What is a birdsmouth cut?

A birdsmouth is a notch cut into the bottom of a rafter so it can sit flat on a wall plate. It typically includes a level seat cut and a plumb cut.

How much can I notch a rafter for a birdsmouth?

A common rule of thumb is to limit the birdsmouth notch depth to about one-third of the rafter depth. Requirements vary by jurisdiction and loading; follow local code or an engineer’s guidance.

What does HAP mean in roof framing?

HAP stands for Height Above Plate. It is the vertical distance from the top of the wall plate to the top of the rafter at the outside edge of the wall.

Birdsmouth Calculator

Birdsmouth Cut Calculator

Use this calculator to estimate birdsmouth notch depth and HAP (height above plate) based on roof pitch, rafter depth, and seat cut length (bearing on the wall plate).

Birdsmouth Cut Calculator

Estimate HAP and notch depth from pitch, rafter size, and seat length

Pitch angle

Seat cut length (plate width)

Rafter depth (actual)

Results

Enter dimensions above to calculate

Formulas used

The calculator converts pitch into an angle, then uses basic right-triangle relationships to estimate notch depth and height above plate.

1. Pitch to Roof Angle

Formula:

θ = arctan(Rise/Run) (e.g., Rise/12 in imperial)

Birdsmouth layout depends on the roof slope. The calculator converts pitch into an angle so it can translate the seat length into a notch depth.

2. Notch Depth from Seat Length

Formula:

Notch depth (vertical) = Seat length × tan(θ)

The seat cut length (bearing on the plate) creates a horizontal run. Over that run, the roof rises by tan(θ), which determines the depth of the birdsmouth notch.

3. Height Above Plate (HAP)

Formula:

HAP = (Rafter depth × cos(θ)) − Notch depth

HAP is the vertical height from the top of the wall plate to the top of the rafter at the outside edge. It helps you check if you have enough rafter depth after cutting the birdsmouth.

4. Notch Depth Rule of Thumb

Formula:

Notch depth ≤ 1/3 of rafter depth

A common framing rule is to keep the birdsmouth notch shallower than one-third of the rafter depth to avoid weakening the rafter. Always follow local code and engineering requirements.

Example

2×8 rafter (7.25 in actual depth) with a 6/12 pitch and a 3.5 in seat cut.

θ = arctan(6/12) ≈ 26.6°
Notch depth = 3.5 × tan(θ) ≈ 1.75 in
HAP = (7.25 × cos(θ)) − 1.75 ≈ 4.74 in

Frequently Asked Questions

What is a birdsmouth cut?: A birdsmouth is a notch cut into the bottom of a rafter so it can sit flat on a wall plate. It typically includes a level seat cut and a plumb cut.
How much can I notch a rafter for a birdsmouth?: A common rule of thumb is to limit the birdsmouth notch depth to about one-third of the rafter depth. Requirements vary by jurisdiction and loading; follow local code or an engineer’s guidance.
What does HAP mean in roof framing?: HAP stands for Height Above Plate. It is the vertical distance from the top of the wall plate to the top of the rafter at the outside edge of the wall.