1. What is Rafter Length Calculation?

The rafter length calculation determines the length of roof rafters needed for construction, accounting for both the horizontal run and vertical rise of the roof, while subtracting the birdsmouth cut at the wall connection.

2. How Does the Calculator Work?

The calculator uses the Pythagorean theorem adjusted for birdsmouth:

Where:

• \( run \) — Horizontal distance from wall to ridge (ft)
• \( rise \) — Vertical distance from wall to ridge (ft)
• \( birdsmouth \) — Depth of the notch cut at wall connection (ft)

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

3. Importance of Accurate Rafter Length

Details: Precise rafter length calculation ensures proper roof framing, prevents material waste, and maintains structural integrity of the building.

4. Using the Calculator

Tips: Enter run and rise in feet (measured from building plans), and birdsmouth depth. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

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

Q2: How do I measure run and rise?
A: Run is half the building width minus any overhang. Rise is determined by roof pitch (e.g., 4:12 means 4" rise per 12" run).

Q3: What's a typical birdsmouth depth?
A: Usually 1-2 inches (0.083-0.167 ft), but varies based on local building codes and rafter size.

Q4: Does this account for rafter overhang?
A: No, this calculates the basic rafter length. Add overhang length separately if needed.

Q5: Can I use this for different roof types?
A: This works for simple gable roofs. Hip roofs and complex designs require additional calculations.