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Some Think DPM Is A Con!

Martin Cummins on moisture and DPMs

ONE of the hardest parts of a technical service or site support role is getting main contractors (and clients) to understand that you are not conning them into using a surface DPM simply to make money.
So often nowadays we are offered add-ons whenever we buy something, such as extended warranties, heated passenger seats for our cars, soft close options for kitchen units, gliding castors for our new settee, all of which may be an improvement but do we really need them.
So I can see where the main contractors are coming from. However, when a DPM is required, then omitting it can have a massive impact in the short and/or long-term, resulting in anything from repairs to isolated areas to full uplift. So how do I go about getting the main contractors to buy in on a DPM?
Take a standard cement and sand screed for starters. Usually if there is time and the full BS hygrometer box method is in use then the results are plain to see.
Similarly when the probe method is used. Readings above 75%RH are higher than that required to lay floors without a surface DPM. The standards say so, not me, not you.
The contractor, however, knows the screed has been in place for three months, it is only 50mm thick and it dries at 1mm a day … all standard beliefs. So surely we are using equipment to give these high results so they have to use our DPM!! Conflict can arise if this is left.
Of course you can explain drying times are based on good conditions, including airflow and temperature and also not covering the screed with all sorts of materials that stop the excess moisture escaping but this can simply question their ability to control the site.
Explain that he/she is right to a degree and that the screed may well have cured and is strong and stable, but it is the excess moisture that will still need to come out of the screed that we are worried about. Electronic meters such as Tramex or MMS are very useful for demonstrating the reality, that residual moisture is present.
If you have used a hygrometer box, it basically seals the subfloor during the test time to gather the moisture vapour and as a consequence the moisture at the surface will probably be higher than surrounding areas, which have been exposed.
Use the electronic meters to show the difference. Explain that it is the moisture down in the screed that will come up and cause problems with adhesives, smoothing compounds and/or floorcoverings.
Take the main contractor to other areas such as a dry lined wall and show him that the meter demonstrates that this is dry…it is not ‘fudged’ to show wet all the time. Sometimes the area where the hygrometer was placed will show a darker surface, possibly even a weaker surface if it is a wet calcium sulphate – both demonstrate that moisture is still significant.
I always start by giving the contractor the option of waiting for the floor to dry and to get heat and dehumidifiers in and remove materials from the floor space. He is then aware it isn’t all about selling a product, but more about getting the floor dry. The reality is that time frames don’t generally allow for this so the option is for a DPM.
Often I get the remark that it is only carpet tiles so it will be fine. This then has to be the main contractor’s instruction to his contractor, not mine, not yours and not the flooring contractor’s decision.
It can be even more difficult on refurbishment projects where the main contractor has a building which has been in place for many years. It hasn’t had any problem with moisture before (as far as he knows), but now I am telling him there is risk … my hygrometer shows it, so do my indicative tests.
My approach here is to ask what previous floorcoverings did it have, was there any timber or vinyl sheet (which are the easiest products for people to understand that moisture can affect)?
It is likely that there wasn’t or that it may have been wood block with bitumen adhesive (both relative stable under moisture variations) or very old low plasticiser vinyl’s stuck with moisture tolerant adhesives.
Use all the elements mentioned above to demonstrate you are not the con man and then get into discussion as to what could be causing these high levels of moisture. Let the main contractor have his say, as he will often answer the questions himself. Such possibilities include:
1. It’s an old building … never had a base DPM in the design … never used to need them as the floorcoverings were old fashioned such as quarry tiles, wood blocks etc.
2. The base visqueen DPM has been breached or damaged when the concrete was laid. Unlikely to have been taped together or much care taken when pouring.
3. So many trades have been in since the refurb started that the amount of water that has been used by them has probably soaked into the subfloor.
4. The roof has been off and/or there have been leaks so again moisture may well have ingressed.
5. If it is a power floated concrete that has had perhaps a carpet tile or an underlay system then it will not have been able to breathe to any degree during all these years so can still have the original residual moisture.
All these scenarios that he/she can realise may help break down the main contractor’s resistance to the possibility that moisture is present.
The subfloor is solid, durable and stood the test of time so it is important to appreciate their position. Finally … do not get arrogant or cocky. Sometimes you could be misinterpreting what you are getting from your tests.
For example, 1. If a surface DPM has been used (now hidden under a smoothing compound), then the subfloor will still show wet … but this moisture is already being checked;
2. If you use plugs and probes you will be testing the moisture further down, so if it has an asphalt or even a very dense terrazzo then you will be reading a point in the subfloor where moisture is may not actually ever rise through and give concerns to floor finished;
3. The indicative meters can also read water in pipes and can also confuse by giving a reading where metal is present.
So happy moisture testing … you won’t win them all, however, so make sure you always put in writing what you believe to be the reality.

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