What a speed square is
A speed square is a right triangle of thick metal or plastic, usually with two equal short sides and a long diagonal edge. One short side has a raised lip, called the fence or lip, which hooks over the edge of a board so the square stays put while you mark. Printed or stamped along the edges are rulers and angle scales.
The tool is popular in framing and roofing because it is fast: you can mark a square line across a board in a single stroke, and the same tool covers 45 degree miters and common roof angles. It is also compact enough to carry in a back pocket or tool belt.
Quick square lines
The most common use is marking a line at 90 degrees to an edge:
- Hook the lip against the edge of the board, and press the body down flat.
- Hold the square firmly with one hand, spreading your fingers across it for steadiness.
- Draw along the edge of the square with a pencil or knife.
Because the lip registers on the edge, the line is square to that edge, assuming the edge is straight and the square itself is true. Test the square before relying on it: draw a line, flip the tool over, and draw again. The lines should coincide.
For cutting, many users hold the square against the board and slide the base of a circular saw along its edge, using the square as a guide. This gives a straight, square crosscut without drawing a line first. Keep fingers clear, and make sure the saw base rides firmly against the square. Because the saw’s blade is offset from the base edge, test the offset on scrap first. A clamped guide gives a steadier result than a hand-held square on thick or long cuts.
45 degree lines
The diagonal edge of a speed square is at 45 degrees to each short side. Hook the lip on the board edge and draw along the long diagonal edge to mark a 45 degree angle. This is useful for miters, braces, and cutting a corner off a board. You can also use it to find the center of a square piece by drawing a diagonal.
Using the pivot point for other angles
At the corner where the long diagonal meets one short edge, there is typically a small pivot point or notch. Place this point on the edge of the board, or on a mark, and rotate the square until the desired angle on the degree scale lines up with the board’s edge. Then draw along the straight side.
The degree scale is stamped along the diagonal and typically runs from 0 to 90 degrees. Read the number that crosses the edge of the board. For example, to mark 30 degrees, pivot until the 30 mark lines up with the board edge, and draw along the straight side of the square. Check the instructions on your own tool, since layouts vary a little between makers.
Rafter-style angles
The speed square was designed with roof framing in mind. The two key terms are:
- Pitch is the slope of the roof, usually stated as rise over run, such as 6 in 12. That means the roof rises 6 inches for every 12 inches of horizontal distance.
- Common rafter is the standard rafter that runs from the ridge to the wall plate at the roof’s pitch.
The scale marked “common” lets you set the square to a pitch. Place the pivot point on the board edge, then rotate the square until the board edge lines up with the number on the common scale that matches your roof’s rise per 12 inches. Draw along the square’s edge to mark the plumb cut at the ridge or the level seat cut at the wall. For a 6 in 12 pitch, the angle works out to roughly 26.6 degrees from horizontal, which you can confirm on the degree scale.
Another scale on many squares is for hip and valley rafters, which sit at a compound angle in the corner of a roof. Their markings follow the same idea but differ because the run is longer. Roof framing involves structural and safety choices, so follow plans or local requirements and ask a qualified person if you are unsure. The speed square helps with marking, but it does not decide the design.
Stairs and other uses
The same angles help with stair stringers, where a square sets the rise and run of each step. Mark one step by aligning two reference numbers on the square with the board edge, then step it along to mark the rest. Check the total rise and run before cutting, because errors repeat on every step.
Speed squares also help with:
- Marking lumber for studs, joists and blocking. Wall framing often relies on quick, consistent square marks.
- Checking if an inside corner is square.
- Marking cut lines on plywood edges when a full-size square is not available.
- Serving as a short straightedge for light layout.
Keeping it accurate
- Check the lip and edges. A burr or dent on the lip changes the angle. File gently or replace the square.
- Keep the surface clean. Sawdust under the tool or on the board edge will tilt the square.
- Do not use it as a pry bar or hammer.
- Test after a drop. Re-do the flip test.
- Read carefully. The degree and rafter scales are small, so good light and a sharp pencil help.
Limits
A speed square has a short reach. For long lines on sheet goods, use a longer square or a straightedge, and for odd angles that do not appear on the scales, a sliding bevel captures the angle more directly. For fine joinery, a combination square or a marking knife is usually more precise.
Within those limits it is a fast, durable tool that rewards practice. This is general information, not professional trade advice.