How to Lay Out and Cut Rafter Birdsmouth Joints

Birdsmouth cut diagram showing seat cut, plumb cut, and HAP dimensions on a rafter sitting on a wall top plate

Understanding the Birdsmouth Joint

The birdsmouth is the notch cut into a rafter where it bears on the top plate of the wall. It consists of two cuts:

Plumb cut (or cheek cut): Vertical cut facing the interior of the building
Seat cut (or heel cut): Horizontal cut that bears on the wall top plate

When properly cut and installed, the birdsmouth distributes rafter load across the plate and prevents the rafter from splitting at the bearing point. A poorly designed birdsmouth can fail—either the rafter cracks above the notch or the bearing becomes inadequate.

The IRC Depth Limit: The Most Critical Rule

IRC R802.7.1 states: The seat cut depth cannot exceed 1/3 of the rafter depth.

For common rafter sizes:

Rafter Size	Full Depth	Max Seat Cut Depth
2x6	5.5”	1.83” (1.75” practical)
2x8	7.25”	2.42” (2.4” practical)
2x10	9.25”	3.08” (3” practical)
2x12	11.25”	3.75”

Why the limit? Cutting too deep removes too much bearing area and leaves insufficient wood above the cut. The rafter becomes a lever that splits at the birdsmouth. Buildings have failed because carpenters cut deep seats to minimize plumb cut angle.

Always verify your rafter size on the plans and check depth limits before cutting.

Calculating HAP (Height Above Plate)

HAP is the vertical distance from the top plate to the bottom of the rafter at the seat cut. It determines the balance of the two birdsmouth cuts.

The relationship is:

Seat cut depth = (Rafter rise/run) × HAP
HAP = Seat cut depth ÷ (Rafter rise/run)

Example: 6/12 pitch rafter, target 1.5” seat cut depth

6/12 pitch = 6” rise per 12” run = 0.5 rise per 1” run

HAP = 1.5” ÷ 0.5 = 3.0”

So the rafter sits 3” above the plate (measured vertically from plate top to rafter bottom).

Common HAP values by pitch:

Pitch	Rise/Run	HAP for 1” Seat	HAP for 1.5” Seat	HAP for 2” Seat
4/12	0.333	3.0”	4.5”	6.0”
5/12	0.417	2.4”	3.6”	4.8”
6/12	0.500	2.0”	3.0”	4.0”
7/12	0.583	1.72”	2.57”	3.43”
8/12	0.667	1.5”	2.25”	3.0”
10/12	0.833	1.2”	1.8”	2.4”
12/12	1.0	1.0”	1.5”	2.0”

Design approach: Pick a seat cut depth that:

Doesn’t exceed 1/3 rafter depth (IRC limit)
Provides at least 3-4” bearing length on the plate
Keeps plumb cut angle reasonable (not excessively steep)

For most residential rafters (6/12 to 8/12 pitch), a 1.5” to 2” seat works well.

Layout on the Rafter: Three-Step Method

Step 1: Mark the Plumb Line (Roof Slope)

Using a speed square and pencil:

Place speed square with the pitch in the corner notch (e.g., 6 for a 6/12 pitch)
The square’s hypotenuse aligns with the rafter top edge
Mark a line down the rafter perpendicular to the top edge
This is the plumb line at the ridge

For the birdsmouth, mark the plumb line at the bearing end of the rafter (same angle as the ridge plumb cut).

Speed square setup:

The square has a corner notch marked with pitch numbers (4, 5, 6, etc.)
Place the notch on the rafter edge
The pitch number should align with the rafter top
The other edge of the square gives you the plumb line

Step 2: Mark the Seat Line (Level Line)

From the plumb line, measure down the rafter a distance equal to the HAP. Draw a line perpendicular to the rafter’s length at this point.

This is the seat line and should be horizontal (level) when the rafter is installed.

Marking with the speed square:

From the plumb line mark, measure down the rafter length by the HAP value
Place the speed square’s level edge at this point
Mark a line across the rafter perpendicular to its length
This is your seat line

Step 3: Mark the Seat Cut Depth

From the bottom edge of the rafter, measure up toward the top a distance equal to the seat cut depth. The point where this vertical measurement meets the seat line is your seat cut depth endpoint.

Simple method:

At the plumb line, mark the seat cut depth vertically (e.g., 1.5”)
From the seat line down, mark the same distance (1.5” for 1.5” seat)
Connect these marks with a diagonal line—this is your seat cut line

The birdsmouth now has:

Plumb cut: From the top of the rafter down to where the 1.5” mark meets the plumb line
Seat cut: From that point diagonally to where the seat line hits the bottom of the rafter

Marking with Speed Square: Detailed Process

For a 6/12 pitch rafter with 3” HAP and 1.5” seat depth:

Identify the bird line (where the birdsmouth is located along the rafter length, usually 16-24” from the bird’s mouth, measured along the rafter):
- Measure along the rafter from the tail
- This distance is determined by overhang and seat support location
At the bird line, mark the plumb line:
- Place speed square with 6 in the notch corner
- Align 6 with the top edge of the rafter
- Draw a vertical line down the rafter
Measure down the rafter 3 inches (HAP):
- From the plumb line, measure 3” down the rafter length
- This is where the seat line intersects the plumb line
Mark the seat line:
- At the 3” mark, draw a perpendicular line across the rafter
- This line represents the top of the bearing (where the plate touches)
Mark seat cut depth:
- From the top of the rafter at the plumb line, measure down 1.5” perpendicular to the rafter top
- From the seat line, measure down 1.5” perpendicular to the rafter length
- Connect these two points with a diagonal line
- This diagonal is your seat cut line
Mark the heel line:
- The heel line is the corner where plumb cut meets seat cut
- It should fall on the plate when installed

Common Birdsmouth Configurations

Standard Roof (6/12 pitch, no overhang):

Plumb cut: 33.7° from vertical (calculated from pitch)
Seat cut: 1.5” depth
HAP: 3”
Bearing on plate: Approximately 4” along the plate surface

Saltbox or Cathedral Roof (Steeper pitch, 10/12):

Plumb cut: 39.8° from vertical (steeper angle)
Seat cut: 1.5” depth (may be only 1” due to rafter depth)
HAP: 1.8”
Bearing: Approximately 2.5” along the plate

Low-pitch Roof (4/12, wide overhangs):

Plumb cut: 18.4° from vertical (nearly flat)
Seat cut: 1.5” to 2” (more latitude for depth)
HAP: 4.5” to 6” (larger vertical distance)
Bearing: Approximately 5-6” along the plate

Avoiding Common Cutting Mistakes

Problem: Plumb cut and seat cut don’t meet

This happens when one line is marked incorrectly. The two cuts must meet at a point (the heel). If they don’t:

Re-mark the plumb line using the speed square
Verify the seat line is truly perpendicular to the rafter length
Ensure HAP measurement is accurate

Problem: Rafter rocks on the plate (bearing area too small)

Seat cut is too shallow or HAP is too large. The rafter needs 3-4 inches of bearing length along the plate top.

If bearing is marginal:

Decrease HAP (but this deepens seat cut—check IRC limit)
Use a larger seat cut depth (but this increases compressive force at the heel)
Add a rafter tie or collar tie for lateral support

Problem: Rafter cracks above the birdsmouth

The seat cut was cut too deep, removing more than 1/3 of rafter depth. This is the most common failure mode.

Measure the rafter depth and verify 1/3 limit
For 2x8, max seat is 2.4” (measure actual, not nominal)
If existing rafters have deep cuts, reinforce with sistering or metal straps per engineer

Problem: Marking is off by 1/8”, rafter won’t fit

Speed square or measuring error. For production cutting (multiple rafters):

Cut one test rafter
Fit it on the building
Mark and adjust if needed
Use that rafter as a template for remaining cuts (trace the birdsmouth onto new stock)

IRC Requirements and Bearing

IRC R802.7 specifies:

Rafters must have a minimum 1.5” bearing on wall plates
Seat cut depth maximum is 1/3 rafter depth
Plumb cuts must be at the appropriate angle for roof pitch

The 1.5” minimum bearing applies to the depth of contact along the plate (not the HAP). A properly designed birdsmouth provides 3-5” of bearing length, which is more than adequate.

Framing Connections at Birdsmouth

After cutting, fasten the rafter to the plate:

Standard connection:

Two 16d nails or 3” screws through the plumb cut into the top plate
Or metal rafter tie/hurricane tie bolted to the plate

For high wind or seismic zones:

Structural connectors required
Verify local code for connector type and fastening

Nails alone may be insufficient in wind/seismic areas. Check your building department.

Using a Template for Efficiency

When framing multiple rafters (20-30 per roof):

Cut one complete rafter and test fit it
Adjust the birdsmouth if needed
Use that rafter as a cutting template:
- Clamp it to raw lumber
- Trace around the birdsmouth with a pencil
- Cut remaining rafters to the traced lines
Speed and accuracy improve dramatically
Ensure consistency across all rafters

Templates save time and eliminate marking errors.

Quick Reference: Standard Configuration

For residential 6/12 roof, 2x8 or 2x10 rafter:

Parameter	Value
Pitch	6/12
Plumb cut angle	33.7°
Seat cut depth	1.5”
HAP (height above plate)	3.0”
Minimum bearing on plate	1.5” (code min)
Typical bearing length	3-4”
Speed square setting	6 in notch corner

Marking sequence:

Speed square plumb line at bird cut location
Measure 3” down rafter from plumb line
Draw seat line perpendicular to rafter
Mark 1.5” down from top at plumb line
Mark 1.5” down from seat line
Connect for diagonal seat cut line
Cut plumb first, then seat
Test fit before cutting remaining rafters

The birdsmouth is critical. Don’t skip the IRC depth check. A rafter that cracks above the notch can collapse and compromise roof integrity. Taking time to layout and cut properly prevents callbacks and building failures.