Most flooring failures over concrete are not the floor’s fault. They are moisture that was in the slab before the floor went down. This guide shows how to test a concrete subfloor for moisture, read the result honestly, and deal with a slab that is too wet, so your new floor does not cup, blister, or lift months later. It is aimed at Newcastle homes where solid ground floors and coastal damp are common.

Why concrete moisture wrecks floors

Concrete holds water long after it looks and feels dry on top. A slab can be years old and still carry moisture from the ground below if there is no working damp-proof membrane, or from a recent screed that has not fully dried. When you seal a floor over the top, that moisture cannot escape upward as easily, so it collects at the interface.

The results are predictable. Wood and laminate cores swell and cup. Glued vinyl and its adhesive break down and the planks lift or bubble. Timber can even attract mould underneath. The floor above may look fine for weeks, which is exactly why people skip the test and pay for it later.

New screed versus old slab

Two different problems share one symptom. A new screed is simply still drying and needs time. An old slab that reads wet usually has a failed or missing damp-proof membrane letting ground moisture through continuously. The test tells you the level; the age and history tell you which problem you have and therefore which fix.

How to test properly

A cheap pin-type meter pressed on the surface is not enough on concrete. It reads only the top few millimetres and misses moisture rising from below. Use one of these instead:

• In-slab relative humidity probe. A sensor is placed in a drilled hole and left sealed for a set period, then read. This measures humidity deep in the slab and is the most reliable field method.
• Surface RH box (hygrometer) test. A sealed box or mat is taped to the slab and left undisturbed for around three days, then the trapped humidity is read.

The exact pass/fail figure depends on the flooring you are installing and its adhesive, so take the target from the flooring or adhesive manufacturer, not a generic number. Test several spots, especially near external walls, doorways, and any area that has been damp before. One good reading in the middle of the room does not clear the corners.

What to do if the slab is too wet

• New screed reading high: wait. Drying takes time and cannot safely be rushed with excessive heat, which can crack the surface.
• Old slab with no working membrane: apply a surface damp-proof membrane, typically a liquid epoxy or moisture-suppressant coating rated for your reading, then lay over it once cured. This is the standard fix and far cheaper than replacing a failed floor.
• Localised damp from a leak: find and fix the source first. A coating over an active plumbing leak just hides it.

A real scenario

A ground-floor kitchen in an older terrace had glued vinyl planks lifting along one wall within a couple of months. The homeowner assumed a bad batch of adhesive. A surface hygrometer test near the affected wall came back clearly wet, while the room centre read acceptable. The slab was old with a patchy original membrane, and the external wall was the entry point. The correct route was to lift the failed area, apply a liquid moisture-suppressant across the floor, and re-lay. Re-gluing the same product to the same slab would simply have failed again.

Common mistakes and how to fix them

• Testing only the surface with a pin meter. Switch to an in-slab or sealed RH test that reads depth.
• Testing one spot. Test multiple points, prioritising edges and previously damp areas.
• Rushing a new screed with heaters. Let it dry at a steady rate; forced heat can crack it and give false low readings on top.
• Using a floor’s own underlay as a “damp-proof” layer. A foam underlay with a plastic film slows vapour but is not a cure for a genuinely wet slab. Deal with the moisture source.
• Ignoring the adhesive’s own moisture limit. The glue often fails before the floor does. Check its spec too.

Action checklist

• Confirm whether you have a new screed or an old slab, and its damp history.
• Get the target moisture figure from the flooring and adhesive maker.
• Use an in-slab or sealed RH test, not just a surface pin meter.
• Test several points, including edges and past-damp zones.
• If wet and new: wait and re-test. If wet and old: apply a rated surface DPM.
• Fix any active leak before covering anything.
• Keep the test record in case of a warranty claim.

Conclusion and next step

A moisture test costs a fraction of a failed floor and takes far less time than a re-lay. Before you buy underlay or open a single box, book or run the correct test on your slab and note the readings against the manufacturer’s limit. If it passes, lay with confidence. If it fails, you have just saved yourself a very expensive redo.

FAQ

My slab is decades old. Does it still need testing?

Yes. Age does not guarantee dryness. Many older ground floors have no membrane or a degraded one, so ground moisture keeps coming up. Test it like any other slab.

Can I just use a plastic-backed underlay instead of testing?

A vapour-retarding underlay helps with normal residual moisture, but it is not a fix for a genuinely wet slab and offers little for glued-down floors. Test first, then choose the underlay to suit the reading.

How long does a new screed take to dry?

It varies widely with thickness, mix, and site conditions, so there is no single figure worth trusting. Test rather than guess, and re-test until it meets the manufacturer’s limit.

Which floors are most sensitive to slab moisture?

Glued vinyl and its adhesive, plus solid and engineered wood, are the least forgiving. Some floating floors tolerate more with the right membrane, but every product has a stated limit.

References

• Flooring and adhesive manufacturer installation data sheets (the binding source for acceptable moisture readings).
• British Standard BS 8203, Code of practice for installation of resilient floor coverings, which covers subfloor moisture testing.