Laying laminate on a concrete floor: moisture, membranes and prep

You can lay laminate on a concrete floor, and concrete is in many ways the ideal base, but only after three checks pass: the slab must be dry, it must be flat to about 3 mm over 1 m, and there must be a damp proof membrane between the concrete and the boards. Moisture is the one that catches people out. Concrete holds and transmits water vapour long after it looks and feels dry, and laminate destroyed from below is the most expensive laminate mistake there is, because the whole floor comes up. The prep itself is modest: one test, one membrane and possibly a bag of levelling compound.

Can you lay laminate directly on concrete?

No. Bare concrete fails a laminate floor twice over: it releases water vapour into the boards, and it gives the click joints nothing to cushion them, so the floor is loud and the joints wear. Every laminate floor on concrete needs two layers under it, which can come as one product or two:

• A damp proof membrane (DPM), a polythene sheet of at least 0.2 mm, laid across the whole slab with joints overlapped by 200 mm and taped, and turned up 30 to 50 mm at the walls where the skirting will hide it.
• An underlay, the foam layer that supports the click joints, evens out minor texture in the slab and quietens the floor underfoot.

Combined underlays with a built-in vapour barrier do both jobs in one pass and are the usual choice on concrete. If you already own standard underlay, a separate DPM sheet underneath it works just as well; what does not work is standard underlay alone, whatever the packaging implies about concrete.

How do you check the concrete is dry enough?

Never by look or touch, a slab can pass both and still be far too wet. Age is the first filter: an old slab in a heated, lived-in room has usually reached equilibrium, while new concrete needs months, not days. The rule of thumb for drying time is 1 day per millimetre for the first 50 mm of thickness and 2 days per millimetre beyond that, so even a 65 mm screed wants around 80 days in good conditions, and cold or humid rooms take longer.

Then test, whatever the age:

The sheet test costs pennies and settles most cases. If a room fails, do not lay: fix the cause first. For new concrete that means more drying time and ventilation. For an old slab that fails, the slab's own damp proofing may be missing or broken, which is a problem to solve before any flooring goes down, a surface DPM helps but rising damp needs diagnosing, not covering.

How flat does the slab need to be?

Laminate tolerates roughly 3 mm of deviation over 1 m. Check with a straight edge (a long spirit level or a straight board) and a torch behind it: light showing under the edge marks a dip. Concrete usually fails in predictable places, a dip in the middle of the room, a ridge where two pours met, and high spots near the walls.

Underlay hides none of this, it is 2 to 3 mm thick and compresses. Boards laid over a dip bridge it, flex every time they are stepped on, and work the click joints loose until the floor creaks and gaps. The fix is cheap and quick on concrete: grind or chisel high spots, and pour self-levelling compound into dips. A bag covers a typical room's low spots, sets overnight, and needs a few days to dry before the moisture test applies to it too.

How do you lay the floor once the slab is ready?

With the slab dry and flat, laying on concrete is the same job as any other subfloor, and the same rules from the rest of this site apply:

1. Acclimatise the boards for 48 hours flat in the room, at normal living temperature, with the heating running as it will after the floor is down. The acclimatisation guide covers why and when longer matters, and new builds with damp air need the extra day.
2. Lay the DPM first, across the whole floor, joints overlapped 200 mm and taped, edges turned up at the walls. Then the underlay on top, butt-jointed and taped, unless a combined product does both.
3. Plan the layout before the first click. Divide the room width by the board width so the last row is not a sliver, and pick the starting wall deliberately, the where to start guide walks through the decision.
4. Keep the expansion gap. 8 to 12 mm on every side, spacers against every wall, nothing fixed through the boards. The full rules, including pipes and doorways, are in the expansion gap guide.
5. Fit skirting or beading to the wall, never through the floor, so the trim covers both the gap and the turned-up edge of the membrane.

None of these steps change because the base is concrete; the membrane and the moisture test are the only additions, and they are the ones that decide whether the floor is still flat in five years.

What does the job cost and how long does it take?

For a typical room, the concrete-specific prep adds little to the bill: a DPM sheet or the upgrade to a combined underlay costs a few pounds per square metre, levelling compound is sold by the bag, and the tests cost almost nothing. Time is the real budget item. The tests take 2 to 3 days of waiting, levelling compound needs a night to set and days to dry, and the boards need their 48 hours in the room, so a well-run job on concrete is a week of elapsed time for a weekend of actual work. Plan the order and the quantities before you start: measure the room, run it through the laminate calculator to get whole packs with a waste allowance built in, and let the slab finish drying while the packs acclimatise beside it.