How To Use A Speed Square - Popular Mechanics

The Speed Square was designed in 1925 as an aid to carpenters framing homes, and nearly a century later has gone on to become a general woodworking aid. The tool was smaller, lighter and easier to pack compared to the L-shaped framing square that ruled construction sites in that era. Its history is fascinating.

Sometimes referred to as a rafter square, the Speed Square facilitates any number of layout operations from marking out and cutting walls to laying out rafters. Its markings and notches help speed up and make more precise the repetitive procedures in these jobs. Though it was originally developed by the Swanson Tool Company, there are dozens of companies making similar rafter squares now. However, Swanson’s Speed Squares are typically packed with more features than others.

If you get the Speed Square Pro, as I did, it comes with Swanson’s Blue Book, which contains instructions and explanations of the geometry, calculation, and layout of virtually any rafter configuration. The book is a fantastic resource, especially if you don't use a Speed Square every day. Aside from house framing, the Speed Square sees use on just about every woodworking project I do—it’s one of my most-used tools. You can use a Speed Square to build sheds, firewood racks, picnic tables, Adirondack chairs, benches, saw horses, even a boat. You name it, and a Speed Square will help you make it.

While some folks find all the markings on the Speed Square a little intimidating at first, they’re not difficult to understand once you see how they’re used. Below, we’ll explain the tool’s eight most common uses.

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Marking Right Angles

Marking Any Angle

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Guiding a Circular Saw

By holding the lip of the speed square against the bottom edge and pressing down on the top, you can use it as a saw guide to get perfectly straight cuts. You can also flip it around to cut at a 45-degree angle or rotate it on the pivot point to cut at any angle.

Squaring Up Joints

The flat base of the Speed Square makes it easy for you to check that studs or joints are square.

Marking a Line Parallel to an Edge

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Laying Out Top and Bottom Plates for a Wall

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Laying Out Plumb Cuts on Ridge and Tail of Rafters

Plumb cuts are the angled cuts on rafters where they meet the ridge beam, and sometimes on the tail that overhangs the exterior wall. They’re called plumb cuts because when the rafter is installed the face of the cut will be perpendicular to level (straight up and down). You mark plumb cut lines by turning the square on the pivot point, and lining up the “common” number with the edge of the rafter. These numbers represent the pitch of the roof, which will be referred to as “X” on 12, with “X” being the number of inches the roof rises over 12 horizontal inches. (This is also known as “rise” over “run.”) A 45-degree angle would be 12 on 12. Here I’m using the 6, for a 6 on 12 pitch.

Marking Birdsmouth Cuts on Rafters

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That’s just a sampling of what you can do with Swanson’s Speed Square. For more complex configurations, check out the brand’s Blue Book I mentioned.

Bradley FordTest EditorBrad Ford has spent most of his life using tools to fix, build, or make things. Growing up he worked on a farm, where he learned to weld, repair, and paint equipment. From the farm he went to work at a classic car dealer, repairing and servicing Rolls Royces, Bentleys, and Jaguars. Today, when he's not testing tools or writing for Popular Mechanics, he's busy keeping up with the projects at his old farmhouse in eastern Pennsylvania.