Mould Under Floor Coverings

Why the surface looks sound while there is moisture beneath

Most modern floor coverings are practically impermeable to water from above: vinyl, sheet flooring, laminate with a dense wear layer, lacquered parquet. Once water has got beneath them — from a burst pipe, a washing machine hose, flooding from a neighbour or an overflowing bath — it can no longer dry upwards. A covering that keeps water out also keeps it in.

The sealed zone underneath

The result is a sealed zone beneath the covering in which moisture stays for weeks. The substrate — a concrete slab, levelling screed, timber boards or chipboard — absorbs it and holds it. These are exactly the conditions in which fungus grows best: constant moisture, no air movement, room temperature and plenty of organic material in the underlay, the substrate material or the lower layer of the laminate.

1A leakWhen the leak was six months ago and the floor "dried out" in two days, all that dried was what you could see. More on how moisture behaves in structures — in the section on indoor humidity.

2Condensation on a cold substrateThis happens on ground floors above an unheated basement and on slabs bordering the outside. Warm indoor air reaches the cool substrate through a non-airtight junction at the wall, and moisture settles where nobody can see it.

3A covering laid over a substrate that had not driedAfter a refurbishment the levelling screed goes down and a few days later the laminate goes on top. The screed still holds a significant amount of water, and it can no longer escape through the new covering. The cause is haste.

All three cases have one thing in common: the moisture is where it cannot be seen, dried or treated while the covering is in place — which is why this problem is found late and its extent almost always turns out to be greater than expected.

How to spot it

Mould under a floor almost always announces itself indirectly. These are the signs worth taking seriously:

A smell. A characteristic basement or earthy smell that intensifies after the room has been unused for a while and in warmer summer weather. It is often stronger closer to the floor and at the edges of the walls.
Springy or moving boards. Laminate planks that used to sit tight begin to "walk" or tilt slightly underfoot.
Swollen joints. Small ridges appear at the laminate joints, and a lift can be felt at the edge of a plank.
Lifting at the edges. Sheet vinyl comes away from the substrate at the wall, and the skirting no longer sits flush.
Dark areas under light vinyl. The material is thin enough for a patch underneath to show through as a grey shadow.
Skirting boards and door frames. A timber component at floor level swells or darkens first, because it draws moisture from the substrate.

None of these signs is proof on its own. The proof is a measurement: the moisture in the substrate material beneath the covering. We measure at several points — in the affected area and at control points in parts of the room where nobody has noticed a problem. If the reading in the affected zone is significantly higher than at the control points, the question is no longer "whether" but "how far".

A frequent question is whether you can simply lift one plank and look. You can, and sometimes that is enough — if there is a visible patch or a clear smell beneath it, the answer is clear.

But a negative result at one point proves nothing: the wet zone may be a metre away, and water under a covering never spreads in an even circle. It takes the easiest route — along the joints, along the wall, through service channels — and it can end up at a completely different edge of the room from where it started.

Sign	What it usually means	How urgent
A smell at floor level with no patches	Fungus is growing under the covering or in the substrate	Needs checking with a measurement
Swollen laminate joints	Water has been under the covering for some time	The covering has to come up
Vinyl lifting at the wall	Moisture in the substrate, the adhesive has lost its grip	The covering has to come up
The floor is springy underfoot	The substrate or an intermediate layer is damp and soft	The covering has to come up
Skirting darkening or swelling	Moisture coming from the substrate at the wall	Needs checking with a measurement
A dark shadow under light vinyl	A visible patch beneath a thin covering	The covering has to come up

Why the covering has to come up

Mould under a floor cannot be treated without removing the covering.

This is the part clients like least, so we will say it plainly. There is no product you can pour over the top that will reach the substrate at the concentration needed. Nor is there any way to dry the substrate through an unbroken layer of covering.

How wide an area has to come up is a matter of assessment. It is often enough to remove the covering in the affected zone and a sufficient band around it, rather than throughout the flat. If the readings show the moisture stops a metre from the leak, there is no sense in taking the whole room apart.

What happens to the covering itself:

Laminate with swollen edges is finished — its core is compressed chipboard, which never returns to its original shape once it has absorbed water. Unaffected planks can often be reused.
Carpet and its backing with fungus have to be thrown away. It is a porous material in which the fungus grows through the full thickness.
Sheet vinyl is not itself a food source, so it can often be cleaned and reused, provided the material has not deformed.
Parquet and board floors have to be judged board by board: a board that has not deformed and where the fungus is only at the surface can usually be treated and sanded.
Acoustic underlay beneath laminate practically always has to be replaced — it is cheap, and it is exactly the material that held the moisture.

Drying the substrate before relaying

Once the covering has come up and the affected zone has been treated, a process begins that no chemical speeds up: drying the concrete or screed substrate. Concrete gives water back slowly, and a thin levelling layer above it can look dry while moisture remains underneath.

Measurement and defining the zone

We measure substrate moisture at several points and determine how wide the affected zone is. That decides how much of the covering has to come up.

Removing the covering and underlay

We expose the substrate in the affected zone and a sufficient band around it. Acoustic underlay and carpet backing are disposed of.

Treating the substrate

We treat the exposed substrate and the lower part of the wall in the affected zone with a professional product, observing the contact time.

Drying with monitoring

We dry the substrate and re-measure the readings. A concrete substrate can take several days — a fan for a couple of hours is no substitute.

Clearance for the covering

We tell you when the moisture readings are normal and the substrate is ready for a new covering.

✓ How to dry properly

Lay a new covering only when the readings show normal levels.
Dry with monitoring and re-measure the readings.
Deal with the moisture source first.

✗ How not to do it

Do not lay a covering because the surface feels dry to the touch — the whole cycle will start again under newly bought material.
Do not run powerful equipment at maximum in a small room — a rapidly dried timber floor and furniture will deform, and doors will start to catch.

At the same time the moisture source has to have been dealt with. If the leak has not been repaired, or if the substrate is damp because of rising damp, drying is only a pause. We have collected the reasons why moisture persists in a structure on the page about the causes of mould.

Prices

Inspection with moisture readings from €40. Treatment from €1.50/m², minimum order from €80. If the substrate has to be dried with equipment, we assess that by area and degree of moisture and agree it before starting work. Lifting and relaying the covering is usually done by a builder — we state how wide an area has to be exposed and when the substrate is ready for a new covering. More about our order of work — in the section on mould removal.

Frequently asked questions

The leak was six months ago and the floor dried out — can it really still be a problem?

Yes, that is one of the most common cases. What dried was the top of the covering, which you could see. Under a dense covering water does not dry out on its own, so the substrate can still be damp long after everything looked fine.

Will the whole floor have to come up?

Not always. A measurement shows how far the moisture has spread. It is often enough to expose the affected zone and a band around it. We only recommend taking a whole room apart when the readings genuinely call for it.

Can laminate planks be laid back down?

Those whose edges have not swollen and whose shape has not changed usually can. Swollen planks are not worth relaying — the joints no longer close tightly. We always recommend replacing the acoustic underlay.