Rafter Length Calculator

Find the length to cut a common rafter from the horizontal run, the roof pitch and any eave overhang.

How the Math Works

The Rafter Length Calculator uses the Pythagorean theorem to determine rafter length, a fundamental principle in geometry. The formula rafter = run × √(1 + (rise/12)²) + overhang breaks down the roof's slope into a right triangle where the run is the horizontal distance and the rise is the vertical height. The term (rise/12) represents the pitch as a ratio per 12 inches of run, and squaring this ratio, adding 1, and taking the square root gives us the slope factor. Multiplying by the total run gives the sloped rafter length, to which we add the overhang extension.

Practical Applications

To use this calculator practically, first measure the horizontal distance from the roof's centerline to the outside wall edge (the run), then determine the roof's pitch by measuring how many inches it rises vertically over a 12-inch horizontal span. Measure the desired overhang length that extends beyond the wall. Input these three values into the calculator to instantly receive the total rafter length needed for cutting lumber to precise specifications, eliminating costly guesswork and material waste during roof construction.

Day-to-Day Use

This calculation proves invaluable when planning roofing projects, whether for a new shed, home addition, or replacing damaged roofing materials. By knowing the exact rafter length needed, you can purchase the correct amount of lumber, avoid multiple trips to the hardware store, and ensure proper roof pitch for drainage. The calculator also helps estimate material costs before starting work and assists DIY enthusiasts in cutting rafters accurately with circular saws or other cutting tools, making roofing projects more efficient and successful.

Worked example

15 ft run at 6/12 → 15 × √(1 + 0.25) = 16.77 ft + 1 ft overhang = 17.77 ft.

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