How to use the rafter calculator
- Run — measure the building width outside-to-outside of the wall plates and divide by two. For a 24' wide building the run is 12'. If the ridge isn't centered, use the horizontal distance from the outside of the wall to the center of the ridge.
- Pitch — the inches of rise per 12 inches of run, like 4/12, 6/12 or 8/12. Type just the first number (
6) or the full ratio (6/12). - Ridge thickness — rafters stop short of the ridge centerline by half the ridge thickness. A 1½" ridge board means a ¾" setback. Use 0 if the rafters meet each other with no ridge.
- Overhang — the level distance from the outside of the wall to where the tail is cut plumb. The calculator converts that to length along the rafter.
The big orange number is the full rafter, ridge plumb cut to tail plumb cut, measured along the top edge. Hook your tape at the long point of the ridge cut and mark the heel of the birdsmouth at the "Ridge → birdsmouth" length.
The math behind it
A common rafter is the hypotenuse of a right triangle. For every foot of run, a rafter at pitch P is √(12² + P²) inches long. At 6/12 that's √(144 + 36) = 13.42". That number is stamped on the first line of a steel framing square's rafter table, and it's the same factor this calculator uses.
Example: a 24' wide building at 6/12 with a 1½" ridge. Run is 144", minus a ¾" setback = 143¼" net run. 143.25 × 13.42 ÷ 12 = 160.16", or 13'-4 3/16" from ridge to birdsmouth. Add 12" of overhang (13.42" along the rafter) and the total is 14'-5 9/16". Plug those numbers into the calculator above to check.
Cut angles
The plumb angle is the angle of the ridge cut and tail cut, measured from the rafter's edge to the cut line the way a speed square's pivot scale reads it. Set your square to the pitch number on the "common" scale and you'll mark the same line. The seat angle is its complement (90° minus plumb), the angle the level seat cut makes with the rafter's edge.
Birdsmouth: the ⅓ rule
The birdsmouth is the notch that sits the rafter on the wall plate. Its level seat cut usually matches the plate width, and the plumb heel cut follows from the pitch. A common rule of thumb is to never notch deeper than one-third of the rafter's depth, so a steep pitch on a wide plate can't always use a full-width seat. When it would, the calculator caps the heel at ⅓ of depth and shows the shorter seat that keeps you there. Some engineers and local codes use ¼ — check your plans.
FAQ
How do I calculate rafter length?
Multiply the run (in feet) by the rafter length per foot of run for your pitch, which is √(144 + pitch²) inches. A 12-foot run at 6/12 is 12 × 13.42" = 161", or 13'-5". Subtract half the ridge thickness from the run first, then add the overhang separately.
What is the run of a rafter?
Run is the level distance the rafter covers, measured from the outside of the wall plate to the center of the ridge. On a symmetrical gable roof it is half the building width.
Do I measure rafter length on the top or bottom edge?
This calculator gives line length along the top edge, which is the traditional framing-square method. Hook your tape on the long point of the ridge plumb cut and measure down the top edge.
How deep can a birdsmouth cut be?
The common rule of thumb is no more than one-third of the rafter depth, so 2⅜" on a 2×8. Some jurisdictions and engineers use one-quarter. Your plans or local building department have the final say.
Why subtract half the ridge board?
Rafter run is calculated to the center of the ridge, but each rafter stops at the face of the ridge board. Taking half the ridge thickness off the run (the setback) makes the rafter the right length to meet that face.