What is a diagonal measurement?
The diagonal measurement is the diagonal dimension of a rectangle, measured from corner to corner. By measuring both diagonals and comparing them, you can check whether a structure is square (90°). If both diagonals are exactly equal, the rectangle is square. If the measurements differ, the structure is out of square.
How does it work?
For any rectangular structure (foundation, floor, frame, window frame):
- Measure diagonal A (top left to bottom right)
- Measure diagonal B (top right to bottom left)
- A = B — the structure is square
- A is not equal to B — the structure is out of square, correct it
When do you check the diagonal measurement?
- Setting out a foundation — Before you start digging or pouring
- Formwork — Before pouring a concrete slab
- Timber frame construction — When erecting walls
- Decking — When laying the frame
- Window frame — After setting, before securing
- Garden house/shed — At every stage of construction
How much deviation is acceptable?
- Foundation/concrete slab: maximum 5 mm difference
- Timber frame construction: maximum 3-5 mm
- Furniture/window frames: maximum 1-2 mm
3-4-5 method
If you cannot measure a diagonal (too large, obstacles), use the 3-4-5 method to set out a right angle:
- Measure 3 metres along one side
- Measure 4 metres along the other side
- The diagonal must be exactly 5 metres — then the angle is 90°
This also works with multiples: 6-8-10, 9-12-15, etc.
Related terms
- Spirit level
- Plumb bob
- Pier
- Strip foundation