Rafter Calculator
Results
Roof Span: ft
Pitch: /12
Rafter Length: ft
How to Use the Rafter Calculator
A Rafter Calculator is a tool that helps builders, carpenters, and DIY enthusiasts determine the correct length of rafters needed for a roof, based on the roof span and pitch.
Why Use a Rafter Calculator?
When constructing a roof, one of the most important tasks is calculating rafter length accurately. Incorrect measurements can lead to wasted materials, structural weaknesses, or a roof that simply doesn’t fit. This calculator simplifies the math by using the Pythagorean theorem—calculating rafter length from the roof span (width of the building) and pitch (rise over run).
How to Use the Rafter Calculator
- Enter the Roof Span (in feet):
This is the total width of the building from one exterior wall to the other. - Enter the Roof Pitch (rise over 12):
Roof pitch is expressed as the rise in inches for every 12 inches of run. For example, a 6/12 pitch means the roof rises 6 inches vertically for every 12 inches horizontally. - Click "Calculate Rafter Length."
The calculator will determine the rafter length using the formula:Rafter Length=(Span2)2+(Rise)2Rafter Length=(2Span)2+(Rise)2where:- Span / 2 = Run
- Rise = Run × (Pitch ÷ 12)
- View the Results:
The calculator displays the original span, the pitch you entered, and the rafter length in feet.
Applications
- Residential roof framing
- Shed and garage roof construction
- Barns and outdoor structures
- DIY carpentry projects
By using this calculator, you save time, reduce mistakes, and ensure your structure is built to precise specifications.
FAQ About the Rafter Calculator
Q1: What is roof pitch?
A: Roof pitch describes the steepness of a roof. It’s written as rise/run, with the run always being 12 inches. For example, a 6/12 pitch means the roof rises 6 inches vertically for every 12 inches horizontally.
Q2: Why divide the span by 2?
A: Because rafters extend from the center ridge of the roof to the outer wall, we only need half of the total span (known as the run) for the calculation.
Q3: Does this calculator include overhangs?
A: No, this calculator only computes the structural rafter length. If you need eave overhangs, you should add that measurement separately.
Q4: Can I use metric measurements?
A: The calculator is set for feet and inches, but you can adapt it to metric by using meters for span and converting pitch into rise per 100 cm.
Q5: Is this calculator suitable for all roof types?
A: It’s designed for gable-style pitched roofs. For hip roofs or complex structures, additional geometry is required.
Q6: Is the calculation precise enough for building codes?
A: Yes, the math is accurate. However, you should always cross-check with local building codes and account for factors like overhang, ridge thickness, and material dimensions.