Common Rafter Calculator: Length & Pitch

Example

Input: 24ft Span, 6/12 Pitch, 12" Overhang

Result: 13ft 5in Rafter

Step-by-Step Guide

1. Input Span

Enter the total width of the building.

2. Select Pitch

Choose the roof slope (e.g., 4/12, 6/12).

3. Add Details

Input ridge board thickness (usually 1.5") and overhang length.

4. Calculate

View the line length and total board length.

What is Common Rafter Calculator: Length & Pitch?

The Common Rafter Calculator determines the length of lumber needed to frame a gable roof. It uses the roof's span and pitch to calculate the diagonal length from the ridge to the wall plate. Correct calculation is essential for ordering lumber and ensuring the roof structure is sound and meets local load requirements.

👷 Safety First: Always adhere to local building codes and regulations. Wear appropriate Personal Protective Equipment (PPE) when performing construction tasks.

How it Works

The calculation uses the Pythagorean theorem. The 'Run' is half the building's span (minus half the ridge board thickness). The 'Rise' is determined by the pitch (e.g., 4 inches rise per 12 inches run). Formula: Rafter Length = √(Run² + Rise²) + Overhang. The tool helps you determine the 'Line Length' (theoretical) and the actual lumber length needed including the tail.

FAQ

What is the 'Run'?

The run is exactly half of the building's span, assuming a centered ridge.

Why deduct the ridge board?

The rafters butt against the ridge board, so you must subtract half its thickness from the run.

What is a Bird's Mouth?

A notch cut into the rafter to allow it to sit flat on the top wall plate.

Does pitch affect length?

Yes, a steeper pitch (e.g., 12/12) requires significantly longer rafters than a shallow pitch (e.g., 4/12).

Can I use this for shed roofs?

Yes, but the run would be the full width of the building, not half.

Conclusion

Accurate rafter calculation minimizes waste and ensures a straight roofline. Remember that the 'bird's mouth' cut will slightly affect how the rafter sits on the wall plate, but the calculated line length along the top edge remains the critical dimension for layout.

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References & Standards

This calculator uses formulas and data standards from Standard References to ensure accuracy.

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