Why Concrete Moisture Testing Matters Before Resin Flooring
Moisture in the concrete is one of the most common reasons a resin floor fails early. The floor can look perfect on handover, then weeks or months later blisters appear, the coating starts lifting at the edges, or whole sections debond. In most cases the problem was in the slab before the first coat went down. This article explains why concrete moisture testing matters, how it is done, and what the options are when a slab is too damp.
Why moisture causes resin floors to fail
Concrete is porous and holds water long after it looks dry. New slabs contain mixing water that takes a long time to leave, and older slabs can draw moisture up from the ground if there is no effective damp-proof membrane underneath. Resin floors are largely impermeable, so when they are laid over a damp slab the moisture has nowhere to go.
Osmotic blistering
Osmotic blistering happens when moisture is drawn through the concrete towards the underside of the resin. Soluble salts in the concrete and at the bond line create pressure differences that pull water in, and over time this builds into liquid-filled blisters. They often start small and grow, and when they are cut open they may contain a liquid that can be alkaline. Once osmotic blistering has started it is rarely solved by patching the blisters alone.
Delamination and poor adhesion
Even without blistering, too much moisture at the surface can stop the primer bonding properly in the first place. Some resins react with water during cure, leaving a weak layer at the interface. The result is a floor that sounds hollow when tapped, peels at edges and joints, or breaks up under traffic.
How concrete moisture is measured
There are several ways to check moisture, and they do not all tell you the same thing. A reliable assessment uses the right method for the job and takes readings in enough places to represent the whole floor.
Relative humidity (hygrometer) testing
The method most commonly referred to in UK flooring guidance, including the BS 8203 and BS 8204 series of standards, is relative humidity (RH) testing with a hygrometer. An insulated hood is sealed to the surface, or a probe is placed in a drilled hole, and left until the air inside reaches equilibrium with the concrete. The RH reading reflects how much moisture the slab is likely to release into a sealed covering. Readings need time to stabilise, so this is not a five-minute test, but it gives a far better picture than a quick surface check.
Many resin manufacturers set a maximum RH for their standard primers, often around 75%, although limits vary by product. The manufacturer’s data sheet is always the reference point for the system being installed.
Surface moisture meters
Handheld meters that read moisture at the surface are useful for a quick comparison between areas, for finding damp patches, and for deciding where to place hygrometers. On their own they are not a substitute for RH testing, because they only see the top few millimetres and can be affected by salts, reinforcement and surface finishes.
Other checks
A survey should also check air and slab temperature, humidity and the dew point, as condensation during application causes similar adhesion problems. It is worth asking how old the slab is and whether it has a membrane underneath.
Why the concrete itself matters
Moisture testing goes hand in hand with checking the quality of the slab. Weak surface concrete, heavy laitance, contamination from oils and old coatings, or poorly finished new concrete all affect how a resin floor will bond and perform. We cover this in more detail in our article on the importance of quality concrete for your resin floor.
What if the slab is too damp?
A high moisture reading does not always mean the project has to wait. The options depend on the programme, the cause of the moisture and the system being installed.
- Allow more drying time. For new slabs, time with good ventilation and heating may be enough, but drying can be slow, particularly for thicker slabs.
- Use a damp-proof membrane (DPM) primer. Specialist epoxy DPM primers are designed to go down on concrete with higher moisture levels and suppress moisture before the main system is applied. They are often finished with an aggregate scatter to give a mechanical key for the following layers.
- Choose a breathable or moisture-tolerant system. Some systems are designed to cope better with moisture vapour, though they still have limits.
- Deal with the source. Leaks, poor drainage or a failed membrane under the slab need investigating, as no coating will overcome a constant supply of water.
In our own projects, DPM primers with an aggregate scatter are a regular part of the build-up under polyurethane screeds, particularly in food production and brewing areas where the slab and the environment are often damp.
Testing as part of proper preparation
Moisture testing should sit alongside the rest of the preparation process: mechanical diamond grinding to remove laitance and contamination, repairs to cracks and joints, and checks on the strength of the surface. You can read about how we approach this on our preparation page. Getting these steps right is far cheaper than lifting and replacing a failed floor.
If your existing resin floor is already blistering or lifting, our guide on how to repair resin flooring explains the common causes and the repair options. Where moisture is the cause, it needs to be dealt with as part of the repair, otherwise the same problem is likely to return.
Frequently asked questions
What moisture level is acceptable for resin flooring?
Many resin primers are limited to a relative humidity of around 75%, measured with a hygrometer, but the limit depends on the product. Always check the manufacturer’s data sheet. Where readings are higher, a damp-proof membrane primer or a moisture-tolerant system may allow work to go ahead.
What causes blisters in a resin floor?
Blisters are most often caused by moisture or osmotic pressure building up beneath the resin. Moisture moves through the concrete and is trapped by the impermeable coating, forming bubbles that can grow and fill with liquid. Contamination, poor preparation and application in cold or humid conditions can also contribute.
How long does concrete need to dry before resin flooring?
There is no fixed time. Drying depends on slab thickness, the concrete mix, temperature, ventilation and whether there is a membrane underneath. Rules of thumb are unreliable, so the slab should be tested with a hygrometer before work starts rather than relying on its age alone.
Can resin flooring be laid on damp concrete?
Not with a standard primer. On damp concrete, a specialist epoxy damp-proof membrane primer can be applied first to suppress moisture, followed by the chosen flooring system. The source of the moisture should still be investigated, especially if water is entering from below or from leaks.
If you are planning a new resin floor and want the slab assessed properly before work starts, call us on 01782 865099 or request a quote to arrange a site survey.
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