Installing flooring over a concrete slab

What flooring can be installed over a concrete slab?

Rigid-core vinyl, porcelain tile, engineered hardwood and carpet can all be installed over a concrete slab. Solid hardwood and standard laminate cannot be installed directly on a slab in most cases: solid hardwood needs to be nailed to a wood subfloor and moves too much with slab moisture, and laminate's wood-based core swells when it absorbs the vapour concrete continuously releases. Before installing anything moisture-sensitive, the slab must be tested, preferably with a relative humidity probe test to ASTM F2170 rather than a surface test. A slab that looks and feels dry can still emit enough vapour to destroy a floor over several years. On below-grade slabs, add a vapour barrier at minimum and ideally a dimpled subfloor panel, which also makes the floor warmer and bridges minor cracks.

Why slabs release moisture

Concrete in contact with soil wicks groundwater and passes it upward as vapour. New slabs also release water from the original mix for months — a general rule is 30 days of curing per inch of thickness before testing. Both sources are invisible and continuous, which is why testing rather than inspection is the standard.

Testing properly

The relative humidity probe test drills into the slab and measures conditions at depth, which is what actually matters. Calcium chloride tests measure only surface emission and can read acceptably on a slab that will still fail a floor. Most manufacturers now specify the probe test for warranty purposes.

Levelling and flatness

Slabs are rarely flat enough as poured. Floating floors need flatness within about an eighth of an inch over six feet, and large-format tile needs better than that. Self-levelling compound or grinding brings the surface into tolerance, typically $1 to $4 per square foot.

Vapour barriers and subfloor panels

A 6 mil poly vapour barrier is the minimum on a below-grade slab. Dimpled subfloor panels do more: they create an air gap that drains and dries any water that arrives, insulate against the cold slab, and bridge minor cracks. At $2 to $4 per square foot they are the standard recommendation in Canadian basements.

Adhesive considerations

Glue-down products depend entirely on the slab's moisture condition, because adhesive fails when vapour pressure builds beneath it. If a slab tests high and you want a glued product, a moisture-mitigation coating is required, adding $2 to $5 per square foot.

Common questions

How long should a new concrete slab cure before flooring?

A common guideline is 30 days per inch of slab thickness, so a four-inch slab needs about four months. That is a rule of thumb, not a substitute for testing. The only reliable answer comes from a relative humidity probe test.

Can I glue vinyl plank directly to concrete?

Yes, if the slab tests within the adhesive manufacturer's moisture limits and is flat and clean. Glue-down gives a very stable result in high-traffic areas. If the slab tests high, use a floating rigid-core product over a vapour barrier instead, or budget for moisture mitigation.

Does a slab in a heated basement still need a vapour barrier?

Yes. Heating the space does not stop the slab from wicking moisture out of the ground beneath it. Warm air can actually increase evaporation from the slab surface. The vapour barrier separates your flooring from that moisture regardless of room temperature.

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