How to Layout Rafters with a Framing Square

The step-off method

Place the framing square on the rafter with the tongue (short leg) representing the rise and the blade (long leg) representing 12 inches of run. For a 6/12 pitch, line up 6 on the tongue and 12 on the blade with the rafter's top edge.

Mark along both legs to scribe the plumb cut at the upper end. Then "step" the square down the rafter — slide it along the top edge so the previous "12" mark on the blade lines up with where the previous step ended.

Each step represents one foot of run. For a 12-foot run, step 12 times. The bottom of the last step marks where the bird's mouth begins.

Laying out the bird's mouth

The bird's mouth is a notch where the rafter sits on the wall plate. Lay out the seat cut (horizontal portion) by extending the blade of the framing square horizontally from the heel of your last step. The seat depth typically matches the wall plate width — 3.5 inches for a 2x4 plate.

The heel cut (vertical portion) drops straight down from the seat to the bottom edge of the rafter. The heel cut is plumb, parallel to the ridge plumb cut.

Adding the overhang

Below the bird's mouth, continue the step-off pattern at the same pitch for the desired overhang. For a 16-inch overhang at 6/12, step the square 16/12 = 1.33 additional steps past the bird's mouth.

At the end of the overhang, mark the tail cut — plumb (matching the ridge cut) for a fascia-friendly eave, or square for an exposed-tail Craftsman style.