Horizontal Run (ft) Enter the horizontal distance from wall plate to ridge centerline (e.g., 12.5)

Vertical Rise (ft) Enter the vertical height from wall plate to ridge (e.g., 6.0)

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Small Residential Roof Steeper Roof Section

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This calculator is for informational purposes only. Verify results with appropriate professionals for important construction decisions.

Use this free online Rafter Length Calculator to calculate your rafter length. Simply enter your horizontal run and vertical rise to instantly get results in feet. The output represents the sloped distance from wall plate to ridge, useful for ordering lumber and planning roof construction.

How Rafter Length Is Calculated

Rafter length is calculated using the Pythagorean theorem, which works for right triangles. The horizontal run and vertical rise form the two shorter sides of a right triangle. The rafter forms the longest side, called the hypotenuse. This method works for any straight common rafter that connects the wall plate to the ridge board.

This formula gives the theoretical rafter length. Actual rafter length may need adjustment for ridge board thickness, birdsmouth cuts, and overhangs beyond the wall plate.

What Your Rafter Length Result Means

The rafter length tells you the sloped distance that your rafter board must span. This number helps you purchase the right lumber length and plan cuts. For example, a result of 13.00 feet means you need lumber at least 13 feet long before accounting for any overhang. Roof pitch is also calculated as a secondary value, showing the steepness of the roof.

Higher pitch values mean steeper roofs that shed snow and water more easily but require more material. Lower pitches are easier to walk on but may need special waterproofing materials.

Accuracy, Limitations & Common Mistakes of the Rafter Length Calculator

How Accurate Is the Rafter Length Calculator?

The calculator provides mathematically precise results based on the Pythagorean theorem. The formula is a standard geometric calculation used in construction. Accuracy depends on correct measurements of run and rise. For most residential projects, the results are accurate within the precision of your input measurements. Professional builders use this same method for initial estimates.

Limitations of the Rafter Length Calculator

This calculator provides the theoretical rafter length and does not account for ridge board thickness, which may reduce the actual rafter length by half the ridge thickness. Birdsmouth cuts at the wall plate also affect where the rafter sits. Overhangs extending past the wall plate are not included in this calculation. Hip rafters, valley rafters, and jack rafters require different formulas. Complex roof shapes may need additional calculations beyond this tool.

Common Mistakes to Avoid

• Measuring run from the outside wall instead of from the wall plate to the ridge centerline. Always measure to the center of the ridge board for accurate results.
• Forgetting to add extra length for overhangs. The calculator gives rafter length to the wall plate only. Add overhang separately if your design includes eaves.
• Confusing roof pitch in degrees with pitch as rise per foot. This calculator uses rise divided by run, not angle measurements.

Frequently Asked Questions

Who is this Rafter Length Calculator for?

This calculator is for homeowners planning DIY projects, contractors estimating materials, architects doing preliminary designs, and students learning construction math. Anyone who needs to find the sloped length of a common rafter can use this tool for quick estimates.

How often should I use this calculator?

Use this calculator whenever you are planning a roofing project, estimating lumber costs, or checking existing roof dimensions. For active construction, you may use it once for initial planning and again when verifying measurements before ordering materials.

Does this calculator work for all roof types?

This calculator works for straight common rafters on gable roofs. It does not directly apply to hip rafters, valley rafters, or curved roofs. For complex roof systems, consult framing tables or a structural engineer for accurate calculations.

Can I use this calculator for metric measurements?

This calculator uses feet for all inputs and outputs. If you have measurements in meters, convert them to feet first by multiplying by 3.281. The mathematical formula works the same regardless of units as long as both inputs use the same unit system.

Is the Rafter Length Calculator free to use?

Yes, this calculator is completely free to use with no sign-up required. It works on any device with a web browser.