Calculate rafter length from roof run, rise, and overhang. Includes birdsmouth cut depth and total rafter stock length needed.
Calculating rafter length accurately is essential for ordering the right lumber and making precise cuts. A rafter's length is determined by the horizontal run (half the building width minus half the ridge thickness), the vertical rise (set by the roof pitch), and any overhang (eave projection) you want beyond the exterior wall.
This rafter length calculator uses the Pythagorean theorem to compute the line length of a common rafter from run and rise, then adds the overhang distance adjusted for pitch. It also shows the total stock length you need to purchase and the birdsmouth cut dimensions.
Whether you're framing a simple gable roof, a shed addition, or a porch, this tool gives you the measurements needed to lay out and cut each rafter accurately.
By quantifying this parameter precisely, construction teams can optimize material orders, reduce on-site waste, and ensure structural requirements are met safely and efficiently. Understanding this metric in quantitative terms allows construction professionals to compare design alternatives, evaluate cost-effectiveness, and select the optimal approach for each project.
Rafter length calculations involve square roots and trigonometry that are error-prone with a tape measure and framing square alone. This calculator gives you the exact line length, overhang adjustment, and stock length so you can order material and set up your saw with confidence. Consistent use of this tool across projects builds a library of reference data that improves estimating accuracy over time and reduces reliance on individual experience alone.
Rafter Line Length = √(Run² + Rise²) Rise = Run × (Pitch / 12) Overhang on Slope = Overhang / cos(pitch angle) Total Stock Length = Line Length + Overhang on Slope Pitch Angle = arctan(Pitch / 12)
Result: 14'-1″ total rafter length
With a 12-ft run at 6:12 pitch, the rise is 6 ft. Line length = √(12²+6²) = √180 = 13.42 ft. The pitch angle is 26.57°, so overhang on slope = 1.5/cos(26.57°) = 1.68 ft. Total stock = 13.42 + 1.68 = 15.10 ft, requiring a 16-ft rafter.
Start by marking the ridge plumb cut at one end, measure the rafter line length along the top edge, mark the birdsmouth location at the wall plate, and finish with the tail cut at the eave. Use a speed square set to your pitch for consistent plumb and level cuts.
The IRC limits birdsmouth depth to one-third of the rafter depth. For a 2×8 rafter (7.25″ actual depth), the maximum seat cut is about 2.4 inches. The seat cut must be long enough to bear fully on the top plate (minimum 1.5″ for a 2× plate).
Steeper pitches produce longer rafters for the same horizontal run. A 12:12 pitch (45°) adds 41% to the rafter length compared to the run, while a 4:12 pitch adds only 5.4%. Always recalculate when pitch changes—don't interpolate.
The run is the horizontal distance from the outside of the wall's top plate to the center of the ridge. For a symmetrical gable roof, the run equals half the building span minus half the ridge board thickness.
Pitch is expressed as rise per 12 inches of run. If the roof rises 6 inches for every 12 inches of run, the pitch is 6:12. You can measure this on an existing roof with a level and tape measure.
A birdsmouth is a notch cut into the bottom of a rafter where it sits on the wall's top plate. It consists of a horizontal seat cut and a vertical plumb cut, allowing the rafter to bear flat on the plate.
Common overhang lengths range from 12–24 inches. Longer overhangs (18–24″) provide better weather protection and shade but may require lookout framing or outriggers for support.
Yes. Subtract half the ridge board thickness from the theoretical run. For a 1.5″ ridge board (2× lumber), subtract 0.75 inches. This shortening ensures the rafters meet the ridge correctly.
Round up to the next standard lumber length (8, 10, 12, 14, 16, 18, or 20 feet). Always buy longer than the calculated total to account for end cuts and any imperfections in the lumber.