How it works
- Run = half the building span, minus half the ridge board thickness.
- Pitch multiplier = √(1 + (pitch ÷ 12)²).
- Ridge to wall = run × pitch multiplier. This is measured along the top edge of the rafter, from the ridge cut to the point above the outside of the wall.
- Overhang tail = overhang × pitch multiplier.
- Total rafter length = ridge to wall + overhang tail.
Example: a 24 foot span at 6 in 12 with a 1.5 inch ridge board. Run = 144 − 0.75 = 143.25 inches. The multiplier is 1.118, so ridge to wall = 143.25 × 1.118 = 160.16 inches, which is 13 ft 4 3/16 in. A 12 inch overhang adds 12 × 1.118 = 13.42 inches. The total is 14 ft 5 9/16 in, so you would buy 16 foot boards.
Cut angles
The plumb cut is the vertical cut at the ridge and at the rafter tail. Its angle equals the roof angle. The seat cut is the level cut of the birdsmouth, where the rafter sits on the wall. Both angles here are measured from a line square across the board, which is how a speed square or miter saw reads them.
What it does not tell you
This calculator gives length, not strength. Whether you need a 2x6, 2x8 or 2x10 depends on the span, the rafter spacing, the wood species and your snow load. Use the rafter span tables in your building code or ask an engineer.
Common questions
How long is a rafter for a 24 foot span at 6/12 pitch?
With a 1.5 inch ridge board, each rafter is 13 ft 4 3/16 in from the ridge to the outside of the wall. A 12 inch overhang adds 13 7/16 in, for a total of 14 ft 5 9/16 in.
How do I calculate rafter length?
Take half the building span, subtract half the ridge board thickness, and multiply by the pitch multiplier for your roof pitch. Then add the overhang multiplied by the same number.
What is the plumb cut angle for a 6/12 roof?
It is 26.6 degrees, the same as the roof angle. The seat cut is 90 minus that, or 63.4 degrees.
Do I measure a rafter along the top or the bottom edge?
Along the top edge. The length runs from the long point of the ridge cut to the plumb line above the outside of the wall.
What size lumber do I need for rafters?
That depends on span, spacing, wood species and snow load, so this calculator does not pick a size. Check the rafter span tables in your local building code.