Roof Rafter Calculator

Rafter Length Formula:

\[ \text{length} = \frac{\text{run}}{\cos(\arctan(\frac{\text{rise}}{\text{run}}))} \]

Rafter Length:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Roof Rafter Calculator?

The Roof Rafter Calculator determines the length of roof rafters based on the run and rise measurements. It's essential for roof construction and framing projects.

2. How Does the Calculator Work?

The calculator uses the rafter length formula:

\[ \text{length} = \frac{\text{run}}{\cos(\arctan(\frac{\text{rise}}{\text{run}}))} \]

Where:

\( \text{run} \) — Horizontal distance from wall to ridge (feet)
\( \text{rise} \) — Vertical distance from wall to ridge (feet)
\( \text{length} \) — Diagonal rafter length (feet)

Explanation: The formula calculates the hypotenuse of a right triangle formed by the run and rise, giving the rafter's actual length.

3. Importance of Rafter Length Calculation

Details: Accurate rafter length calculation ensures proper roof framing, prevents material waste, and maintains structural integrity.

4. Using the Calculator

Tips: Enter run and rise in feet. Both values must be positive numbers. The run is typically half the building's width for a gable roof.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between run and span?
A: Run is half the span. For a 20-foot wide building, the run would be 10 feet (from wall to center ridge).

Q2: How does pitch relate to rise/run?
A: Roof pitch is the rise over run (e.g., 4/12 means 4" rise per 12" run).

Q3: Should I add overhang to this calculation?
A: This calculates the basic rafter length. Add overhang length separately if needed.

Q4: What about birdsmouth cuts?
A: This gives the full length. Birdsmouth cuts (where rafter meets wall) require additional measurements.

Q5: Can this be used for hip rafters?
A: No, hip rafters require different calculations as they run at 45° to common rafters.