Does Microcement Crack?

Why the substrate decides everything

Microcement is not a slab. It is a thin cement-and-polymer coating troweled over something else — a concrete floor, a tiled bathroom, a sheeted timber floor. At that thickness it cannot span or resist movement; it inherits it. If the surface underneath moves, flexes or splits, the finish on top eventually shows it.

That is why a good installer spends longer on your substrate than on the finish coats. Checking what you actually have is not a formality before the interesting part — it is the part that decides whether your floor is still perfect in ten years.

What actually causes microcement to crack

• —A moving substrate — a timber or particleboard floor that flexes underfoot will telegraph straight through a coating. It needs rigid sheeting or a decoupling layer first.
• —A slab that was still curing — new concrete shrinks for months. Coat it too early and the slab's own shrinkage cracks become your cracks.
• —Existing cracks that were skimmed over instead of treated — a live crack in a slab will reopen through anything laid over it.
• —Movement and expansion joints that were bridged — those joints exist because the structure needs somewhere to move. Cover one with microcement and the microcement becomes the joint.
• —No reinforcing mesh — fibreglass mesh bedded into the base coats is what distributes stress and bridges hairline movement. Skipping it is the most common corner cut we see.
• —Drummy or loose tiles — tiles are an excellent base when they are solidly stuck down, and a poor one when they are not.
• —Rushed application — coats laid too thick, or dried too fast in direct sun, wind or a hot closed-up room, can craze while they cure.

Hairline crazing is not structural cracking

These two get confused constantly, and they are not the same problem. Crazing is a fine, shallow web of surface lines left by a coat that dried faster than it cured — cosmetic, usually preventable with site control, and addressed in the finish layers.

A structural crack follows something real underneath: a slab crack, a joint, a sheet edge, a doorway where two different floors meet. It runs reasonably straight, it reappears in the same place if you simply patch it, and the fix belongs in the substrate rather than the surface.

How we stop it before a trowel comes out

• —We assess and moisture-test the substrate, and say so honestly if it is not ready — including when that means waiting.
• —Cracks in the base are cut out, filled and stabilised rather than coated over.
• —Drummy tiles get lifted and made good; flexing timber floors get sheeted or decoupled.
• —Every job is primed for its specific substrate — tile, concrete and sheeting do not take the same primer.
• —Fibreglass mesh goes into the base coats as standard, not as an upgrade.
• —Structural and perimeter movement joints are carried through the finish, not buried.
• —Thin, controlled coats with proper cure time between them — which is why our minimum is three days on site, even for a small bathroom.

The Gold Coast bit

Two local conditions matter here. The first is new build: a lot of Gold Coast work happens on slabs poured months rather than years ago, and coating a young slab is the fastest way to inherit its shrinkage. The second is our older raised and high-set houses, where the floor is timber over bearers and joists — beautiful homes, but the floor moves, and it has to be made rigid before microcement goes anywhere near it.

Then there is summer. A closed-up house in February, or a coat catching afternoon sun through a wall of glass, dries far faster than it should. Controlling those site conditions during cure is part of the job, not an afterthought.

If your microcement has already cracked

It is repairable. Cracks can be opened up, stabilised, refilled and re-troweled, then re-sealed — and because microcement is a coating rather than a slab, a repair blends far better than patching tile or concrete ever will.

But a repair only holds if the cause is dealt with first. If the crack is a moving slab or a bridged joint, filling the surface just resets the clock. We are happy to look at someone else's work and tell you which of the two you have.

Common questions