Builder's Squares: Which Type You Need, How to Check It, and What to Buy

A kitchen extension wall that meets the existing house at 88 degrees instead of 90 creates a two-degree wedge that gets wider as the wall gets longer. Over a 4-metre run, that's roughly 140mm of drift at the far end. Your kitchen fitter can't square base units into a corner that isn't square. The worktop doesn't fit. The tiler has to cut progressively wider slivers along the skirting. All because nobody checked the angle before the first course of blockwork went down.

The "builder's square" is the tool that prevents this. But walk into Screwfix or Toolstation and search for "builder's square" and you'll find five different product types, all called some variation of "square," none of them interchangeable. Framing squares. Combination squares. Speed squares. Try squares. The naming is chaotic and nobody explains which one does what.

This page does.

What "builder's square" actually means

"Builder's square" isn't a standardised product name in UK retail. It's an umbrella term covering any L-shaped tool used to check or mark a right angle. In practice, there are four types you'll encounter, and they solve different problems at different scales.

A framing square is a large steel L-shape, typically 400x600mm, with metric markings along both arms. It's the tool most people picture when they hear "builder's square." You press it into a corner to check 90 degrees, or lay it on sheet material to mark a square cut line. Simple, no moving parts, cheap.

A combination square has a steel rule that slides through a cast metal head. The head has a flat face for checking 90 degrees and an angled face for 45 degrees. Most have a small spirit bubble built in. The rule slides and locks at any position, which means you can use it as a depth gauge, a marking gauge, and a straightedge as well as a square. It's the most versatile single square you can own.

A speed square (also called a rafter square) is a triangular aluminium or plastic tool, typically 7 inches (180mm). Roofers and carpenters carry them for marking angle cuts on timber. The hypotenuse edge has degree markings for setting common roof pitches. Useful for timber work but not for checking blockwork corners.

A try square is a fixed 90-degree L-shape, usually smaller (150-230mm), with a thick stock and a thinner blade. Woodworkers use them. For construction work, a combination square does everything a try square does and more.

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For a homeowner managing an extension or renovation, you need two of these four: a combination square and a framing square. The combination square handles marking and measuring tasks. The framing square handles checking corners at a larger scale. The speed square is only worth buying if you're doing timber framing work yourself. The try square is redundant if you own a combination square.

How to use them properly

The combination square

This is the tool you'll reach for most often. Hold it with the head pressed firmly against the edge of whatever you're checking or marking. The head's flat face gives you 90 degrees; the angled face gives you 45 degrees.

Marking a square line on timber. Press the head against the edge of the board. The rule extends across the face. Run a pencil along the end of the rule to scribe a perfect 90-degree line. For marking a consistent distance from an edge, slide the rule out to your measurement, lock it, then run the pencil point at the end of the rule while sliding the head along the edge. That's a parallel line at exact distance. Cabinet makers and kitchen fitters use this constantly.

Checking a surface for square. Press the head tight against one face and look for a gap between the rule and the adjacent face. Light visible between the rule and the surface means the corner isn't 90 degrees. No light, no gap, it's square.

Using the depth gauge. Slide the rule through the head until the end touches the bottom of whatever you're measuring (a rebate, a mortise, a recess in a wall). Lock the rule. Read the measurement from the head. Faster than poking a tape measure into a narrow slot.

The framing square

A framing square is simpler. Press the shorter arm (called the tongue) against one surface, and the longer arm (the body or blade) should sit flat against the perpendicular surface. Gap means out of square.

For checking blockwork corners as a wall rises, hold the shorter arm against the course you've just laid and see whether the longer arm sits flush against the return wall. Any gap tells you the corner isn't true. On a brickwork or blockwork corner, check every three or four courses while the mortar is still wet enough to adjust.

The critical limitation: a framing square is 600mm on its longest arm. That's fine for checking individual corners and marking cuts. It is not accurate enough to set out a building-scale structure. If you're verifying that your extension footprint (say, 4m x 6m) has right-angle corners, a 600mm square simply cannot do that job. A small error invisible at 600mm becomes a large error at 6,000mm.

For setting out foundations, you need the 3-4-5 method. More on that below.

The 3-4-5 method: how to check right angles at building scale

This is the technique your builder uses (or should use) when setting out an extension. It works at any scale and requires only a tape measure and some string lines.

The principle is Pythagoras: in a right-angled triangle, sides of 3, 4, and 5 units always produce a perfect 90-degree corner. Scale it up to whatever units suit the job. For a typical extension, use metres: 3m, 4m, and 5m. For tighter work, use 600mm, 800mm, and 1,000mm.

The method: mark your two wall lines on the ground with string. From the corner point, measure exactly 3 metres along one line and mark it. Measure exactly 4 metres along the other line and mark it. Now measure the diagonal between those two marks. If it reads exactly 5 metres, the corner is a true right angle. If it's short, the angle is less than 90 degrees. If it's long, the angle is more than 90 degrees. Adjust the strings and recheck.

You can also check by measuring both diagonals of a rectangle. If your extension footprint is rectangular, the two diagonals should be identical. Any difference means the rectangle isn't true.

How to check accuracy

Every square you buy should be tested before you trust it. Budget squares are regularly out of true straight out of the box, and even mid-range squares can be knocked out in transit. The test takes thirty seconds.

The flip test for combination squares. Find a board or surface with a straight edge. Press the head against the edge and scribe a fine pencil line along the rule. Now flip the square (rotate the head 180 degrees so it's on the other side of the line) and scribe a second line from the same starting point. If both lines are perfectly parallel (or overlap), the square is accurate. If they diverge, the square is out. The gap between the lines at the far end shows you double the actual error.

The flip test for framing squares. Same principle, larger scale. Place the square on a sheet of plywood with the tongue against a factory edge. Draw a line along the body. Flip the square (put the tongue on the other side of the same factory edge). Draw a second line. Divergence means the square isn't true.

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What to buy

Two squares cover every task a homeowner managing a build will face: a 300mm combination square for marking and measuring, and a 400x600mm framing square for checking corners.

Combination squares

The Bahco CS300 is the square to buy if you're buying one combination square and want it to last. UK Workshop forum users describe it as "one of the few true bargains still to be had," praising the heavier body, better machining, and clearer graduation marks compared to Stanley at a similar price. The scribing pin screws in rather than push-fitting, so it doesn't fall out and vanish on site.

The OX Pro 300mm at around £11 – £12 is surprisingly good for the money. One independent review found it within "less than a sheet of paper's thickness" of a Starrett. At a third of the Bahco's price, it's worth considering if budget is tight.

Avoid the Axminster "Precision" range. Despite the name, forum users have reported units arriving 2.5mm out over 300mm. The Axminster "Workshop" range (different product line, around £15) is better received.

Framing squares

Framing squares are simple tools. There's less to go wrong compared to a combination square, so the price difference between budget and mid-range buys you heavier steel and slightly clearer markings rather than a meaningful accuracy difference. A Screwfix own-brand steel framing square tested against a reference sheet has been found square out of the box. At under £9 it's hard to justify spending more unless you specifically want the angle-finding feature of the OX Pro.

Speed squares (if you need one)

If you're doing any timber framing, rafter marking, or angled cuts, a 7-inch (180mm) speed square earns its place. The OX Pro 7" at TBC from Screwfix is the best value. DEWALT's 7" at around £13.5 is the mid-range pick. Most professionals carry both a 7" in their apron and a 12" on the bench, but for a homeowner, a single 7" is enough.

Alternatives

If you already own a tape measure and some string, the 3-4-5 method replaces a framing square for checking corners at any scale. For a homeowner who will only ever need to check a few corners during a build, a tape measure, a pencil, and the 3-4-5 technique might be all you need for large-scale squareness checks.

A spirit level with a plumb vial tells you whether a surface is vertical, but it doesn't tell you whether two surfaces meet at 90 degrees. The two tools answer different questions. You need both.

For precision woodworking (fitting shelving, scribing worktops, or marking joinery), an engineer's square is more accurate than any combination square because the blade is permanently fixed to the stock. But engineer's squares are small (typically 100-150mm), single-purpose, and unnecessary for construction-scale work.

Where you'll need this

• Foundations and Footings - checking corners are at true right angles when setting out the foundation trench
• Walls and Blockwork - checking wall corners are at true 90 degrees as the blockwork rises, marking cut lines on blocks
• First Fix Electrics - marking square positions for back boxes and switch plates on walls and studs
• First Fix Plumbing - marking pipe bracket positions and checking waste pipe angles
• Kitchen Installation - checking room corners for square before fitting base units, marking scribe lines on end panels
• Snagging Checklist - verifying wall corners, door frames, and window frames are square during final inspection

Squares are used across every stage of any extension or renovation project. If a surface is supposed to be at 90 degrees, you need a way to verify it.

Common mistakes

Trusting a new square without testing it. This is the single most consistent warning from UK trade forums. Budget combination squares and framing squares are regularly out of true from the factory. Test every square with the flip test before you use it on anything that matters. It takes thirty seconds and can save hours of rework.

Using a framing square to set out a foundation. A 600mm arm cannot verify a 4-metre wall line. The error is invisible at the square's scale and obvious at the wall's scale. Use the 3-4-5 method for anything larger than a single corner check.

Not locking the combination square rule. An unlocked rule slides under pencil pressure, producing a line that's in the wrong place. Lock it every time. Every single time.

Pressing too lightly against the reference edge. A combination square head that isn't pressed firmly against the edge will rock slightly, introducing an angular error. Push it flat. Use your other hand to hold the workpiece if needed.

Buying aluminium when you need steel. Aluminium framing squares are lighter, but they can't be corrected if they go out of true. Steel squares can be adjusted with a hammer and punch technique. For a tool that lives on a building site and takes knocks, steel is the right material.