Birdsmouth Cut Calculator
Calculate birdsmouth cut dimensions for rafter construction with precise measurements
Calculate Birdsmouth Cut
Building Dimensions
Total width of the building from wall to wall
Thickness of the ridge board material
Roof Pitch
Birdsmouth Cut Specifications
Horizontal cut length (typically matches wall plate width)
Length of rafter extending beyond the wall
Birdsmouth Cut Results
Please Complete Required Fields
- ⚠️Please enter a valid building width
- ⚠️Please enter a valid ridge board thickness
- ⚠️Please enter a valid roof pitch
- ⚠️Please enter a valid seat cut length
Rafter Measurements
Cut Classification
Material Requirements
Safety Recommendations
• Low pitch angle may require special drainage considerations
Key Formulas:
Rafter Run: (Building Width - Ridge Thickness) ÷ 2
Heel Cut Depth: Seat Cut Length × tan(Pitch Angle)
Minimum Rafter Depth: 4 × Heel Cut Depth × cos(Pitch Angle)
Example Calculation
Standard Residential Roof
Building Width: 8m (26.2 feet)
Ridge Board: 5cm (2 inches)
Roof Pitch: 30° (57.7% slope, 6.9:12 ratio)
Seat Cut: 10cm (4 inches)
Calculation Steps
1. Rafter Run = (8m - 0.05m) ÷ 2 = 3.975m
2. Heel Cut Depth = 0.10m × tan(30°) = 0.058m = 5.8cm
3. Min Rafter Depth = 4 × 0.058m × cos(30°) = 0.20m = 20cm
4. Rafter Segment = 3.975m ÷ cos(30°) = 4.59m
Result: 5.8cm heel cut, 20cm minimum rafter depth
Cut Classification Guide
Shallow Cut
Less than 25mm (1")
Good for light loads
Standard
25-50mm (1-2")
Most residential use
Deep Cut
50-75mm (2-3")
Use with caution
Too Deep
Over 75mm (3")
May compromise integrity
Construction Tips
Use a speed square for accurate angle cuts
Mark both heel and seat cuts before cutting
Test fit on a small piece first
Follow International Residential Code guidelines
Ensure adequate rafter depth for structural integrity
Safety: Always check local building codes and consider professional consultation for structural elements.
Understanding Birdsmouth Cuts
What is a Birdsmouth Cut?
A birdsmouth cut is a notch cut into the underside of a rafter that allows it to sit securely on the top plate of a wall. It consists of two cuts: a horizontal seat cut and a vertical heel cut.
Why Use Birdsmouth Cuts?
- •Stability: Prevents rafters from sliding during installation
- •Load Transfer: Distributes roof loads evenly to the wall
- •Professional Appearance: Creates clean, precise joints
- •Code Compliance: Often required by building codes
Key Formulas
Rafter Run:
X = (W - t_ridge) ÷ 2
Heel Cut Depth:
c_h = c_s × tan(θ)
Min Rafter Depth:
d_min = 4 × c_h × cos(θ)
Height Above Plate:
y = 3 × c_h
IRC Rule: Rafter depth must be at least 4 times the heel cut depth to maintain structural integrity.
Cutting Process
Step 1: Mark the Rafter
Measure and mark the rafter segment length from the ridge end. Mark the seat cut location and heel cut depth.
Step 2: Make the Cuts
Cut the horizontal seat cut first, then the vertical heel cut. Use a circular saw or hand saw for precision.