🔨 Birdsmouth Cut Calculator
Calculate birdsmouth cut dimensions for rafters
Actual width of rafter (e.g., 3.5 for 2×4, 5.5 for 2×6)
Rise in inches per 12 inches of run (e.g., 6 for 6/12 pitch)
Leave blank for automatic calculation (typically 1/3 of rafter width)
Width of wall plate (e.g., 3.5 for 2×4, 5.5 for 2×6)
Leave blank for automatic calculation based on seat depth and pitch
How to Use This Calculator
Enter Rafter Dimensions
Input the rafter width in inches (actual width, not nominal). For example, enter "3.5" for a 2×4 (actual width 3.5 inches) or "5.5" for a 2×6.
Enter Roof Pitch
Input the roof pitch in inches per 12 inches of run. For example, enter "6" for a 6/12 pitch (6 inches rise per 12 inches run).
Enter Wall Plate Width
Input the width of the wall plate in inches. This is typically the same as the rafter width if using same-sized lumber.
Optional: Specify Seat Depth or Heel Height
You can optionally specify seat depth or heel height. Leave blank for automatic calculation. Click "Calculate Birdsmouth Cut" to see cut dimensions and angles.
Formula
Pitch Angle = arctan(Pitch / 12)
Heel Height = Seat Depth × tan(Pitch Angle)
Seat Depth ≈ 1/3 of Rafter Width (or user-specified)
Plumb Cut Angle = Pitch Angle
Example: 2×6 rafter (5.5"), 6/12 pitch, wall plate 3.5"
Step 1: Pitch angle = arctan(6/12) = 26.6°
Step 2: Seat depth = 5.5 / 3 = 1.83" (or use wall plate: 3.5")
Step 3: Heel height = 3.5 × tan(26.6°) = 1.75 inches
Step 4: Plumb cut angle = 26.6°
Step 5: Seat cut = 3.5 inches (horizontal)
About Birdsmouth Cut Calculator
The Birdsmouth Cut Calculator is an essential tool for carpenters, roofers, contractors, and DIY enthusiasts who need to calculate birdsmouth cut dimensions for rafters. This calculator eliminates guesswork by providing precise cut dimensions based on rafter width, roof pitch, and wall plate dimensions, helping you create accurate birdsmouth cuts for proper rafter installation.
When to Use This Calculator
- Rafter Installation: Calculate birdsmouth cuts when installing roof rafters
- Roof Construction: Determine cut dimensions for proper rafter seating on wall plates
- Framing Projects: Calculate cuts for DIY framing and construction projects
- Material Planning: Determine cut dimensions before cutting rafters
- Professional Carpentry: Quick reference for accurate birdsmouth cuts
Why Use Our Calculator?
- ✅ Accurate Calculations: Precise birdsmouth cut calculations using standard carpentry formulas
- ✅ Automatic Calculations: Automatically calculates seat depth and heel height if not specified
- ✅ Multiple Dimensions: Calculates seat cut, heel height, plumb cut angle, and birdsmouth depth
- ✅ Time Savings: Instant calculations eliminate manual math and lookups
- ✅ Easy to Use: Simple input fields for rafter width, pitch, and wall plate width
Understanding Birdsmouth Cuts
Birdsmouth Definition: A birdsmouth cut is a notch cut in the bottom of a rafter that allows it to sit securely on the wall plate. It consists of a horizontal "seat cut" and a vertical "heel cut" that creates a notch in the rafter.
Seat Cut: The horizontal cut of the birdsmouth. This is typically 1/3 to 1/2 of the rafter width, but should not exceed the wall plate width. The seat cut provides a horizontal bearing surface on the wall plate.
Heel Cut: The vertical cut of the birdsmouth. This is calculated from the seat depth and roof pitch. The heel height should not exceed 1/2 of the rafter width to maintain structural integrity.
Plumb Cut: The angle cut at the top of the rafter where it meets the ridge. This angle equals the roof pitch angle, calculated from the roof pitch.
Tips for Best Results
- Use Actual Dimensions: Always use actual lumber dimensions, not nominal sizes (e.g., use 3.5" for 2×4, not 4")
- Verify Pitch: Confirm roof pitch before making cuts—incorrect pitch results in improper cuts
- Check Seat Depth: Seat depth should not exceed wall plate width—calculator automatically limits this
- Maintain Structural Integrity: Heel height should not exceed 1/2 of rafter width—calculator enforces this
- Cut Accurately: Use the calculated angles and dimensions for precise cuts—accurate cuts ensure proper rafter seating
Frequently Asked Questions
What is a birdsmouth cut?
A birdsmouth cut is a notch cut in the bottom of a rafter that allows it to sit securely on the wall plate. It consists of a horizontal "seat cut" and a vertical "heel cut" that creates a notch in the rafter.
How do I calculate birdsmouth cut dimensions?
Calculate seat depth (typically 1/3 of rafter width or equal to wall plate width), then calculate heel height = seat depth × tan(pitch angle). The plumb cut angle equals the pitch angle. The calculator performs these calculations automatically.
What is the seat cut?
The seat cut is the horizontal cut of the birdsmouth. It's typically 1/3 to 1/2 of the rafter width, but should not exceed the wall plate width. It provides a horizontal bearing surface on the wall plate.
What is the heel height?
The heel height is the vertical cut of the birdsmouth—the height of the notch. It's calculated from the seat depth and roof pitch: heel height = seat depth × tan(pitch angle). The heel height should not exceed 1/2 of the rafter width.
What is the plumb cut angle?
The plumb cut angle is the angle cut at the top of the rafter where it meets the ridge. This angle equals the roof pitch angle. For example, a 6/12 pitch has a plumb cut angle of approximately 26.6°.
Do I use actual or nominal lumber dimensions?
Always use actual lumber dimensions, not nominal sizes. For example, use 3.5 inches (actual) for a 2×4 (nominal), not 4 inches. The calculator uses actual dimensions for accurate calculations.