Installing SPC Over a Concrete Subfloor

Concrete is a great base for SPC — it's solid, it doesn't flex, and rigid planks float over it happily. Two things trip people up though: moisture and flatness. Slabs hold and release water, and a new pour can look bone dry on top while it's still curing underneath. And a slab that looks flat by eye can hide dips a plank will bridge unevenly. Sort both before the first plank goes down and you'll have a floor that stays put.

Test for moisture before anything else

A slab that hasn't fully cured, or one with no barrier below, keeps pushing vapour upward. SPC itself has a 100% waterproof core, but trapped moisture can still gather under the floor and reach whatever's at the walls. New concrete typically needs weeks of drying before it's ready. Tape a square of poly to the slab overnight and look for condensation, or better, use a moisture meter or a calcium chloride test to get a real reading.

Add a moisture barrier over slab

Even a slab that tests acceptable can benefit from a thin polyethylene moisture barrier over concrete. It's cheap protection against seasonal damp. On ground floors and basements it's close to standard practice; upstairs over timber it isn't needed.

Check the slab is flat

Rigid SPC bridges minor imperfections but not large ones — a big dip leaves a plank unsupported, which flexes the joints and can crack the lock. Drag a long straightedge across the floor in several directions and mark any daylight underneath. A common trade tolerance is about 3 mm under a 2 m straightedge; grind down high spots and fill hollows with a levelling compound. Follow MILAT's installation guidance for the exact deviation your floor is rated to handle.

• Vacuum the slab — grit under click joints causes gaps and squeaks
• Fill cracks and hollows, grind high ridges flat
• Prime before using a self-levelling compound so it bonds
• Lay a moisture barrier on ground-contact slabs
• Acclimate the planks in the room before you start

Why MILAT SPC works well on concrete

The waterproof core shrugs off the odd spill, and the licensed Unilin click floats the floor over the slab with no glue and no nails — up to 60% faster than a glue-down, and reversible if you ever need to lift it. The IXPE acoustic backing (up to 21 dB) softens the hard, echoey feel of bare concrete underfoot.

Old slabs: deal with what previous floors left behind

Renovation slabs are rarely bare, clean concrete. They carry ridges of old adhesive, paint splashes, plaster droppings and sometimes curing compounds that were sprayed when the slab was poured. All of it has to go, not for cosmetic reasons but because every high spot becomes a rocking point under a floating floor and every soft residue is a layer that can shear. Scrape and grind the surface back to sound concrete, vacuum thoroughly, and only then judge flatness. An hour of preparation here saves years of clicking noises later.

Using self-levelling compound properly

Where a slab dips beyond tolerance, self-levelling compound is the standard fix, and it works well when respected. Prime the slab first so the compound bonds and does not dry too fast, mix to the manufacturer's water ratio exactly, and let it cure fully before laying anything over it — a levelling layer holds mixing water that must leave. Feather-finish products handle shallow dips; deeper hollows need a compound rated for the fill depth. Do not be tempted to bridge a dip with underlay instead; foam compresses, and the dip returns as joint movement.

Basements and ground-bearing slabs are a special case

Below-grade and ground-bearing slabs live closest to ground moisture, and older buildings may have no damp-proof membrane at all. SPC itself does not care — the core is 100% waterproof — but persistent vapour trapped beneath any floor covering can feed mould and musty odours at the slab surface. In basements, treat a vapour barrier as mandatory regardless of the moisture reading on the day you test, because ground conditions change with seasons. It is a small cost against a very annoying problem.

Acclimatisation and expansion gaps still apply

SPC is dimensionally stable, but stable is not inert. Bring cartons into the room and let them settle to its temperature before laying, especially if they arrived from a cold vehicle onto a warm slab, and keep the expansion gap at every wall, pipe and fixed unit. Concrete rooms are often the large, open ones — living areas, basements, commercial space — and large floating areas are exactly where perimeter gaps and, in very long runs, transition joints do their quiet work.

The mistakes we see most on concrete

• Skipping the moisture test because the slab looks dry on the surface
• Laying over adhesive residue or paint instead of grinding back to concrete
• Checking flatness by eye instead of with a straightedge
• Filling dips with underlay rather than levelling compound
• Closing the expansion gap with skirting nailed through the planks
• Laying over a fresh levelling layer before it has fully cured