Laying a Timber Floor on a Concrete Slab: What Happens First

You can lay a timber floor on a concrete slab, and it is one of the most common installs in Melbourne homes. The work that decides whether it lasts happens before the first board goes down: testing the slab for moisture, grinding and levelling it to the manufacturer’s flatness limits, and choosing between gluing the boards down and floating them over underlay. Then there is the height the new floor adds, which shows up at every door, tile edge and stair. This guide takes those in the order they come up on a real job.

Step one: moisture

Concrete holds water for a long time after it is poured, and a ground-floor slab can keep drawing moisture up from the soil for years. A slab can look and feel dry and still carry enough moisture to make timber cup, swell, or let go of the adhesive. That is why flooring manufacturers generally require the slab to be tested before timber is installed.

How the slab is tested

A quick surface meter is useful for a first look. Run it along the walls and doorways and it will pick up a damp corner or the patch where the old shower leaked. It can’t see deeper into the concrete, though. For that, installers drill small holes, set humidity probes into them and leave them to settle before taking a reading, which tells you what is going on inside the slab. The board and adhesive manufacturers decide which of these tests is required and what number counts as dry enough, so the test is chosen to suit the products going down.

Slabs that need the most care

• New builds and new extensions, where the concrete is still drying.
• Ground-floor slabs generally, and especially where the garden or paving outside sits higher than the slab.
• Houses with a history of leaks, such as a shower that leaked into the next room or a washing machine overflow.
• Slabs that have been under tiles or vinyl for years, which can hold moisture that shows up once the covering comes off.

If the reading is too high, the choice is a moisture barrier system or more drying time. Laying timber over a wet slab isn’t a suitable option. Many manufacturers ask for a barrier on ground-floor slabs whatever the reading.

Step two: flatness

Slabs are rarely as flat as they look. Manufacturers set a limit on how much the subfloor can dip or rise over a given distance, and installers check it with a long straight edge laid in different directions across each room.

High spots, such as a ridge where two pours meet or old tile adhesive, are ground back. Low spots are filled with levelling compound, a cement-based product poured or trowelled on that sets hard. A small dip might need a patch. An old slab with a lot of movement might need a whole room levelled. Levelling compound needs time to cure before boards go down, which can add a day or more.

How much levelling is needed is hard to know until the old floor is up. Carpet in particular hides a lot. A good quote says what has been allowed for and what happens if more turns up. Our floor removal and subfloor page explains this work in more detail.

Step three: glue down or float?

With a dry, flat slab, there are two ways to lay the boards.

	Direct stick (glued)	Floating
How it works	Each board glued to the slab with flooring adhesive	Boards joined to each other and laid over underlay, not fixed to the slab
Feel and sound	Solid, close to a traditional timber floor	Lighter, a little more click in hard shoes
Flatness needed	High; the board follows the slab	High; hollows make joints flex and creak
Time	Slower, adhesive needs curing time	Quicker
Suits	Engineered oak and hardwood, herringbone	Laminate, some engineered boards, many apartments
Fixed joinery on top	Possible	Avoid; the floor needs to move

Our engineered oak and hardwood boards are tongue and groove, and they can go either way depending on the board, the slab and, in apartments, the building’s acoustic rules. Herringbone is almost always glued. Our laminate uses a click joint and is always floated. For a fuller comparison of the two methods, see our guide to floating floors vs glue-down.

Step four: the height you are adding

A new floor on a slab raises the floor level, and it is easy to overlook until a door won’t close. Our engineered oak is 15 mm thick and our laminate 12 mm, plus adhesive or underlay. The height matters in a few places.

• Internal doors. Doors usually need trimming at the bottom, and door jambs are undercut so the boards slide underneath for a clean finish.
• Tiles. Where the new floor meets a tiled bathroom, laundry or kitchen that is staying, a reducer or transition trim takes up the difference.
• Sliding doors. Check the new floor sits correctly against the sliding door track and still lets the door run.
• Stairs. The bottom step of a staircase gets shorter by the added height unless the stairs are clad as well. Uneven steps are a trip hazard.
• Appliances. A dishwasher under the bench can be trapped if the floor is laid in front of it. Plan whether it comes out first.

Before installation day

Once the slab is prepared, the boards are delivered and left to acclimatise to the house, following the manufacturer’s instructions for that product. Keep the house at normal living conditions during this time. If you normally run heating or air conditioning, please keep it running.

On the day, the installers check the slab is clean and dry, lay the barrier if one is needed, then the underlay or adhesive, then the boards. They leave an expansion gap around the edges, hidden under skirting or scotia. More on the full sequence is on our supply and install page.

If there is heating in the slab

Hydronic or electric heating in the slab affects which boards can be used, how they are laid, and how the heating is run before and after installation. Each product has its own limits. Please let us know about in-slab heating at the start, and see our guide to engineered timber over underfloor heating.

Upstairs slabs in apartments

An apartment floor above ground level is usually a suspended concrete slab. It doesn’t draw moisture from the soil the way a ground-floor slab does, although a recent build or a past leak from above can still leave it damp, so it is still checked. The bigger issue is noise. Hard floors carry footsteps to the apartment below, and most owners corporations set acoustic rules for timber, laminate and hybrid, usually through the underlay or the adhesive system. Some also require written approval before work starts.

We recommend getting the building’s rules in writing before you choose a board, and bring them to the measure. Access matters too: lifts may need booking and protecting, and in older walk-up blocks the boards are carried up the stairs. Our Box Hill page covers apartment jobs in more detail.

Common mistakes on slabs

1. Skipping the moisture test on a new build because the slab “looks dry”.
2. Floating over a slab with dips, which leaves joints that creak or open within months.
3. Laying against the walls with no expansion gap, so the floor has nowhere to go in a humid summer.
4. Pinning a floating floor under a kitchen island or heavy built-in joinery.
5. Forgetting the bottom stair and the sliding door until the boards are down.

Each of these is avoidable, and all of them are cheaper to fix before installation than after. Manufacturer warranties also usually depend on the slab being prepared to their instructions, so skipping steps can affect your warranty cover as well as the floor.

Next step

If you know anything about your slab, such as its age, past leaks or in-slab heating, write it down. Then book a free measure and quote. We check the slab for flatness and signs of moisture, measure the rooms and list the preparation on the quote. You can call us on 1300 690 160 or visit the showroom at 2/1386 Toorak Road, Camberwell.