A builder’s square is a large set-square tool used to lay out right angles (90°) on a building site. It is mainly used when setting out foundations, walls and buildings. Unlike a small try square, a builder’s square is large enough to work accurately over distances of several metres.
How Does a Builder’s Square Work?
A builder’s square is based on Pythagoras’ theorem (the 3-4-5 method):
- The two legs are in a ratio of 3:4
- The diagonal (hypotenuse) is then exactly 5
- Common sizes are 600×800×1000 mm or 900×1200×1500 mm
Application
The builder’s square is used when setting out:
- Foundations — the corners of a strip or pad foundation must be exactly square
- Walls — when building, it is checked that walls meet at right angles
- Paving — when laying patios and driveways
- Sheds and outbuildings — even small projects need square corners
Alternatives
- 3-4-5 method with tape measure — without a builder’s square, use a tape measure to apply the Pythagorean method
- Cross-line laser — a modern laser that projects perpendicular lines
- Theodolite — a professional surveying instrument for large projects
Related Terms
- Spirit level
- Plumb line
- Foundation
- Setting out
- Square (right angle)
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