How To Use A Carpenter Square

What You'll Need
Framing square
SPEED(R) Square
Rafter material
Measuring tape

A carpenter square is virtually indispensable for doing almost any type of woodwork.

Types of Carpenter Squares

There are 2 types of carpenter’s squares: the SPEED® Square, a registered trademark of Swanson Tool Co., Inc, and the framing square. The SPEED® Square is designed to provide a quick and easy method for laying out and cutting roof rafters. It can also be used to ensure that right angle cuts are cut square, and as a guide for cutting with a circular saw.

A framing square is a flat one-piece tool made of aluminum or steel that consists of 2 arms: a larger, longer arm called the 'blade' set at 90 degrees to a shorter, narrower arm called the 'tongue.'

Both squares are simple but versatile tools that are essential for cutting roof rafters. Read on to learn the basics of how to cut a roof rafter using both a SPEED® Square and a framing square in 3 simple steps.

Using a Framing Square

The framing square has a ruled edge used for measuring as well as a diagonal scale, a board foot scale and an octagonal scale. It can also be used for making square cuts and for checking 90 degree angles.

Step 1 - Find the Roof Pitch

Lay a framing square flat on the board near one end, so that the horizontal blade aligns with the edge of the board at 12-inches and the vertical tongue is aligned at the roof pitch.

Step 2 - Trace the Rafter Tail and Bird’s Mouth

First, trace the plumb cut and the level cut for the eave onto the board along the blade and the tongue. Knowing the width of the overhang, you can slide the square along the edge of the board and lay out the notch—or 'bird’s mouth'—in the same way.

Keep in mind that when the rafter is installed onto the roof, a 'level' cut becomes horizontal and a 'plumb' cut is vertical.

Step 3 - Cut the Rafter to Length

Continue to slide the square along the board at the same angle so that the run (the total span of the rafter) is stepped off in horizontal 1-foot increments. Use the vertical tongue to scribe the plumb cut for the ridge board, and cut out the rafter pattern.

Using a SPEED® Square

The SPEED® Square is a triangular-shaped tool with ruler markings along the short side perpendicular to the fence. There are also degrees marked in 1-degree increments, and markings that correspond to roof pitch for common and hip / valley rafters along the hypotenuse.

Step 1 - Find the Roof Pitch

To use the SPEED® Square to make an angled cut on a roof rafter, simply align the pivot point at the short point of the cut. Rotate the square about the pivot point until the edge of the board aligns with the desired roof pitch.

Step 2 - Find the Angle for the Level and Plumb Cuts

A line is scribed along the ruler edge, which will give a plumb cut. To make a level cut, use the SPEED® Square’s right angle to scribe a line at 90 degrees anywhere along this line.

Step 3 - Use the Original Lines as References to Create a Pattern

You now have both level and plumb lines from which you can create a pattern for the other parallel cuts. Knowing the length of the overhang and the distance from the inside corner of the top plate (at the bird’s mouth) to the long point of the rafter (top of the ridge), you can trace out the complete rafter pattern.