Movement, control and expansion joints

Frequently asked questions

What is the difference between a control joint and an expansion joint?

They solve opposite problems, and the words are used interchangeably by almost everybody. A CONTROL or contraction joint is a deliberate line of weakness — a sawcut, a formed groove, a bead — cut into something that is going to SHRINK. Concrete shrinks as it cures and a slab will crack; the joint decides where, so the crack happens in a straight tooled line instead of wandering across the floor. It carries no gap and it is not meant to open much. An EXPANSION joint is a genuine gap, filled with a compressible material, in something that is going to GROW — clay brickwork expands irreversibly for years after firing, and a long elevation with nowhere to grow will spall or bow. Cutting a control joint where an expansion joint is needed leaves no room; filling an expansion joint solidly removes the room it was providing.

Why does clay brickwork expand and concrete block shrink?

Because they start from opposite states. Clay bricks come out of the kiln bone dry and then slowly take moisture back out of the air, expanding irreversibly over years — most of it in the first months, but never quite stopping. Concrete blocks are made wet and cure by hydrating, then dry out and SHRINK. So a clay elevation needs room to grow and a blockwork one needs somewhere to crack, and a wall with both leaves needs both details in different places. It is also why joint spacing guidance differs by material rather than being one number, and why a long clay elevation built hard against a return with no joint is one of the most reliable ways to produce vertical cracking near the corner.

Why is an isolation joint different again?

Because it separates two elements entirely rather than accommodating movement within one. A slab cast hard against a column, a wall, a machine base or a drain runs into a restraint that does not move with it, so the slab cracks at that point whatever joints it has elsewhere. An isolation joint — a compressible strip full depth around the obstruction — lets the slab move independently of it. The distinguishing question is whether the joint is INSIDE one element controlling its own behaviour, or BETWEEN two elements that will behave differently. Getting that wrong produces the classic radial cracks running away from a column in an otherwise well-jointed floor.