Moisture is one of the most common hidden problems in concrete floors — and one of the most expensive if ignored. Trapped water vapour or rising damp can cause resin delamination, tile lifting, bubbling paint, or persistent surface staining. Before any floor finish, coating, or overlay goes down, it’s critical to assess moisture levels correctly. This guide walks you through how to test, detect, and diagnose concrete floor moisture problems like a pro.
Start with a thorough visual inspection. Look for wet patches, surface darkening, blistered coatings, lifted corners of flooring, or bubbling in resin finishes. Moisture can also show up as chalky white powder (efflorescence), especially near walls or corners. Note whether the signs are isolated, widespread, or appear seasonally — this gives clues to whether the issue is rising damp, retained construction moisture, or a broken vapour barrier.
This test measures the **Moisture Vapour Emission Rate (MVER)** — the amount of moisture escaping the slab over time. Place a pre-weighed dish of calcium chloride under a sealed plastic dome for 60–72 hours, then weigh it again. Results are in lbs/1000ft²/24hrs. Most flooring products require under 3 lbs; high readings mean the slab isn’t dry enough for adhesives or coatings. Perform at least 3 tests per 100 m².
For deeper readings, drill a 40–50 mm hole into the slab and insert an in-slab RH probe. This measures the internal moisture content of the slab — not just surface evaporation. Most coatings require RH to be below 75–85% depending on manufacturer. This test is critical for slabs poured in the last 6–12 months, or where there’s no vapour barrier underneath.
While not as scientific as MVER or RH testing, a taped plastic sheet can help indicate ongoing vapour transmission. Tape a 600x600 mm square of plastic sheeting to the slab with duct tape. After 24–48 hours, check underneath. If condensation has formed, moisture is being released from the slab. Use this as a red flag for further testing — it won’t give you exact numbers but is useful for quick surveys.
If you suspect rising damp from underneath, investigate whether the slab has a Damp Proof Membrane (DPM). For older buildings (pre-1980s), this may be missing or degraded. Drill a test core or look at exposed edges near expansion joints or thresholds. If the slab is laid directly onto soil or hardcore with no membrane, rising damp is likely — especially in winter months or in poorly ventilated interiors.
Surface moisture meters (electrical resistance or impedance types) can help detect damp zones, but they only measure the top few mm of concrete. They're useful for mapping affected areas but shouldn’t be relied on alone for decisions about coating or flooring readiness. Use them in tandem with deeper RH or MVER testing to get a full picture of the slab’s condition.
Not all moisture problems come from inside the slab. Check for external sources too: poor drainage near foundations, lack of guttering, HVAC condensate on cool slabs, or seasonal slab sweating (common on warehouse floors). High indoor humidity can also drive condensation if the slab is cooler than the dew point. Fixing these external sources is often cheaper than mitigation membranes or overlays.
Once your testing is complete, compare the results against the technical data sheets of any planned flooring or coating systems. Resin floors, levellers, tile adhesives, and polished concrete sealers all have moisture limits. If the slab is too wet, consider mitigation like vapour barriers, epoxy primers, or waiting longer. Skipping this step is the #1 cause of coating failures and rework.
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