How to Prepare a Subfloor for Click-Lock Flooring

When you're laying new planks in a Brisbane renovation or a Perth extension, the success of click-lock flooring depends almost entirely on what's underneath it. Australian homes throw a few curveballs at flooring projects — reactive clay soils under Melbourne slabs, salty coastal air in Sydney's eastern suburbs, and humidity swings across the Top End — so the subfloor has to be properly assessed before any board clicks into place.

The good news is that a waterproof, rigid click-lock system forgives a lot of sins that timber and traditional laminates cannot. WPC and SPC cores handle small surface variations better than solid hardwood, but they still need a flat, dry, and clean foundation. Skipping the prep work leads to soft spots, peaking joints, and that hollow drumming sound nobody wants underfoot. Here's how tradies and serious DIYers across Australia get the base right the first time.

Knowing what lies beneath

Australian houses are predominantly built on a concrete slab-on-ground, especially anything constructed after the 1970s. Older weatherboard homes in places like Hobart or Adelaide's inner suburbs often sit on timber joists with floorboards or yellow tongue particleboard over the top. Each subfloor type has its own preparation pathway, and recognising yours sets the tone for everything else.

Concrete slabs need to be fully cured before any flooring goes down. The general rule is sixty days per inch of thickness, though most residential slabs in a project home are cured long before handover. Timber subfloors require a check for squeaks, loose boards, and signs of borer or termite activity — a particular concern in regional New South Wales and along the Queensland coast where subterranean termites are most active. If you're weighing up which rigid core product suits your project, the WPC versus SPC comparison breaks down the structural differences that matter during installation.

The relevant Australian Standard is AS 1884-2021 for resilient floor coverings, which most click-lock installations follow as a benchmark. It specifies a maximum variation of 4mm over a 2 metre span for the substrate. Anything outside that and you're heading toward levelling compound territory rather than hoping the underlay will fix it.

Inspecting the existing surface

Before any tools come out, walk the floor slowly and look for the obvious tells. Cracks running diagonally across a slab in Melbourne's western suburbs often point to reactive clay movement and may need more than a cosmetic filler. Surface pitting, efflorescence (that white salty bloom), or crumbling edges near doorways suggest moisture wicking through the slab — common in older homes around Fremantle or Geelong where drainage hasn't aged gracefully.

For timber floors, probe suspect areas with a screwdriver. Soft spots mean rot, which means opening up the floor to find out how far it spreads. Squeaky boards can usually be fixed by screwing down into the joists, but a creaky particleboard sheet might need replacing entirely. Pay special attention to areas near external walls, bathrooms, and laundry doors where water splash has historically been tolerated for decades.

Don't forget to lift any existing floor coverings to see what's underneath. Vinyl tiles from the 1980s often hide a glue layer that's brittle, flaky, and impossible to bond new flooring to. Old carpet on concrete in a rental renovation might be hiding a slab that needs grinding back to sound material. The goal is a substrate that's solid, stable, and ready to take the new system.

Clearing and cleaning the workspace

Once the surface is sound, everything has to come off the floor. Skirting boards can usually stay if you're happy to undercut them, but door jambs typically need trimming to allow the new floor to slide underneath. A jamb saw or a multi-tool with a flush-cutting blade makes quick work of Australian pine jambs, which are softer than the meranti or kwila used in older homes.

Dust control matters more than most people realise. Abrasive dust under floating flooring creates a grinding paste that slowly destroys the click-lock profile and wears through the wear layer. Vacuum the entire area with a shop vac, then mop the slab with clean water — no chemicals needed, especially on concrete where residues can interfere with underlay adhesion. For timber subfloors, a damp microfibre mop picks up the fine sawdust that ordinary brooms leave behind.

If the renovation is happening during a knockdown-rebuild or extension in a dusty part of town like the inner west of Sydney or western Melbourne, hang plastic sheeting over the doorway to keep construction dust out of adjacent rooms. It's a small step that saves a deep clean later, and it keeps the site safer for any little ones still living in the house during the works.

Levelling and smoothing the surface

A straight edge across the floor reveals more than the eye notices. Lay a 2-metre spirit level or a long aluminium screed on the slab and check for high spots and low spots. Anything beyond a 4mm deviation under the straight edge needs correction — the click-lock profile can flex over a small dip, but it will eventually fatigue at the joint and start showing gaps along the long edge.

For concrete, a pour-on self-levelling compound like the Ardex or Selleys products available at Bunnings or a local trade supplier handles most residential issues. Prime the slab first, mix the compound to a pancake-batter consistency, and pour in ribbons, working into corners with a trowel. The compound seeks its own level, hence the name. Allow twenty-four hours of curing time before walking on it, longer if the weather is cool — something to factor in during a Tasmanian winter or a Perth evening in July when temperatures drop overnight.

Timber subfloors often need sanding to knock down high spots at board edges or joints between sheets. A floor sander with a coarse grit makes short work of yellow tongue, but be cautious of older Baltic pine floorboards which can be paper-thin in places after decades of sanding. Spot-level any low spots with a fibre-reinforced leveller designed for timber substrates, and sweep thoroughly between coats.

Moisture testing and protecting against humidity

Moisture is the silent killer of floating floors. SPC and WPC cores are waterproof, which protects them from spills above, but moisture migrating up through a slab from underneath will corrode the click profiles and encourage mould growth beneath the planks. This is especially relevant in tropical North Queensland, the coastal strip from Cairns to the Gold Coast, and even parts of Sydney where summer humidity pushes the dew point well above comfortable levels.

Conduct a moisture test on any concrete slab. The plastic sheet test — taping a square of clear plastic to the floor for twenty-four hours — gives a rough indication. Condensation underneath signals trouble. For accurate readings, a calcium chloride moisture test or a capacitance meter used by most flooring inspectors gives a parts-per-million figure against the limits in AS 1884.

Once the slab passes muster, lay a 200-micron vapour barrier sheeting across the concrete, lapping the seams by 200mm and taping them down. Many click-lock products come with a pre-attached underlay that includes a moisture barrier, but on a known-damp slab the extra layer is cheap insurance. For installations over ground-floor timber, standard foam or acoustic underlay is usually sufficient, although upper-storey apartments in Brisbane towers often require an acoustic underlay that achieves the NCC's impact isolation rating for body corporate compliance.

Acclimatising the click-lock planks

Floating floors expand and contract with temperature and humidity, even if the SPC or WPC core reduces the movement compared to solid timber. Manufacturers recommend acclimatising the planks in the room where they'll be installed for at least forty-eight hours, longer if the product has been stored in a cold warehouse. A Melbourne winter delivery sitting in a garage at 5°C won't perform the same as one that's sat in a 22°C living room for two days.

Lay the cartons flat, opened at the ends, and distribute them around the room to encourage even air circulation. Stack them so air can move between layers, and keep them away from direct sunlight through west-facing windows. The room should be at normal living conditions — heated in winter in places like Canberra and the ACT, or air-conditioned in the humid tropics during a Darwin wet season.

While the boards settle, this is also the time to plan the layout. Measure the room, mark the centre lines, and dry-lay a row to see how the last plank in each row will fall. Aim for a minimum 200mm end joint offset between rows and avoid narrow slivers of plank against walls, which look awkward and don't lock in securely.

Final checks and installation readiness

With the subfloor flat, dry, and clean, and the boards acclimatised, run through a final checklist before cracking open the first pack. Confirm that door clearances still allow the new floor height — typically 6 to 8mm with underlay — and undercut any jambs you missed earlier. Check that the right underlay is on site if it isn't pre-attached, and that you have spacers for the 8 to 10mm expansion gap around the perimeter.

Plan the direction of the planks. Running them parallel to the longest wall or to the main light source makes most rooms feel larger, but in narrow hallways running them lengthwise visually elongates the space. Around tricky spots like kitchen islands in open-plan homes or bathroom doorways, plan your cuts in advance so you don't end up with a fiddly wedge under a vanity that fails to click home properly.

Browse a few recent installation case studies to see how others have tackled similar layouts — particularly relevant if your subfloor involved levelling compound or acoustic underlay. They often reveal small tricks, like tapping the long edge with a pull bar in tight spots, that save time and broken planks. Click-lock flooring rewards careful preparation; get the subfloor right and the rest of the install falls into place without drama.