How to Stop Moisture Coming Through Your Concrete Floor
Why moisture comes through concrete
Concrete is porous. Water moves through it as vapour, driven by the difference in humidity between the ground below and the air above, and it will keep moving as long as that difference exists. The technical term is moisture vapour transmission. In practice, it means a slab can look and feel completely dry while still passing a steady stream of water vapour up into whatever is bonded to it.
Damp concrete floor sources
Residual construction moisture
Concrete takes a long time to dry. A rough working guide is around one day per millimetre of thickness for the first 50mm under good drying conditions, and considerably longer beyond that. A 150mm slab poured in autumn in a building with no heating on will not be ready for a floor covering in six weeks, whatever the programme says.
Groundwater rising through an unprotected slab
Where there is no damp-proof membrane beneath the concrete, or where it has been punctured or was never lapped properly, groundwater has a direct route up. Rising damp in a concrete floor is common in older industrial buildings, and it does not go away on its own.
A failed original membrane
Membranes get damaged during construction, cut through by later service runs, or simply fail with age. The symptoms often appear years after the building was finished, sometimes triggered by a change in drainage or landscaping outside.
Whichever the source, the effect on a resin floor is the same. Vapour cannot escape through a sealed coating, so pressure builds underneath it until the bond gives way.
Damp concrete floor signs to look out for
- Dark patches on the concrete that come and go with the weather, or that stay damp when the rest of the slab has dried.
- Bubbling or blistering coatings. Small domes in a resin floor, often filled with moisture, usually appearing in the middle of open bays rather than at the edges.
- Adhesive failure. Vinyl tiles lifting at the corners, sheet flooring bubbling, or a resin coating coming away cleanly with damp concrete visible underneath.
- A persistent damp smell, particularly in a closed area or one that is not used often.
- Efflorescence, the white powdery salt deposit left on the surface as water evaporates out of the slab.
Spotting these before you commit to a new floor is worth a great deal. Once a resin system has been laid over a damp slab, the only reliable remedy is to remove it and start again, and you will have paid for the floor twice as well as losing the use of the area twice.
How to stop moisture coming through your concrete
Test first
Hygrometer testing to BS 8203 gives you a relative humidity reading at the surface of the slab, which is the figure resin manufacturers write their limits against. The usual threshold is 75% RH. Below it, most systems can go straight down. Above it, you need to do something about it.
Wait, where waiting is realistic
If the slab is new and simply still drying, and the programme has room, drying is free. Improve the conditions by heating the building and moving the air, and re-test. This is often the right answer on a new build and rarely the right answer on a twenty-year-old warehouse, because a slab drawing moisture from the ground will never dry out.
Apply a surface damp proof membrane
This is the standard solution for the great majority of industrial cases. A liquid damp proof membrane, typically a low viscosity epoxy, is applied over the prepared concrete and cures into a continuous barrier that stops vapour reaching the resin floor above it. It is thin, it becomes part of the build-up, and once it is down it is invisible.
Use an epoxy moisture suppression system for higher readings
Where readings are very high or where there is genuine hydrostatic pressure, a heavier-duty system is specified, sometimes in more than one coat. The product and the number of coats follow the reading, not a rule of thumb.
Deal with the external cause where there is one
Blocked gullies, ground levels above the slab, a leaking drain or failed external tanking will keep feeding the problem. A membrane manages the symptom well, but it is worth fixing the cause where one is identifiable.
The point running through all of this is that sealing the surface without testing is guesswork. Apply a membrane that is not needed and you have spent money for nothing. Apply one rated below the reading and you have bought yourself a delayed failure.
How to test for moisture in concrete
Different methods measure different things, and they are not interchangeable.
Hygrometer testing to BS 8203
This is the BS 8203 standard for the UK flooring industry and the method most resin manufacturers write their limits against. An insulated hygrometer box is sealed to the surface of the concrete and left in place, usually for a minimum of 72 hours, until the air trapped inside reaches equilibrium with the slab. You should not take a reading until then.
The relative humidity reading at equilibrium tells you the moisture condition of the concrete, not just the surface.
Surface moisture meters
A hand-held concrete moisture meter, either pin or pinless, gives an immediate indicative reading. These are genuinely useful for a first look, for comparing one area against another, and for spotting a wet corner that needs proper investigation. What they cannot do is tell you what is happening deeper in the slab, and their readings vary with surface finish, aggregate type, reinforcement, salts and temperature.
This is why a concrete moisture meter alone is not enough for a commercial specification. It is a screening tool. It tells you where to put the hygrometers.
In-situ relative humidity probes
Probes inserted into drilled holes at a set depth within the slab. Common in some specifications and useful where you need to understand what a thick slab is doing at depth rather than only at the surface.
Whichever method is used, it needs to be done in enough places. One reading in the middle of a 5,000 square metre warehouse tells you about that spot. Testing should cover the extremities, the areas near external walls and doors, anywhere with a history of damp, and a representative spread across the rest.
What you get once moisture is under control
A stable substrate and a resin floor that behaves. No blistering, no lifting, no discolouration coming through from below, and no repeat of the problem that took out the last floor.
The membrane itself you will never see. What you will notice is a floor that is still tight to the slab in five years, which is the entire point of specifying one.
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