How to Use a Framing Square

A framing square, also called a steel square, is a simple but versatile carpenter’s tool that is indispensable for doing wood frame construction. It is a flat, single piece of aluminum or steel that consists of 2 arms—a larger arm, 24 inches long, called the "blade" set at 90 degrees to a shorter, narrower arm called the tongue.

The framing square has ruled edges used for measuring with a level of accuracy up to 1/32 inch. It also has a diagonal scale, a board foot scale and an octagonal scale. Most framing squares have rafter tables inscribed on the face that give rafter information for various roof pitches. A framing square is essential for cutting and assembling the roof of a wood frame house. It is also useful for laying out and cutting stair stringers.

Laying Out a Common Rafter

This article will explain the basics of how to cut a roof rafter using a framing square in 4 easy steps. For this project, you will need:

  • Framing square
  • Rafter boards
  • 2 Sawhorses
  • Pencil

Step 1 - Determine the Roof Pitch

The slope or pitch of a roof is measured in inches of (vertical) rise per foot of (horizontal) run. A roof that slopes upward vertically 6 inches for every 12 inches measured horizontally is said to have a 6 inch pitch.

Once the roof pitch has been selected, you can layout and cut a rafter pattern from which other rafters can be traced and cut.

Step 2 - Layout the Rafter Tail

Begin by laying out the rafter tail. When the rafter is installed onto the roof, the bottom “level” cut is now horizontal, and the end “plumb” cut is now vertical. All level cuts are parallel and all plumb cuts are parallel.

Lay a framing square flat on the board to be used as a rafter near one end. The horizontal blade should align with the bottom edge of the board at 12 inches, and the vertical tongue should align at the roof pitch.

Layout the plumb cut and level cut for the rafter tail. The length of the plumb cut is determined by the width of the fascia board.

Step 3 - Layout the Bird’s Mouth

Slide the framing square along the edge of the board and lay out the plumb cut and level cut for the notch or “bird’s mouth.” The width of the overhang is the horizontal distance between the 2 plumb cuts. The level cut for the bird’s mouth is a minimum of 3½ inches and bears directly on the top plate of the exterior wall.

Step 4 - Determine the Overall Length of the Rafter

Continue to slide the framing square along the board, keeping the blade at 12 inches and the tongue at the roof pitch until the run (the total span of the rafter) is stepped off in horizontal one-foot increments. When you have measured off the total span of the rafter, scribe the plumb cut where the rafter connects to the ridge board along the vertical tongue.

The rafter is now completely laid out. Use a circular saw to cut out the rafter and use it as a pattern or template to trace out and cut the remaining rafters.