Why Concrete Slabs Behave Differently

Concrete is a superb base for flooring — flat, solid and permanent — but it is also porous and thermally greedy. A ground-floor slab sits in contact with the earth, drawing moisture upwards by capillary action, while its high thermal mass pulls warmth out of a room far more quickly than timber or a suspended floor. That combination of damp and cold is what ruins otherwise excellent installations: laminate swells along the joints, engineered boards cup at the edges, vinyl lifts and adhesives let go.

All three problems are solvable, and the underlay does most of the solving. Get it right and your new floor will sit flat, feel warmer underfoot and stay that way for years. Get it wrong and you will be pulling it up within a season.

Test the Slab Before You Buy Anything

No underlay can compensate for a genuinely wet slab, so start with a moisture test. A cheap hygrometer and a sealed polythene sheet left taped down for 24 hours will give you a rough reading; a calcium carbide or electrical impedance meter gives you figures you can act on. As a working rule, most floor coverings want the slab below 75% relative humidity, and some adhesives and solid timber want it lower still.

New concrete needs time. Allow roughly one month of drying per 25mm of slab thickness, and remember that drying slows dramatically in cold weather. If readings are high, the sensible options are a surface damp-proof membrane, a liquid epoxy coating, or simply waiting.

If you are laying a floating floor, choose an underlay with an integrated damp-proof membrane — often sold as a three-in-one or five-in-one product with a foil backing and a polythene film. Where you use a separate membrane, lap the joints by at least 200mm, tape them with waterproof tape, and turn the edges up the wall by 50mm so any moisture is guided back under the floor rather than into the skirting.

Taking the Chill Off a Concrete Floor

Concrete feels cold because it conducts heat away from your feet quickly, not because it is actually colder than the air. Underlay slows that transfer, and its thermal resistance is measured in tog — the same unit used for duvets. For normal use, aim for around 1.0 to 1.5 tog. Go thicker and you may make the floor feel spongy rather than warm.

• Foil-backed foam underlays are thin, inexpensive and reflect heat back into the room. Good for laminate and engineered wood.
• Extruded polystyrene (XPS) boards of 5–10mm give real insulation and add a little firmness. Useful where a floor is noticeably cold.
• Wood fibre boards of 5–8mm offer warmth, sound deadening and a stable base for timber floating floors.
• Rubber crumb underlays feel warmer underfoot and absorb impact, though they add little thermal insulation on their own.

If you have underfloor heating, keep the total tog below 1.5 and follow the heating manufacturer's guidance — too much insulation and the system will struggle to reach temperature.

Match the Underlay to the Flooring

The wrong underlay for the covering above it causes more failures than any other single mistake. Work backwards from the floor you have chosen.

• Laminate: 3–5mm foam or combined foam-and-membrane, firm rather than soft. Too much give and the click joints will flex and eventually crack.
• Engineered wood: 3–5mm wood fibre or a dedicated timber underlay with a built-in vapour barrier. Avoid bouncy foam, which stresses the locking system.
• Luxury vinyl tile: 1–3mm only. LVT needs a dead-flat, firm base, so a thin membrane plus a plywood or self-levelling overlay is often the correct answer rather than a thick comfort underlay.
• Carpet: PU foam, felt or crumb rubber at 6–10mm works well over concrete, with a separate damp-proof membrane beneath if the slab is at all suspect.
• Solid timber: usually glued down or fixed to battens, with insulation boards between the battens and a membrane below.

Details That Decide Whether the Job Lasts

Preparation accounts for most of the quality of the finished floor. Sweep the slab thoroughly, fill cracks and hollows, and use a self-levelling compound where the surface varies by more than 3mm over a two-metre straightedge. Dust is the enemy of adhesives, so prime and clean before you lay anything.

Let your flooring acclimatise in the room for at least 48 hours, ideally longer for timber. Leave a 10mm expansion gap around the perimeter and cover it with skirting rather than filling it. Never sandwich two vapour barriers together — trapping moisture between them turns a small problem into a large one.

Finally, think about thresholds and doorways. Underlay should stop short of the door bar rather than run beneath it, and a slightly firmer strip at the transition keeps the edges of the floor from rocking as you walk through. Small touches, but they are what make an installation feel properly finished.