Movement Joints Inside a Building

Materials change size with temperature and with moisture, by different amounts and sometimes in opposite directions. A movement joint lets that happen without the material being forced to crack somewhere nobody chose. The word covers joints that allow expansion, joints that control where shrinkage cracks form, and joints separating elements expected to move independently.

It is not a construction joint, which is where work stopped and restarted and is intended to act as continuous. It is not a crack, which is where movement occurred without provision. And it is not a sealant: the joint is the gap, and the sealant is one component within it.

A joint has to be positioned where movement actually concentrates, which follows from the materials and the geometry rather than from convenience. That is why its position cannot be moved to a tidier place, and why a joint in a floor becomes a visible line that cannot be routed for looks alone.

The consequence for finishes is immediate. Where a joint is set out with the slab, the line falls where the structure needs it, and any finish chosen for its seamless appearance has to live with that. Discovering the joint layout after choosing the finish is the wrong order, and it is the usual one.

A joint interrupted by a rigid finish is not a joint. Where a joint continues through a screed or a topping, that layer stops at the same line; where tiling runs over it, the tiling has to be interrupted in the same way; where a resin or terrazzo finish crosses it, the same applies. A hard finish carried continuously across a moving joint will crack along it whatever its quality.

In terrazzo the divider strips are doing exactly this job, and they are routinely read as decoration. In a thin trowelled wall finish the joint has to be carried through rather than covered over, and that decision belongs at specification rather than on the day.

Most floor movement joint profiles are cast in or bedded as the floor is built, which puts the decision at slab stage. Retrofitting one into a finished floor means cutting out and rebuilding both edges, which is a very different exercise from fitting a surface strip.

The profile is also only as good as the concrete or screed holding it. Where the arrises are weak, contaminated or already broken, they are cut back and rebuilt before the section is set, otherwise the same failure returns at the same line. Many systems are built around a replaceable centre insert that nobody ever replaces because nobody knows it is meant to be.

A sealed joint is a joint plus a sealant, and the sealant accommodates movement while closing the line. Joint sealants have a finite life and are among the components most likely to need replacing during a building's life, which makes a sealed joint a consumable element rather than a permanent one.

There is a specific trap in decoration. A painted bead becomes invisible, so nobody knows where the movement joints are a few years later, and paint over a moving joint can crack even when the sealant underneath is perfectly sound. Recording the joint positions is what prevents the next redecoration from filling them solid.

The trades most likely to close a joint are the ones arriving last, chasing a flat line, and having been told nothing. A gap in an otherwise continuous surface looks like unfinished work, and filling it with something rigid is a reasonable thing to do if you do not know why it is there.

That makes communication part of the detail rather than an afterthought. The joint layout drawing, the reason for each joint, and an instruction that no later finish bridges them belong in the information given to every finishing trade and in the handover record for the building.

Where movement concentrates, how much provision a construction needs and where the joints therefore go are determined by the engineer or designer responsible for the floor or wall. What movement a given profile or sealant can accommodate repeatedly is stated by its manufacturer. Whether a crack is reporting a movement problem or something else is assessed at the building by a qualified professional.