A cross measurement is the diagonal measurement of a rectangle, taken from corner to corner. By measuring and comparing both diagonals, 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):
1. Measure diagonal A (top left → bottom right)
2. Measure diagonal B (top right → bottom left)
3. A = B → the structure is square
4. A ≠ B → the structure is out of square, adjust accordingly
When should you check the cross measurement?
- Setting out a foundation — Before you start digging or pouring
- Formwork — Before pouring a concrete slab
- Timber frame construction — When erecting walls
- Deck — When laying the frame
- Window/door frame — After positioning, before fixing
- Garden shed/outbuilding — At every stage of construction
How much deviation is acceptable?
- Foundation/concrete slab: maximum 5 mm difference
- Timber frame: maximum 3-5 mm
- Furniture/window frames: maximum 1-2 mm
3-4-5 method
If you can’t measure the diagonal (too large, obstacles), use the 3-4-5 method to set out a right angle:
1. Measure 3 metres along one side
2. Measure 4 metres along the other side
3. 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 foundation
- Shallow foundation