Moisture Risk Assessment of an Existing Floor Before Screeding

A new-build slab is generally cast with a DPM specified and installed as part of the design, so its baseline moisture protection is known. An existing floor in a renovation project may predate current DPM standards altogether, or have a membrane that's degraded, been penetrated by later works, or was never designed with the intended new floor finish in mind. None of this is unusual in older housing stock, but it does mean the moisture risk has to be actively assessed rather than assumed.

This is particularly relevant where a liquid screed is being introduced as part of upgrading an older solid floor, since the finished build-up may behave differently to the original construction.

A sensible moisture risk assessment usually combines a visual inspection of the existing floor and surrounding walls, an understanding of the building's history and location, and appropriate testing suited to the substrate. The findings then feed into decisions about whether a DPM needs adding or renewing, whether a floating screed system with its own separating layer is preferable to a bonded approach, and what floor finish above the screed is realistic.

Getting this right at the assessment stage avoids specifying a screed build-up that looks correct on paper but doesn't suit the actual moisture conditions of the specific floor.

Where the assessment leads to introducing or renewing a damp-proof membrane, the detailing at the edges and around any penetrations — pipework, service entries, or steps between rooms — matters as much as the membrane material itself. A well-specified DPM can still let moisture through at an unsealed lap or an unrecorded penetration, undermining the protection the rest of the floor relies on.

Taking time to identify every penetration through the existing floor before the membrane goes down, rather than working around them as they're discovered, gives a far more reliable result than trying to retrofit a seal once the screed is already scheduled.

On-site testing gives a snapshot of current conditions, but it's usefully read alongside context — the building's age, any known history of dampness or flooding, and local ground conditions in the area. A property with a known history of groundwater issues, for instance, may warrant a more cautious approach than a straightforward reading would suggest on its own.

This wider context is particularly useful in older Devon and Somerset housing stock, where construction methods and DPM standards varied considerably over different eras and aren't always documented in any surviving paperwork.