How to Check for Square

How to Check for Square

Using the 3 4 5 Rule to build L shape & Square walls, rooms, roofs and more!

Tips on this page cover:

  • Why you need to check for square when you build things (mainly to save yourself loads of grief later on..)
  • How to square a four sided frame/roof/room/building
  • Squaring two sides with the 3 4 5 rule (pythagoras theorum)
  • Types of carpenters squares (tools) available


How do I make sure my patio is square?

If your patio is a rectangle, then you can measure from one corner to the opposite diagonal corner, and when the measurements are equal, the patio is square. Another method to ensure a patio corner is square is to use the Pythagorean theorem, or the 3, 4, 5 rule.

Learn how to do every step yourself

Here at Western Interlock, we’re on a mission to take the hard out of hardscaping. That’s why we publish DIY tips, tricks, and step-by-step tutorials at and host a live DIY seminar every month during the summer at one of our manufacturing or distribution facilities.

If you have any questions, you can visit our Paving Stone Display at our Manufacturing Facility in Rickreall, OR. Or, give our team of hardscape experts a call at 503-623-9084.

Not all squares are reliable

Never trust any tool right out of the box. Even minor bumps or dings during packaging or shipping could alter the tool’s accuracy. Always check it and set it up with squares, rules, and calipers that you know to be accurate, as we’ll show you in the following steps.

What does a try square look like?

A try square is made of two key parts, the blade (also known as a beam or tongue) and the stock, which are fixed together at 90° to form an ‘L’ shape. Typically the blade and the stock will be rectangular in profile, though on some wooden squares the ends of the blade and the stock might be cut to a decorative shape.

Hardwood frame adds strength

The table shown on the previous slide was built from a 4 x 8′ sheet of 114 ” particleboard, and edged with 2″-wide hardwood attached with #20 biscuits, as shown in the drawing. Plastic laminate was glued to the top because it resists finishes and glue better than a porous wood surface.

How to Use a Level

level, tape, pencil

Use a carpenter's level that is 4 feet long to level and plumb your construction whenever you have room for its length. Shorter levels may be affected by warps or waves in the boards themselves and might not prove as accurate. Boards are level or plumb when the bubble is centered in the appropriate vial.

Technology has improved leveling devices so much that they make the task virtually goofproof. For a modest investment you can purchase an automated water level—it beeps when the water is stabilized in the tube. Or you can buy a laser level that indicates level and projects a visible level line across long distance. A few years ago tools like these were only for the pros. Today's prices put them within reach of the average homeowner.

To level on-site, just extend the length of your level with straight boards. Get in the habit of checking each piece as you install it and use the widest board possible—narrower boards (like 2x4s) may flex and give you a false reading. Center the level on the board to minimize the effect of flexing or crowning.

When you need to level objects or surfaces within 6 to 8 feet of each other, a carpenter's level set on a straight board will do. But you can make almost any leveling task easier and more accurate with a water level (available at many hardware stores). Essentially two pieces of clear plastic tubing that fasten to the ends of a hose filled with water, this tool relies on the principle that water will seek its own level over any distance. Hold the ends of the level against both surfaces, and mark each board at the water line.

Step 2: Trick #2 – the Equal Diagonals Trick

This method is a little easier than the 3,4,5 trick but requires access to all 4 corners of a rectangle or square. This would work great for a picture frame. Just measure across the diagonals from corner to corner. If the measurements are equal then the corners are square.

How to check for square on a four sided frame

To form a square or rectangle frame both the plates (top and bottom) should be the same length, and all the studs or joists should be the same length as each other as well. Once nailed up, to check the structure is square simply measure from corner to corner.

If the measurements aren’t the same, pull the long corner towards the middle of the structure until they even out. Once identical, the framework is perfectly squared. Nail a batten temporarily across three or four joists or studs to hold it square until secured in place.

This is also a good way of checking wall plates are square before fixing them in place before you start cutting roof rafters.



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