Lay Out a Common Rafter With a 12-Inch Square

The Problem

Rafter layout is traditionally framing-square territory, but a framing square on a jobsite is a delicate instrument that bends the first time it's dropped or packed wrong. A veteran framer's preference for two decades: the oversized 12-inch rafter square — basically indestructible, and it lays out the same rafter.

The Technique

Working a 6/12-pitch rafter as the example:

1. Plumb cut first. At the top of the rafter — where it meets the ridge — pivot the square on its pivot point and rotate until 6 on the common scale lines up. Mark along the edge: that's the ridge/plumb cut, vertical once the rafter's in place.
2. Measure the rafter length. Hook your tape on the long point of that plumb cut and pull the rafter length down the board (four feet in the demo). Mark, then draw a second plumb line there — same pivot, same 6 on the common scale.
3. Seat cut. From that lower plumb line, slide the square downhill until 3-1/2 inches reads through the square's slot, back it off a hair, and mark the level seat cut. Plumb line plus seat line is the birdsmouth that lands on the wall plate.
4. Tail and fascia. Continue past the birdsmouth for the overhang — 16 inches in the demo, marked to the outside of the fascia — and mark the back side of the fascia with the same 6/12 plumb angle if the tail gets a vertical cut.
5. Cut the lines and the rafter's done — fewer tools to baby, fewer scraps to handle.