1. What is the Rafter Length Calculation?

The rafter length calculation determines the length of roof rafters needed for construction, accounting for the roof pitch and birdsmouth cut adjustment. This ensures proper roof framing and structural integrity.

2. How Does the Calculator Work?

The calculator uses the rafter length formula:

Where:

• \( Run \) — Horizontal distance from wall to ridge (feet)
• \( Pitch \) — Angle of roof slope (degrees)
• \( \cos \) — Trigonometric cosine function
• \( Birdsmouth Adjustment \) — Length reduction for notch at wall connection (feet)

Explanation: The formula calculates the hypotenuse of the right triangle formed by the run and rise, then subtracts the birdsmouth cut length.

3. Importance of Accurate Rafter Length

Details: Precise rafter length calculation is essential for proper roof framing, preventing material waste, and ensuring structural stability of the building.

4. Using the Calculator

Tips: Enter run (horizontal distance), roof pitch in degrees, and birdsmouth adjustment length. All values must be positive numbers with pitch between 0-90 degrees.

5. Frequently Asked Questions (FAQ)

Q1: What exactly is a birdsmouth cut?
A: A birdsmouth is a triangular notch cut into the rafter where it meets the wall plate, providing better load transfer and stability.

Q2: How do I measure the run?
A: Run is the horizontal distance from the outside of the wall to the centerline of the ridge board.

Q3: What's a typical roof pitch?
A: Common residential pitches range from 4/12 (18.43°) to 12/12 (45°), but can vary based on design and climate.

Q4: How much should I allow for birdsmouth?
A: Typically 1.5-3 inches (0.125-0.25 feet), but consult local building codes for specific requirements.

Q5: Does this account for overhang?
A: No, this calculates the basic rafter length. Add overhang length separately if needed.