Birdsmouth Cut Calculator

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Expert Verified by: David Chen, Senior Construction Consultant & Licensed General Contractor

Accurately calculate the dimensions for your rafter’s birdsmouth cut. This professional tool helps carpenters and DIY builders determine the seat cut, heel cut, and HAP (Height Above Plate) based on roof pitch and rafter size.

Birdsmouth Cut Calculator

Please enter values to calculate results.

Birdsmouth Cut Calculator Formula

Angle (θ): arctan(Pitch / 12)

Heel Cut (Vertical): Seat Cut × tan(θ)

HAP (Height Above Plate): (Rafter Depth / cos(θ)) – Heel Cut

Formula Source: Fine Homebuilding, International Code Council (ICC)

Variables Explanation

  • Roof Pitch: The rise of the roof for every 12 inches of run.
  • Rafter Depth: The actual width of the lumber (e.g., a 2×8 is 7.25″).
  • Seat Cut Width: The horizontal length of the cut that sits on the wall plate.

What is a Birdsmouth Cut?

A birdsmouth cut is a structural notch cut into a roof rafter where it meets the top plate of a wall. It consists of two distinct cuts: the seat cut (horizontal) and the heel cut (vertical or plumb).

Ensuring the correct depth of the birdsmouth cut is critical for structural integrity. Code usually dictates that no more than one-third of the rafter’s depth should be removed to maintain its load-bearing capacity.

How to Calculate Birdsmouth Cut (Example)

  1. Determine your roof pitch (e.g., 6/12).
  2. Measure your rafter’s actual depth (e.g., 7.25″).
  3. Decide your seat cut width based on wall thickness (e.g., 3.5″).
  4. Calculate the angle: atan(6/12) = 26.57°.
  5. Calculate Heel Cut: 3.5 × tan(26.57°) = 1.75″.

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Frequently Asked Questions (FAQ)

How deep should a birdsmouth cut be? Typically, the heel cut should not exceed 1/3 of the rafter’s depth to ensure the rafter remains strong enough to support the roof load.

What is HAP in framing? HAP stands for “Height Above Plate.” It is the vertical distance from the top of the wall plate to the top edge of the rafter.

Can I use a birdsmouth cut on a truss? No, you should never cut into a pre-engineered roof truss without consulting the manufacturer or a structural engineer.

What tools are needed for a birdsmouth cut? A framing square and a circular saw are standard, often finished with a hand saw for precision in the corner.

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