)

Results

  1. the length of the seat cut in inches
  2. the vertical depth of the cut into the rafter (notch depth) in inches
  3. the remaining depth of the rafter measured from the top of the rafter to the deepest point of the birdsmouth notch into the rafter in inches
Birdsmouth Calculator - Calculate Seat and Heel Cut Lengths

Birdsmouth Calculator for Heel Cut

Rise :12
Seat Cut in
Rafter Depth in

Results:

Heel Cut in
Notch Size in
Remaining Depth in

Calculator

  1. enter the vertical rise of the roof per 12 inches of horizontal run
  2. the length of the seat cut in inches (e.g. 2 1/2)
  3. the size of the rafter in inches (e.g. 11 1/4)

Results

  1. the length of the heel cut in inches
  2. the vertical depth of the cut into the rafter (notch depth) in inches
  3. the remaining depth of the rafter measured from the top of the rafter to the deepest point of the birdsmouth notch into the rafter in inches

Vertical Depth Explained:

Vertical Depth of Notch:
  • This is the vertical distance from the rafter's bottom edge to the birdsmouth cut's deepest point. It determines how much of the rafter's depth is used to sit the rafter flatly on the support structure (e.g., a wall top plate).
  • The vertical depth is part of the birdsmouth, which consists of two cuts: the seat cut (horizontal cut) and the heel cut (angled cut).

Importance in Construction:

Structural Integrity:
  • The vertical depth is crucial for ensuring the rafter maintains adequate structural integrity after cutting the birdsmouth. It needs to be deep enough to allow for a stable fit on the supporting structure without compromising too much of the rafter's strength.
Building Codes:
  • Local building codes often specify maximum allowable depths for notches in rafters to prevent the structure from weakening. The notch depth should be a fraction of the rafter's total depth (often one-third or one-fourth).
Practical Considerations:
  • When making the birdsmouth cut, carpenters carefully measure the vertical depth to ensure it allows the rafter to fit securely while maintaining enough rafter material above the cut for strength.
  • The vertical depth, along with the seat cut and the heel cut, contributes to the correct positioning and angle of the rafter in relation to the roof slope.

Roof Framing

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