A Skill Everyone Should Have-How to Use a Speed Square

In this tutorial, we’re going to take a quick look at the speed square, a tool that you may already have, but may not fully understand. While made for framing carpentry, a speed square can be indispensable in a lot of measuring and marking situations. Let’s have a look.

 A speed square (aka “rafter square” or “triangle square”) is a measuring multitool. Made of steel, aluminum, or plastic, this common carpenter’s tool combines a ruler, a Try square, a protractor, a line scribing tool, a common-width board ripping guide, and a saw guide for making accurate 90° and 45° cuts with a hand or circular saw. A speed square’s main uses are

• as a Try square, for quickly measuring a line perpendicular to the edge of a board (hence the “square” in the name),
• as a Miter square, for accurately marking 45° angles, and
• as a protractor, for easily finding and marking various common angles, especially roof rafter pitches and angles for stair stringers (the vertical supports on a stairway).

The speed square was invented in 1925 by Albert Swanson. Swanson was a carpenter in a small town outside of Chicago. He wanted to create a device to make it easier to quickly determine roof pitches. After he created the speed square, other carpenters began asking him for one, and the Swanson Tool Company was born. Speed Square is actually a trademarked name, but like Kleenex, it is often used as the generic name for this class of tool. Stanley’s similar tool is called a Quick Square and Irwin’s is simply called the Rafter Square.

How Does it Work?

This video clearly runs through most of the key features of the speed square and how one might use it in day-to-day angle measuring and cutting, and in determining pitches.

Here is a little more detail on the various functions of the square and how they are applied.
Marking – The most commonly used application of the speed square is as a Try square. The tool has a lipped fence along one right-angled edge that allows you to hold the square firmly against a board. From there, you can easily and accurately scribe a line along the other right-angled edge to create a line that is perfectly perpendicular to the board’s “factory edge.” You can also use it in this mode to scribe 45° angles along the angled edge of the tool (i.e. the base of the triangle).
Measuring – Along one of the right-angled edges of the square is a conventional English ruler which is either 7″ or 12″ (depending on the model of speed square). This ruler is used for measuring cuts. Below this ruler, in a triangular cut-out, is something called the “scribe bar,” a series of notches at ¼” intervals. To easily scribe a rip or trim line along a board, you simply hold your pencil in the desired notch and run the fence along the edge of the board to make an accurate mark down its length. You can also use the right angle of the square to check internal angles for accuracy on a square.

Saw Guide – One of the other common functions of the speed square is as a guide for cross-cutting lumber at an accurate 90° or 45° angle. By firmly holding the fence against the edge of a board, you can use the other right-angle or 45° angle edge as a fence for your hand or circular saw.

Protracting – To use the protractor feature, you first find the Pivot point marked in the 90° corner of the square. With the Pivot mark on the factory edge of your board, you pivot the bottom of the tool (where the protractor degrees are marked) and move it to the desired degree (let’s say 45°) on the same factory edge. The pivoting edge of your square is now at a 45° angle to the factory edge. Mark that angle and you’re ready to cut.

Measuring Pitches – Part of the speed square’s role as a protractor, and its original mission as a tool, is in quickly finding common (and uncommon) rafter angles for the pitches on roofs. This is accomplished by use of the main protractor and two specialty protractor angle guides on the inside of the tool. The guide directly above the protractor is marked “Common.” On it, common roof pitches are indicated as the rise (in inches, over a 12″ run), for common rafters from 1″ to 30″. Above this guide is another, marked “HIP-VAL.” This stands for “Hip-Valley” and refers to the rise over a 12″ run for Hip or Valley-type rafters.

The above video shows one example of how to use a speed square for finding rafter pitches. Explaining the entire process for doing this quickly becomes too convoluted for this overview. Stanley has the manual for their QuickSquare online and I found a public copy of the Swanson Speed Square manual (see links below). The Stanley manual does a great job of explaining all of the different roof types, includes a glossary of terms, and shows you how to use a square to find the correct pitch for different types of roofs. Classic Work, the YouTuber who did the above speed square basics video, also has a video demonstrating how to frame rafters with a speed square.
Other Cool Features – The Swanson Speed Square has a patented feature — the diamond cut, a diamond-shaped cut-out on the ruler edge of the tool. This is used for squaring on a line scribed across the board so that you can then make a perfect 90° line from your scribed angled to the edge of the board.

You can also use a speed square as a make-do level when you find yourself without a proper bubble level. You do need a plumb-bob, but you can improvise one with a string and a nut (or some other suitable weight). Here’s a video showing this in action.

For More Information

I managed to find a copy of the infamous little blue Speed Square manual [PDF] that ships with every Swanson Speed Square. I found it in the archives of the Oak Lawn public library, Oak Lawn, Illinois, being the home of the Swanson Tool Company.
A little more accessible for the newbie is Stanley’s Quick Square Instruction Manual [PDF].
There’s also a brief how-to on The Family Handyman which shows you a simple method of finding a roof pitch with a level and a speed square.

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