Birdsmouth Cut Calculator

Birdsmouth Cut Calculator. A birdsmouth cut is the notch carved into a rafter so it sits flush and stable on top of a wall plate — and getting the angles and depths right is critical for a structurally sound roof. Enter your roof pitch, rafter depth, wall plate width, and desired seat cut depth into the Birdsmouth Cut Calculator to get the Plumb Cut Length. Secondary outputs include heel cut depth, actual seat depth, and remaining rafter depth — everything you need to mark and cut your rafters accurately. Also try the Rolls Needed — Roof Underlayment.

Roof Pitch (X in 12) *

/12

Rise in inches for every 12 inches of run

Rafter Depth *

Depth of the rafter (e.g., 2x8 = 7.25", 2x10 = 9.25")

Wall Plate Width *

Width of the wall plate the rafter sits on

Desired Seat Cut Depth *

Horizontal depth of the seat cut (typically equal to wall plate width)

Overhang Length

Length of rafter extending beyond the wall

Measurement Units *

Results

Plumb Cut Length

Heel Cut Depth

Actual Seat Depth

Remaining Rafter Depth

What is a birdsmouth cut?

A birdsmouth cut is a notch cut on the underside of a rafter where it sits on the wall plate. It consists of a horizontal seat cut and a vertical plumb cut (heel cut) that allows the rafter to rest securely on the wall while maintaining proper roof pitch. See also our Roof Shingle Calculator.

What are the purposes of a birdsmouth cut?

The birdsmouth cut provides a stable bearing surface for the rafter on the wall plate, prevents the rafter from sliding down the roof, transfers the roof load directly to the wall, and maintains the structural integrity of the rafter system.

How do I find the heel cut depth of a birdsmouth cut?

The heel cut depth is calculated using the roof pitch and seat cut depth. It equals the seat cut depth multiplied by the rise over run ratio (pitch/12). This ensures the cuts are perpendicular and the rafter sits properly on the wall plate.

What is the minimum ridge board depth for different rafter sizes?

The ridge board should be at least as deep as the rafter cut depth at the ridge. For 2x8 rafters, use at least a 2x8 ridge board. For 2x10 rafters, use at least a 2x10 ridge board to provide adequate bearing surface.

Is a birdsmouth cut necessary?

While not always required by code for smaller structures, birdsmouth cuts are highly recommended for proper load transfer and structural stability. They're essential for spans over 8 feet and required by most building codes for residential construction.

How deep can I cut the birdsmouth without weakening the rafter?

The birdsmouth cut should not exceed 1/3 of the rafter depth. For a 2x8 rafter (7.25" deep), the maximum cut depth is about 2.4 inches. Deeper cuts can significantly weaken the rafter and may violate building codes.

What tools do I need to make a birdsmouth cut?

You'll need a circular saw or hand saw for the initial cuts, and a jigsaw or handsaw to complete the notch. A speed square helps mark accurate angles, and a measuring tape ensures precise dimensions.

How do I mark the birdsmouth cut on the rafter?

Use a framing square to mark the plumb cut line at the correct angle, then measure and mark the seat cut depth perpendicular to the bottom of the rafter. The intersection of these lines defines your birdsmouth cut.