Concrete Moisture Testing Before Resin Flooring
How concrete moisture is properly measured before a resin floor, what the readings mean, and why surface meters alone are not enough.
Every resin system has a moisture limit for the substrate it is applied to. Testing tells you whether the slab is inside it. Done properly it takes a little planning; done badly, or not at all, it is one of the most expensive omissions on a flooring project.
The methods
Hygrometer box (relative humidity) testing An insulated box is sealed to the floor and left to equilibrate — typically for at least 24 hours, longer on thick or slow-drying slabs. The RH inside then reflects the moisture in the slab. This is the reference method most flooring specifications are written around.
In-situ probe testing A sleeved probe is set into a drilled hole to a set depth, sealed and left to equilibrate. It measures conditions within the slab rather than at the surface, and suits thick slabs and situations where several depths matter.
Carbide (calcium carbide) testing A sample is taken from the slab and reacted in a pressure vessel to give a moisture content by weight. Fast and destructive, useful for a rapid check on a screed.
Surface moisture meters Fast, non-destructive, useful for screening a large area to find the wet spots and decide where to place proper tests. They read only the surface zone and must never be the sole basis for a decision.
Reading the results
Manufacturers state their own limits, so the system decides the threshold, not a rule of thumb. As a general guide:
- Around 75% RH or below suits most epoxy and PU resin systems applied directly
- Above roughly 75% RH normally requires an epoxy damp proof membrane build-up
- Polyurethane cementitious screeds tolerate considerably higher moisture, which is why they suit food production and wet process areas
Practical points that catch people out
- New concrete dries at roughly one millimetre per day in good conditions. A 150 mm slab is not ready in a fortnight.
- Power-floated and sealed surfaces dry more slowly than open ones.
- Test where the risk is: near external walls, low points, doorways, cores and old drainage lines, not just in the middle.
- Test after preparation, not before. Grinding opens the surface and changes what you are measuring.
- Heating and drying a building before testing gives a false low reading if the slab has not actually dried through.
What we do
Where there is any doubt over the slab, we test before quoting and state the readings, the method, the date and the resulting recommendation. If a DPM is required, it is priced as a visible line item. If the slab is genuinely dry, you do not pay for a membrane you do not need.