Polished concrete on an existing slab: can it be done?

Most of the floors we polish in Bunbury and the South West started as an ordinary slab under carpet, tiles or vinyl. So yes, an existing slab can usually be polished. The useful question is what the slab will give you, and what it will not.

What we check first

• Cracks and movement. Hairline cracks can be filled and will show as fine lines. Cracks that are moving, or slabs that have settled unevenly, need a structural look before any finishing.
• Flatness. Grinding flattens a slab a little, but it cannot take out big humps or dips. A floor that was screeded for tiles is often fine; a rough shed slab may need more work.
• Old coverings and glue. Tile adhesive, carpet glue, paint and old sealers all have to come off. This is the slow, dusty part and it is where most of the extra cost sits.
• Moisture. Slabs without a vapour barrier can push moisture up through a polished surface. We test before sealing.
• Patches and trenches. Plumbing trenches, old repairs and different pours show as different colours once ground. They can be a feature or a problem, depending on taste.

What the slab decides for you

On an existing floor the cement colour and the stone are already set. We choose how deep to grind and how much sheen to leave, and that is about it. Many older slabs have a lot of large stone, so a deep grind gives a busy, stone-rich floor; a light grind keeps more of the cream top. If you want a particular colour, read our guide to polished concrete colours before deciding.

For big, open commercial floors where a full polish is more than needed, burnished concrete can be a lighter option on a sound slab.

When an overlay or topping is the answer

If the slab is sound but you want a colour, a stone or a level the old concrete cannot give, a bonded topping poured over it solves that. It needs enough thickness to be ground safely, a properly prepared base so it bonds, and it raises the floor level, which matters at doors and stairs. It is not a cure for a slab that is cracking or moving.

Why a new slab is planned differently

When we pour a house or shed slab that will be polished, the mix, the stone, the placement and the surface are all chosen for the finish. The slab is placed flatter, the joints are laid out with the finished floor in mind, and the colour is decided before the truck arrives. One crew from structure to finish removes most of the surprises that an existing slab can throw up.

Getting an answer for your floor

Send photos of the bare slab if you can see it, tell us what is on top of it now, and the rough area. We will say whether it suits polished concrete, a honed finish, a topping, or a new slab. This applies to residential renovations and commercial fit-outs alike.

Where we work: Bunbury and the South West of Western Australia.