Rafter Length Calculator

To calculate rafter length using pitch, first determine the run (half the span), then use the formula: rafter length = run × √(1 + (pitch/12)²). For example, with a 12-foot run and 6/12 pitch, the calculation would be 12 × √(1 + (6/12)²) = 13.42 feet.

Use the Pythagorean theorem: rafter length = √(rise² + run²). This gives you the basic rafter length before adding overhang. For a 6-foot rise and 12-foot run, the calculation is √(6² + 12²) = √(36 + 144) = 13.42 feet.

For a 12-foot span, you typically need 2x6 or 2x8 rafters depending on the load and spacing. With 16-inch spacing, 2x6 rafters can span up to 9 feet, while 2x8 rafters can span up to 12 feet. Always consult local building codes for specific requirements.

Common roof pitches range from 4/12 to 12/12. A 4/12 pitch is considered low slope, 6/12 to 9/12 is standard for most homes, and 12/12 creates a 45-degree angle. Steeper pitches shed water better but require more materials and labor.

Yes, always add overhang to your basic rafter length. Typical overhangs range from 6 to 24 inches. The overhang protects your walls from weather and provides aesthetic appeal. Remember to account for this when ordering lumber.

Ridge board thickness affects the actual rafter length needed. Subtract half the ridge board thickness from your calculated run before computing rafter length. Most ridge boards are 1.5 inches thick, so subtract 0.75 inches from each rafter's horizontal run.

Rafter length is the actual measurement along the rafter from the ridge to the wall plate. Board length is slightly longer to account for the angled cuts at both ends. You'll need to buy lumber that's longer than the calculated rafter length to accommodate these cuts.