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Beware: Moisture Is A Movable Beast

Martin Cummins on the use of surface DPMs

THE age old question of the relevance and use of surface DPMs recently reared its ugly head on few sites. The discussion, however, was not the basic: ’Do we really need a DPM or are you just trying to con us into spending more money?’ But do we really need to DPM the entire floor?

Strictly speaking, he meant a surface applied damp proof membrane and wherever DPM is included below this is what he is referring to.

Let me explain further. The first site visit was after an area of vinyl started ‘blowing’. It was not a random area and was not repeated elsewhere on the project. On investigation it became apparent that the latex had blown, the adhesive had suffered plasticiser migration (lovely sweet odour) causing the vinyl to blister. The first instinct of the main contractor is, of course, that the products are faulty. The flooring contractor was confident about the products he had used so couldn’t fathom why this happened. On further investigation, discussion and moisture tests using an electronic meter showed that moisture was undoubtedly the reason for the failure (isn’t it always!!).

It was eventually realised that the flooring contractor had only used DPM up to a certain point on the floor. Initial moisture tests showed the floor to be dry in the area where it was now failing, so no DPM had been required. However, DPM had been used on the rest of the corridor because moisture readings there were high.

As I hadn’t been party to the original moisture assessment I couldn’t immediately comment on the reasons for such differences, but after probing I was told that this was where the new extension met the existing building. So one area was an original screed and another a new screed married up to it. One area was initially dry and one area wet. Bingo!!

We now understood what had occurred. A similar scenario could easily be encountered if screeds are laid at different times, at different thicknesses and with different drying conditions. After identifying the cause there would no doubt follow an argument on who was responsible and who would pay to get the works rectified, but thankfully it was clear that the products themselves were not faulty.

Damp subfloors are a flooring problem simply because the moisture is always trying to reach a level of equilibrium. In layman’s terms this means that the moisture is always trying to get out of the subfloor into a drier environment.

The purpose of a surface DPM is to suppress or control the release of moisture into the levelling compounds, adhesives and floorcoverings, but it is important to understand that moisture does not only move up vertically but will track along a floor until it can find points of no/low resistance. In this instance the evidence of this happening was stark – a line of bubbling vinyl.

The second site I visited still needs a report to be drafted, but it is in a ‘back of house’ in a retail outlet where an extension had been added. The new extension has a concrete subfloor and everyone accepted this was wet. However, the existing floor also shows high moisture levels, but not consistently across the floor.

The subfloor moisture levels, tested by surface hygrometer and by indicative means, are high for the first 3m or so into building and drop to ‘dry’ beyond this. It is highly likely to be because this was where the loading doors were; it is also the area of subfloor that will have been exposed to all the elements whilst the extension was being built.

The main contractor was asking the flooring contractor up to where the DPM needs to be laid as not all the floor was wet. My answer was: ‘He needs to DPM all the floor to ensure moisture from the wet area does not migrate into the dry area and cause a failure’.

In both the above cases the use of a surface DPM for several metres into the dry areas may have been sufficient to act as a buffer, but this would have to be a judgement call.

The third site was slightly different. This was a luxury apartment block where the screed was still not sufficiently dry to lay vinyl. The moisture levels were in the mid to high 80s % RH. The main contractor was happy for the flooring contractor to DPM the floor, but only where vinyl was to go. The adjacent areas, on the same screed, would have a ceramic/porcelain tile in the kitchen and carpet in the lounge area.

The main contractor’s thinking was that moisture would have minimal effect on these two materials, so a DPM would not be needed. He understood the potential for migration, but was prepared to take the risk.

And he is probably right in this instance. The carpet is not being bonded so no adhesives can be affected and the tiled area is to be fixed with a cement adhesive suitable for use outside so shouldn’t be affected by moisture. I did labour the point that moisture may track back along the smoothing compound, so he agreed to DPM for a metre or so into the adjacent areas. I hope this has given you an insight into why simply taking moisture readings may not be the full story. The entire floor span must be considered as moisture is not a static entity, but very mobile and will search for a way out of the floor. Make sure that your floorcovering won’t be vulnerable.

Martin Cummins is Ultra Floor technical sales manager

T: 01827 871871

This article has been reproduced from the Contract Flooring Journal website. You can find them at