Rafter-Birdsmouth-Cut
The birdsmouth is the notch where a rafter sits on the wall plate. Plumb cut follows roof pitch; seat cut is horizontal. IRC limits seat depth to 1/3 of rafter depth.
Why This Construction Metric Matters
Why: Correct birdsmouth ensures structural integrity and code compliance. Exceeding 1/3 seat depth weakens the rafter.
How: Plumb angle = arctan(pitch/12). Seat angle = 90ยฐ โ plumb. Max seat depth = rafter depth รท 3. HAP = depth โ seat depth.
- โ4:12 pitch โ 18.43ยฐ plumb; 6:12 โ 26.57ยฐ; 12:12 โ 45ยฐ.
- โIRC and IBC limit seat cut to 1/3 of rafter depth.
- โMinimum HAP typically 2" for insulation clearance.
- โMark plumb cut first, then seat cut; use speed square.
Birdsmouth Cut Calculator
Plumb & seat angles โข HAP โข Code compliance โข Roof framing
Quick Examples โ Click to Load
2ร6 Rafter 4:12 Pitch
Standard residential rafter
2ร8 Rafter 6:12 Pitch
Medium pitch roof
2ร10 Rafter 8:12 Pitch
Steeper residential roof
2ร12 Rafter 12:12 Pitch
45ยฐ steep roof
Low Slope 3:12
Shallow pitch roof
Maximum Seat Cut
Maximum allowed depth (1/3)
๐ Rafter Size
๐ Roof Pitch
Common pitches: 3:12 (shallow), 4:12 (standard), 6:12 (medium), 8:12 (steep), 12:12 (45ยฐ)
๐งฑ Wall Plate
โ๏ธ Cut Specifications
๐ Building Code
Plumb: 18.43ยฐ | Seat: 71.57ยฐ | HAP: 3.667 in | Non-Compliant โ
๐ Results
Plumb Cut Angle
18.43ยฐ
Seat Cut Angle
71.57ยฐ
Seat Cut Length
3.689 in
HAP
3.667 in
Bevel Angle
18.43ยฐ
For compound miter saw
Max Seat Depth
1.833 in
Code limit (1/3 rule)
Code Compliance
โ ๏ธ Non-Compliant
โ ๏ธ Code Warnings
- Seat depth limited to maximum allowed (1/3 of rafter depth)
- โ ๏ธ Seat cut depth (1.833") exceeds code maximum (1.833")
Step-by-Step Calculation
=== Birdsmouth Cut Calculation === Rafter Size: 1.50" ร 5.50" Roof Pitch: 4:12 Wall Plate Width: 3.50" === Plumb Cut Angle === Plumb Angle = arctan(Pitch/12) Plumb Angle = arctan(4/12) Plumb Angle = arctan(0.3333) Plumb Angle = 18.43ยฐ === Seat Cut Angle === Seat Angle = 90ยฐ - Plumb Angle Seat Angle = 90ยฐ - 18.43ยฐ Seat Angle = 71.57ยฐ === Maximum Seat Cut Depth === Max Seat Depth = Rafter Depth / 3 Max Seat Depth = 5.50" / 3 Max Seat Depth = 1.833" Building Code Limit: Maximum 1/3 of rafter depth === Seat Cut Length === Seat Length = Wall Plate Width / cos(Plumb Angle) Seat Length = 3.50" / cos(18.43ยฐ) Seat Length = 3.50" / 0.9487 Seat Length = 3.689" === HAP (Height Above Plate) === Seat Cut Depth (calculated): 1.833" Based on HAP requirement: 2.00" HAP = Rafter Depth - Seat Depth HAP = 5.50" - 1.833" HAP = 3.667" === Bevel Angle (Compound Miter) === Bevel Angle = Plumb Cut Angle Bevel Angle = 18.43ยฐ Set your compound miter saw to this angle
๐ Visualizations
๐ HAP Comparison
๐ Cut Angles
๐ Seat Depth Comparison
Planning estimates only. Verify with a licensed engineer or contractor before construction.
๐ Construction Industry Facts
Plumb cut angle = arctan(Pitch/12); 4:12 = 18.43ยฐ.
โ IRC
Max seat cut depth = 1/3 of rafter depth per IRC.
โ IRC R802
HAP (Height Above Plate) ensures structural depth remains.
โ AWC
Named for resemblance to a bird's open mouth.
โ Framing tradition
๐ Key Takeaways
- โข Plumb angle = arctan(Pitch/12)
- โข Seat angle = 90ยฐ โ plumb angle
- โข Max seat depth = rafter depth รท 3 (1/3 rule)
- โข HAP = Height Above Plate = rafter depth โ seat depth
What is a Birdsmouth Cut?
A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the wall plate. This cut consists of two parts: the plumb cut (vertical cut that follows the roof pitch) and the seat cut (horizontal cut that sits on the plate). The birdsmouth allows the rafter to sit securely on the wall plate while maintaining proper roof pitch and structural integrity.
Plumb Cut
Vertical cut matching roof pitch
Seat Cut
Horizontal cut for plate contact
HAP
Height Above Plate measurement
Code Compliance
Verify building code limits
How to Calculate Birdsmouth Cuts
Key Formulas
Convert roof pitch to angle
Complementary angle for seat cut
Building code limit (1/3 rule)
Quick Reference
When to Use Birdsmouth Cuts
Residential Roofs
Standard rafter-to-plate connections
Framing Projects
Traditional stick framing
DIY Roofing
Homeowner roof projects
Precision Cuts
Accurate angle calculations
Birdsmouth Formulas
Building Code Requirements
| Code | Max Seat Depth | Min HAP | Description |
|---|---|---|---|
| International Residential Code | 33.33% | 2" | Standard residential code - max 1/3 depth cut |
| International Building Code | 33.33% | 2" | Commercial building code |
| Conservative (25%) | 25% | 2.5" | More conservative approach |
โ ๏ธ Important: Max seat cut depth is typically 1/3 of rafter depth. Verify local codes before cutting.
Common Pitch Angles
Frequently Asked Questions
Why is it called a birdsmouth?
The notch resembles a bird's open mouth when viewed from the sideโthe plumb and seat cuts form a V shape.
Can I cut deeper than 1/3?
Generally no. IRC and IBC limit seat depth to 1/3 of rafter depth. Deeper cuts weaken the rafter.
What is HAP used for?
HAP (Height Above Plate) ensures adequate rafter depth remains for structural strength and insulation clearance.
Cutting Tips
- โข Mark plumb cut first, then seat cut
- โข Use a speed square or rafter square for accuracy
- โข Cut seat depth slightly shallow and test fit
- โข Verify local building codes before cutting
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