How do I use a speed square?

A speed square looks like a marking square and is really a jig. Its thick cast body is the working surface: it is tall enough for a circular saw shoe or a handsaw blade to bear against, and the lip on the fence stops it sliding. Used that way it makes square crosscuts faster and more reliably than measuring and marking, which is why site carpenters keep one in a pouch.

What you'll need

• Speed square
• Circular saw or handsaw
• Clamp (optional)

Step by step

1. Understand the three parts.The fence with its lip, which hooks over the board edge and sets the reference. The perpendicular leg, which gives 90 degrees. And the hypotenuse carrying the degree scale, with a pivot notch at the corner where the fence and the hypotenuse meet.
2. Make a square crosscut with a circular saw.Hook the fence over the edge on the side your saw shoe will run along, position the square so the saw blade lands on the cut line, hold it firmly or clamp it, and run the shoe against the thick body. Because the square cannot rotate, the cut cannot wander.
3. Allow for the offset between the shoe edge and the blade.The blade is set in from the edge of the shoe by a fixed distance, typically 30 to 40mm depending on the saw. Position the square that distance back from your line, or measure it once and remember it — it is the only arithmetic in the method.
4. Use it as a handsaw guide too.Hold the square against the line and start the cut with the saw plate touching the square's body. The first few strokes set the direction of the whole cut, so a guide for those is worth more than a guide for the rest.
5. Mark or cut an angle with the pivot.Put the pivot notch on the board edge, hold it there, and swing the square until the degree figure you want lines up with the same edge. The square is now set at that angle and the body still guides the saw.
6. Read the roof scales only if you need them.The common and hip-valley scales are for rafter layout at a given pitch. They are genuinely useful for roof work and completely ignorable for everything else — the degree scale and the 90 degree leg cover ordinary carpentry.
7. Clamp it for anything where your hand cannot hold it steady.One hand on the square and one on the saw leaves nothing holding the workpiece. On a narrow batten a clamp on the square is the difference between a square cut and a kicked saw.
8. Check the fence for damage.The lip is what registers the whole tool. A dropped square with a burred or bent fence lip no longer sits flat against the edge, and the error goes straight into every cut.

Tips

• Aluminium speed squares are lighter and easier to knock out of true; cast or composite ones survive site abuse better. Test either one with the flip test.
• The scribing notches along the edge of some squares let you mark a line parallel to an edge by holding a pencil in the notch and running the fence along the board — the same trick as a combination square.
• For repeated cuts to the same length, clamp a block on the workpiece as a stop rather than measuring each one. The speed square gets the angle right; the stop gets the length right.
• It makes a decent small straightedge and a very good saw-blade-square check against a circular saw shoe.

Common mistakes

• Using it as the only guide without holding it firmly — It is not a fixed jig. Any sideways force from the saw pushes it out of position, and a square that moves mid-cut is worse than no guide at all.
• Forgetting the shoe-to-blade offset — The cut lands 30 to 40mm from where you intended. This is the single commonest mistake with the tool and it is entirely predictable.
• Marking fine joinery lines with it — The body is thick and the edges are cast rather than ground, so a knife cannot register cleanly against it. Use a combination square for fine marking.

Questions people ask