Why the Subfloor Decides How Your Laminate Performs

Laminate is only as good as what sits beneath it. A concrete slab that is dusty, uneven or still releasing moisture will undo even the most careful installation within a season. Boards will rock, click joints will open, and that hollow tap underfoot will follow you around the room. The good news is that concrete is a genuinely excellent base for laminate — it is stable, hard-wearing and doesn't flex — provided you give it the preparation it deserves. That usually means a weekend of work, a few specialist products, and a clear plan.

Before you lift a single plank, set aside time for four jobs: moisture testing, cleaning, repairing, and levelling. Skipping any of them is the most common reason laminate installations fail.

Test the Slab for Moisture Before Anything Else

Concrete is porous and can stay damp for months, particularly in new builds, extensions and any slab with a ground bearing beneath it. Trapped moisture has nowhere to go once laminate and underlay are sealed over the top, so it migrates upwards and attacks the core board. The result is swelling along the seams and a floor that feels spongy in patches.

You have two practical options for testing:

• Plastic sheet test. Tape a 500mm square of polythene to the bare slab with the edges sealed. Leave it for 24 to 48 hours. If condensation appears on the underside or the concrete beneath looks darker, there is moisture on the move.
• Electronic meter or calcium carbide test. A hygrometer gives a reading in relative humidity, while a carbide test gives a percentage by weight. For most laminate, aim for readings below roughly 75% RH or around 2% CM — always check the manufacturer's stated limit, as it varies.

As a rule of thumb, a new slab needs about one month of drying per 25mm of thickness, and longer in cold, poorly ventilated conditions. Six weeks is the bare minimum for most pours; 100mm slabs often want three months or more. If readings are high, either wait, improve ventilation, or plan on a surface damp-proof membrane — a 250 micron (1000 gauge) polythene sheet laid with 200mm overlaps and taped seams.

Clean, Repair and Deal with Contaminants

Once you know the slab is dry enough, strip it back to bare concrete. Old adhesive, paint splashes, plaster droppings, carpet gripper residue and general builders' grit all need to go. A scraper, a stiff brush and a vacuum will handle most of it, and a floor scarifier or grinder helps with stubborn residues. Never rely on sweeping alone — a single stray screw or lump of hardened mortar will telegraph through the underlay and dent the board above it.

Then walk the floor and deal with imperfections:

• Fill cracks and small holes with a cement-based repair mortar, feathered flush with the surface.
• Chase out and fill any shallow divots left by old fixings.
• Remove any protruding lumps with a chisel or grinder rather than trying to bridge over them.
• Check for oil, grease or silicone patches, which will stop levelling compound bonding. Clean these and prime the area generously.

If there is a painted or sealed surface, test a small area for adhesion. Levelling compound will not grip a glossy coating and may peel away in sheets.

Levelling: Flat Beats Level Every Time

Laminate needs a flat plane, not necessarily a perfectly horizontal one. The accepted tolerance is generally around 3mm deviation over a 2m straight edge. Slide a long spirit level or straight edge across the slab in several directions and mark every high and low spot with a pencil.

Shallow dips of a few millimetres are best dealt with using a self-levelling compound. Prime the slab first — most compounds need it, and priming stops the concrete sucking water out of the mix too quickly, which causes a weak, crumbly finish. Mix to the exact ratio on the bag, pour in manageable sections, and spread with a trowel and a spiked roller to release air bubbles. Work in one continuous push per area and let it cure fully, usually 24 hours before light foot traffic and longer before laying.

For deeper hollows, build up in layers rather than one thick pour, and consider a patching mortar for anything over 20mm. High spots are trickier — grind them down if they are small, or accept that a raised area may need a thin plywood overlay, though this adds height at doorways and skirting.

Underlay, Acclimatising and the Expansion Gap

With a flat, dry, clean slab, you are nearly there. Bring the laminate packs into the room at least 48 hours before installation so the boards adjust to the ambient temperature and humidity. Stack them flat, not on their edges.

Lay your underlay directly onto the concrete, butt the edges and tape the joints so it cannot shift. If you are using a polythene damp-proof membrane, put it down first, then the underlay. Foam underlay is fine over a flat slab; a combined underlay with a built-in membrane saves a step if moisture readings are borderline.

Leave a minimum 10mm expansion gap all the way around the perimeter, including at door frames, pipes and any fixed obstacles. Laminate moves with the seasons, and a tight fit against a wall is a guaranteed buckle. Underlay door architraves and skirting rather than cutting the boards around them where you can.

Final Checks Before the First Plank

Run your hand across the prepared floor. It should be dry, dust-free and smooth, with no grit and no soft patches. Recheck your straight edge one last time. Confirm the room's humidity is stable and that heating, if fitted, has been running normally for a few days. Once those boxes are ticked, the installation itself becomes the easy part — and the floor you walk on in five years will still feel solid, silent and flat.