You pull up the old carpet and there it is: a line running across the slab. Some are hairline, some are wide enough to catch a fingernail, and some sit in a neat straight groove. The question we get on site is always the same. Will it show through the new floor?
The honest answer is that it depends on what kind of crack it is and whether it is still moving. Here is how we sort them out before any finish goes down.
Cracks and control joints are not the same thing
Concrete shrinks as it dries. Because it is strong when squeezed and weak when stretched, it cracks. Builders cut or tool control joints into a slab to give that cracking somewhere to go, so it forms inside a straight line instead of wandering across your living room.
That means a control joint is planned. A random crack is not. Both can telegraph through a floor, but they need different treatment.
- Control joints: straight, regular, often lining up with doorways or the centre of a room.
- Shrinkage cracks: fine, often hairline, usually appear while the slab is young.
- Moving cracks: open and close with the seasons, or show a step where one side sits higher than the other.
Why they show through
A floating hybrid or vinyl plank sits over the slab. If the surface has a ridge, a lip or an open joint, the planks can flex over it. Over time that can mean a soft spot underfoot, a creak or a click joint that starts to lift.
Glued floors tell on you differently. Thin vinyl will mirror a ridge along the joint line. Timber glued to a slab that is still moving can open up along the same line. Microcement is rigid, so a crack that keeps moving underneath will often come back up through the finish.
In every case the floor is only as stable as what it sits on. That is why the crack gets dealt with first, not after the flooring is chosen.
How we assess a crack on site
At the site inspection Mitch looks at the whole slab, not just the crack you pointed out. He is checking a few things:
- Is the crack hairline, or can you feel an edge?
- Is there a height difference from one side to the other?
- Does it run straight like a joint, or does it branch?
- Are there signs it has moved before, such as old patching that has cracked again?
- Is there any moisture or contamination along it?
That last one matters. A crack can be a path for moisture to rise, so it gets read alongside the moisture testing, not separately. If you want the full list of what we look at, the eight checks are in our free Floor Ready Guide.
How cracks and joints get treated
Treatment depends on what the crack is doing and what finish is going over it. The manufacturer's specification for your flooring has the final say, and we work to it so your warranty stays valid.
- Stable hairline cracks: usually opened up a little, cleaned of dust and filled so the surface is sound before levelling.
- Wider or uneven cracks: need proper repair and the surface brought back flush, then checked with a straight edge.
- Moving joints and cracks: should not be filled with something rigid. A rigid fill just cracks again beside the repair. These generally need a flexible treatment, and many manufacturers do not allow a floor to simply bridge a true movement joint.
Grinding comes before any filling. We use dust-extracted grinding, so the crack is clean and the house stays clean. Then the levelling and the readings are recorded before the finish goes down.
When it is not a flooring job
Around Newcastle and Lake Macquarie, slabs can sit on reactive clay, sand or ground affected by old mine workings. Some cracks are a sign the building itself is moving. If a crack is growing, is stepped, runs up the walls, or you have doors that have started to jam, that is a question for your builder or a structural engineer first. We will tell you if we think a crack is beyond a flooring fix. A floor laid over a moving slab will only repeat the problem.
The same goes for cracks on a repair job. If a floor has failed along a joint line, replacing it without finding the cause usually fails the same way. Our repairs and rectification work starts with finding that cause.
Check it before you buy flooring
Do a quick walk of the slab with a torch held low along the surface. Shadows show ridges and steps. Note where lines run and take a photo of anything that looks like movement. It gives you something specific to show whoever is quoting.
Crack treatment is one part of our preparation and levelling work, and it is much easier to get right before the floor is chosen than after it is laid. If you have a line across your slab and you are not sure what it means, book the site inspection and Mitch will read it for you.