How to Use a Rafter Square How to Use a Rafter Square

What You'll Need
Rafter square
Roof frame

Rafter squares, also known as framing or steel squares, are tools used in carpentry and are pivotal for their usefulness and versatility. Made of steel, they consist of two arms joined at a right angle and are used to determine right angles and, in some instances, take measurements. One of its arms, known as the blade or body, is longer than the other, which is called the tongue. To look at the face of the rafter square, hold the body with your left hand with the tongue pointing to your right. To use this tool, you will need to do some rudimentary math. Ensure that the wood surfaces are well-planed and sturdily attach to one another before you begin. The instructions below refer to a roof under construction to elaborate the use of a rafter square.

Step 1 - The Width

Measure the entire width of the building. For this example, the width is 30 feet.

Step 2 - The Run

Determine the run of the common rafter. This should be measured as half the width of the building, so 30 feet / 2 = 15 feet.

Step 3 - The Pitch

Now determine the pitch of the roof. This will be calculated as for every 12-inch mark you come across when measuring straight up, the pitch is 6 feet. This is usually a standard figure.

Step 4 - How to Read the Rafter Scale

The edge of the body and the tongue of the rafter square are marked out by graduations that mostly consist of fractions, from 1/8 to 1/32. The most important figures are the ones located on the face side of the body (the longer arm). The body is used to measure the horizontal planes of the joint, while the tongue (the shorter arm) is used to measure the vertical ones.

Common Rafter

This row represents the common rafter per foot run of the angle you are measuring. The number is under the number of the pitch i.e. under the 6-foot pitch marking, the common rafter is marked out as 13.42.

Hip or Valley Rafter

This row represents the hip or valley length per foot run. Like the common rafter above, this figure is under the 6' pitch reading. Under the 6' pitch marking, the hip or valley rafter is marked out as 18.

The 16 Foot and 24 Foot Centers

Represented by one row each, this two scales tell you how much shorter or longer the next jack rafter is from the previous one. Like before, look under the 6-foot marking. For 16 feet, you will find the reading to be 17 feet 7/8 inches, and for 24 feet, you will find the reading to be 13/16.


The best rafter squares are those found at hardware stores and not those in the retail stores, which are cheap versions as they are made from plastic and are less durable than their steel counterparts are.

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