Birdsmouth Calculator
Calculate birdsmouth seat cut and plumb cut dimensions for rafters. Get precise notch measurements based on roof pitch and rafter size.
Roof Pitch
Rafter Size
Wall Plate Width
Birdsmouth Diagram
Seat Cut Width
3-1/2"
Set your speed square to 6 on the COMMON rafter scale. The plumb cut mark shows 26.6° from vertical.
- Cut a test rafter first and verify fit before cutting the batch
- Mark the birdsmouth location from the same reference on all rafters
- Use a handsaw or jigsaw to complete the inside corner - don't overcut with circular saw
- Consistent HAP is critical for a flat roof deck - check each rafter
- The seat cut rests ON the plate, the plumb cut sits AGAINST it
Related Calculators
About This Calculator
The Birdsmouth Calculator determines the precise dimensions for cutting the birdsmouth notch in roof rafters—the V-shaped notch where the rafter sits on the wall's top plate. A properly cut birdsmouth consists of a horizontal "seat cut" and a vertical "plumb cut" that work together to transfer roof loads directly into the wall structure while preventing the rafter from sliding off the plate.
Getting birdsmouth dimensions right is critical for structural integrity, consistent HAP (Height Above Plate) across all rafters, and proper load transfer. A birdsmouth cut too deep weakens the rafter; too shallow creates insufficient bearing and potential uplift problems. The IRC limits birdsmouth depth to 1/3 of rafter depth to preserve structural capacity.
This calculator provides exact measurements based on your roof pitch, rafter size, and wall plate width, including the seat cut width, plumb cut height, HAP remaining, and warnings if your configuration violates the 1/3 rule. In 2026, a single mis-cut rafter wastes $15-45 in lumber—accurate calculations before cutting save money and ensure a structurally sound roof.
Trusted Sources
How to Use the Birdsmouth Calculator
- 1Select your roof pitch (rise per 12" of run, e.g., 6/12).
- 2Choose your rafter lumber size (2×6, 2×8, 2×10, or 2×12).
- 3Enter your wall plate width (3.5" for 2×4 walls, 5.5" for 2×6 walls).
- 4Review the calculated seat cut width and plumb cut height.
- 5Verify the HAP meets your requirements and energy code compliance.
- 6Check that the seat cut does not exceed 1/3 of rafter depth (calculator will warn you).
- 7Use the visual diagram to mark and cut your rafters accurately.
Formula
Seat Cut Depth = Plate Width × tan(Pitch Angle)The seat cut depth is determined by the geometry of the roof pitch angle and the width of the wall plate. A steeper pitch creates a deeper seat cut for the same plate width. The plumb cut height equals the seat cut depth, and HAP (Height Above Plate) is the rafter depth minus the seat cut depth.
Understanding the Birdsmouth Cut
A birdsmouth is the notch cut into a rafter that allows it to sit flat on the top plate of a wall. The name comes from its resemblance to a bird's open beak when viewed from the side.
Components of a Birdsmouth:
| Component | Description | Function |
|---|---|---|
| Seat Cut | Horizontal cut resting on plate | Provides bearing surface |
| Plumb Cut (Heel Cut) | Vertical cut against wall | Resists horizontal thrust |
| HAP | Height Above Plate | Determines roof deck height |
| Heel | Triangular cutout portion | Material removed |
Why Birdsmouths Are Essential: Without a birdsmouth, rafters would only make point contact with the wall—a tiny area that can't distribute loads properly. The birdsmouth creates:
- Full bearing surface for vertical load transfer
- Positive resistance against outward thrust
- Consistent rafter height for flat roof decking
- Solid nailing surface at the plate connection
Birdsmouth vs. Alternative Methods:
| Method | Pros | Cons |
|---|---|---|
| Birdsmouth in rafter | Standard, proven, code-approved | Reduces rafter depth |
| Notched plate | Keeps full rafter depth | Weakens plate, not for load-bearing |
| Bird blocks | Allows raised heel | Extra material and labor |
| Rafter hangers | No notching | Expensive hardware |
The Critical 1/3 Rule (IRC R802.6)
Building codes require that the birdsmouth seat cut not exceed 1/3 of the rafter's total depth. This preserves the rafter's structural integrity where bending forces are highest.
Maximum Seat Cuts by Rafter Size:
| Rafter Size | Actual Depth | Max Seat Cut | Min HAP Remaining |
|---|---|---|---|
| 2×4 | 3.5" | 1.17" | 2.33" |
| 2×6 | 5.5" | 1.83" | 3.67" |
| 2×8 | 7.25" | 2.42" | 4.83" |
| 2×10 | 9.25" | 3.08" | 6.17" |
| 2×12 | 11.25" | 3.75" | 7.50" |
Why the 1/3 Limit Matters: The bottom 1/3 of a rafter is in tension under load, the top 1/3 in compression, and the middle 1/3 (neutral axis) experiences minimal stress. Cutting into the bottom tension zone creates a weak point where the rafter can crack under heavy snow or wind uplift.
When Seat Cut Would Exceed 1/3:
| Pitch | 3.5" Plate | Exceeds 1/3 on: |
|---|---|---|
| 4/12 | 1.17" | Nothing |
| 6/12 | 1.75" | 2×6 (barely OK) |
| 8/12 | 2.33" | 2×6 and smaller |
| 10/12 | 2.92" | 2×8 and smaller |
| 12/12 | 3.50" | 2×10 and smaller |
Solutions When Seat Cut Would Be Too Deep:
- Use larger rafters (upgrade 2×8 to 2×10)
- Reduce seat cut width and add beveled blocking
- Use a raised heel detail with vertical blocking
- Switch to engineered trusses
- Consult structural engineer for alternatives
Calculating Birdsmouth Dimensions
Birdsmouth dimensions are determined by roof pitch, plate width, and rafter size. Here are the formulas and example calculations:
Key Formulas:
Pitch Angle (degrees):
Pitch Angle = arctan(Rise ÷ 12) × (180 ÷ π)
Example: 6/12 pitch = arctan(0.5) × 57.3 = 26.57°
Seat Cut Depth (plumb cut height):
Seat Cut Depth = Plate Width × tan(Pitch Angle)
Example: 3.5" plate at 6/12 = 3.5" × tan(26.57°) = 3.5" × 0.5 = 1.75"
Height Above Plate (HAP):
HAP = Rafter Depth - Seat Cut Depth
Example: 2×8 rafter = 7.25" - 1.75" = 5.5"
Birdsmouth Dimensions by Pitch (3.5" plate):
| Pitch | Pitch Angle | Seat Cut Depth | HAP (2×6) | HAP (2×8) | HAP (2×10) |
|---|---|---|---|---|---|
| 3/12 | 14.0° | 0.87" | 4.63" | 6.38" | 8.38" |
| 4/12 | 18.4° | 1.17" | 4.33" | 6.08" | 8.08" |
| 5/12 | 22.6° | 1.46" | 4.04" | 5.79" | 7.79" |
| 6/12 | 26.6° | 1.75" | 3.75" | 5.50" | 7.50" |
| 7/12 | 30.3° | 2.04" | 3.46" | 5.21" | 7.21" |
| 8/12 | 33.7° | 2.33" | 3.17" | 4.92" | 6.92" |
| 9/12 | 36.9° | 2.63" | 2.87"⚠️ | 4.62" | 6.62" |
| 10/12 | 39.8° | 2.92" | 2.58"⚠️ | 4.33" | 6.33" |
| 12/12 | 45.0° | 3.50" | 2.00"⚠️ | 3.75"⚠️ | 5.75" |
⚠️ = Exceeds 1/3 rule, use larger rafter or alternative detail
Marking and Cutting Techniques
Accurate birdsmouth layout is critical—a 1/8" error on one rafter becomes visible waviness when multiplied across an entire roof.
Layout Procedure:
- Establish the birdsmouth location by measuring from the ridge plumb cut
- Mark the plumb line (heel cut) using speed square at pitch setting
- Mark the seat cut (level line) perpendicular to plumb line
- Verify seat cut width equals plate width (3.5" or 5.5")
- Double-check that seat cut depth does not exceed 1/3 rule
- Cut the pattern rafter and test-fit before cutting remaining rafters
Tools Required:
| Tool | Purpose | Quality Level |
|---|---|---|
| Speed square | Marking pitch angles | Swanson or Empire |
| Framing square | Layout on wide stock | Steel, not aluminum |
| Stair gauges | Consistent angle setting | Brass preferred |
| Tape measure | Measuring locations | 25' minimum |
| Pencil | Sharp layout lines | Carpenter's pencil |
| Circular saw | Primary cutting tool | 7-1/4" minimum |
| Reciprocating saw | Completing inside corner | For tight cuts |
| Chisel | Cleaning up corners | 3/4" or 1" width |
Cutting Steps:
- Set up saw depth - Just through the lumber thickness
- Cut plumb line first - Stop at intersection point
- Cut seat line second - Stop at intersection point
- Complete corner - Use reciprocating saw or handsaw
- Clean up - Chisel if needed for clean corner
- Test fit - Check on actual wall plate before mass cutting
Common Errors and Prevention:
| Error | Result | Prevention |
|---|---|---|
| Wrong pitch setting | All angles off | Verify with pitch gauge |
| Overcut with circular saw | Weakened rafter | Stop 1/2" short, finish with handsaw |
| Inconsistent seat cut width | Varying HAP | Use stop block or template |
| Measuring from wrong edge | Length errors | Always measure along TOP edge |
Raised Heel and Energy Code Compliance
Modern energy codes often require raised heel details to accommodate full insulation depth at eaves:
Standard vs. Raised Heel Birdsmouth:
| Configuration | HAP | Insulation Space | Code Compliance |
|---|---|---|---|
| Standard 6/12 on 2×8 | 5.5" | R-19 to R-22 | Often fails |
| Raised 6" | 11.5" | R-30 | Marginal |
| Raised 10" | 15.5" | R-38 | Most codes |
| Raised 14" | 19.5" | R-49+ | Cold climates |
2026 Energy Code Requirements (IECC):
| Climate Zone | Required Ceiling R | Minimum Heel Height |
|---|---|---|
| Zone 1-2 | R-38 | 10-11" |
| Zone 3 | R-38 | 10-11" |
| Zone 4-5 | R-49 | 14-15" |
| Zone 6-8 | R-49 | 14-15" |
Methods to Achieve Raised Heel:
1. Raised Birdsmouth:
- Cut seat higher on rafter (more HAP)
- Limited by 1/3 rule
- Works for moderate raises
2. Vertical Block Method:
- Standard birdsmouth
- Add vertical 2× block above plate
- Rafter sits on block top
- Most common raised heel method
3. Bird Block Method:
- Beveled blocks between rafters at plate
- Creates airflow baffles
- Supports raised insulation
4. Energy Truss:
- Factory-built with raised heel
- Consistent quality
- Most efficient for new construction
Cost Impact of Raised Heel:
| Item | Additional Cost |
|---|---|
| Extra blocking lumber | $30-80 |
| Taller fascia boards | $40-100 |
| Additional labor | $100-300 |
| Energy savings (annual) | $150-400 |
| Payback period | 6-18 months |
2026 Rafter Lumber Costs
Accurate birdsmouth calculations prevent costly mistakes. Current lumber pricing:
Dimensional Lumber for Rafters (2026):
| Size | Price/LF | 12' Length | 16' Length | 20' Length |
|---|---|---|---|---|
| 2×6 | $0.80-1.20 | $10-14 | $13-19 | $16-24 |
| 2×8 | $1.10-1.60 | $13-19 | $18-26 | $22-32 |
| 2×10 | $1.50-2.10 | $18-25 | $24-34 | $30-42 |
| 2×12 | $2.00-2.80 | $24-34 | $32-45 | $40-56 |
Cost of Cutting Errors:
| Error Type | Wasted Material | Cost |
|---|---|---|
| Wrong pitch angle | 1 rafter | $15-45 |
| Birdsmouth too deep | 1 rafter | $15-45 |
| Wrong rafter length | 1 rafter | $15-45 |
| Pattern rafter error | All rafters (60-100) | $900-4,500 |
Material Savings from Accurate Calculations:
| Approach | Waste Factor | On 60-Rafter Roof |
|---|---|---|
| Estimate and cut | 15-20% waste | $200-400 extra |
| Calculate once | 8-10% waste | $100-200 extra |
| Pattern rafter method | 3-5% waste | $50-100 extra |
When to Upgrade Rafter Size:
| Situation | Standard Size | Upgrade To | Cost Increase |
|---|---|---|---|
| Steep pitch (10+/12) | 2×6 | 2×8 | $3-5/rafter |
| Heavy snow loads | 2×8 | 2×10 | $5-8/rafter |
| Long spans (16'+) | 2×8 | 2×10 or 2×12 | $5-12/rafter |
| Raised heel required | 2×6 | 2×8 | $3-5/rafter |
Birdsmouth Variations for Special Situations
Not all birdsmouths are created equal—different situations require modified approaches:
Hip and Valley Rafters:
- Deeper birdsmouth due to compound angles
- Backing cut may be needed at birdsmouth
- Seat cut typically wider for stability
- Requires careful layout with compound angles
Shed Roof (Single Slope):
- Birdsmouth at low wall only (usually)
- High wall may use ledger board instead
- Simpler than gable roof birdsmouths
- Same 1/3 rule applies
Cathedral Ceiling Rafters:
- May need deeper birdsmouth for adequate HAP
- Often combined with structural ridge beam
- Insulation considerations affect birdsmouth height
- Consider raised heel for R-value at eaves
Porch and Overhang Rafters:
- May have birdsmouth AND notch for beam
- Multiple bearing points require careful layout
- Lookout rafters may not need birdsmouth
- Verify structural path for each rafter type
Existing Roof Tie-Ins:
- Must match existing birdsmouth height (HAP)
- Measure existing rafters carefully
- May require non-standard birdsmouth depth
- Shimming at plate may be needed to match
Compound Pitch Intersections:
- Where two different pitches meet
- Birdsmouth heights may not match
- Requires valley or hip detail to transition
- Consider pre-built trusses for complex situations
Steel Plate Connection Alternative: For situations where birdsmouth isn't ideal:
| Product | Application | Cost |
|---|---|---|
| Simpson LSU | Light loads | $5-8 each |
| Simpson LUS | Heavier loads | $8-12 each |
| Simpson?"U | Adjustable | $10-15 each |
Birdsmouth Layout with Speed Square
The speed square is the most efficient tool for marking birdsmouths. Here's how to use it:
Speed Square Basics:
- "Pivot point" at corner for rotating around board edge
- "Common" scale gives pitch angles
- "Hip/Valley" scale for compound cuts
- Lip hooks over board edge for consistent positioning
Setting Up for Birdsmouth:
- Locate pitch number on "Common" scale
- Position pivot point on top edge of rafter
- Rotate until pitch mark aligns with board edge
- Draw plumb line along square edge
Marking the Seat Cut:
- Draw plumb line at birdsmouth location
- Measure seat cut width (3.5" or 5.5") from plumb line
- Draw level line perpendicular to plumb line
- Connect to form the birdsmouth notch outline
Using a Framing Square: For wider rafters or more precision:
- Set stair gauges at 12 and pitch rise (e.g., 6 for 6/12)
- Position body (24" side) along rafter edge
- Tongue (16" side) gives plumb cut direction
- Body gives level cut direction
- Clamp gauges for consistent, repeatable layout
Creating a Birdsmouth Template: For production cutting, make a plywood template:
- Cut birdsmouth shape from 1/4" plywood
- Include seat cut and plumb cut lines
- Add positioning marks for rafter edge
- Drill holes for quick pencil marking
- Use for all identical rafters
Digital Tools:
| App/Tool | Platform | Features |
|---|---|---|
| Construction Master | iOS/Android | All rafter calculations |
| Calculated Industries | Calculator | Dedicated framing model |
| Speed Square Pro | iOS | Visual layout assistance |
Pro Tips
- 💡Always cut and test-fit ONE pattern rafter before cutting the batch—a birdsmouth error multiplied across 60 rafters is an expensive mistake.
- 💡Mark all birdsmouth locations from the same reference point (usually the ridge plumb cut) for consistency—never measure from the tail.
- 💡Keep all seat cuts exactly equal depth for consistent HAP—inconsistent cuts create a wavy roof deck that shows through shingles.
- 💡For steep pitches (over 8/12), verify the seat cut against the 1/3 rule before cutting—upgrade rafter size if needed.
- 💡Use a handsaw or reciprocating saw to complete the inside corner of the notch—circular saws overcut and weaken the rafter.
- 💡Apply construction adhesive at the birdsmouth seat before nailing—this dramatically improves uplift resistance in high-wind areas.
- 💡Check your speed square accuracy before starting—a dropped square can bend the pivot, throwing off all pitch angles.
- 💡For raised heel details, cut a taller birdsmouth on a deeper rafter rather than adding blocking—it's faster and stronger.
- 💡Mark the "crown" on each rafter and install crown-up—this prevents sagging and ensures the birdsmouth seats properly.
- 💡When cutting multiple identical rafters, clamp a stop block for consistent birdsmouth location—this is faster and more accurate than measuring each one.
- 💡Seal birdsmouth cuts on pressure-treated lumber with end-cut preservative—the exposed end grain is vulnerable to rot.
- 💡For retrofitting rafters to match existing, measure HAP on several existing rafters and use the average—old rafters may have settled unevenly.
Frequently Asked Questions
The name comes from the notch's resemblance to a bird's open beak when viewed from the side. The horizontal seat cut forms the bottom of the "beak" and the vertical plumb cut forms the back, creating a distinctive V-shape that has been used in timber construction for over 2,000 years.
