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Never Use A Moisture Suppressant As A DPM

Sid Bourne on why a wood floor was cupping

NOT long ago I inspected a solid wood floor. The contractor wanted me to find out why the floor had cupped badly after several months. He believed he had installed a dpm (or so he thought).

The installation – large for a domestic job (over 75m) – was on a ground floor level with a concrete subfloor. The contractor installed what he said was a dpm over the concrete subfloor and then fixed 18mm plywood so that he could secret nail down the solid wood flooring.

I conducted my usual test with relative humidity of the air (52%) and the temperature (around 20degC), all well within the recommended guidelines. I then did moisture checks on the wood flooring.

Using an insulated pin meter I got surface readings of 13%; I then inserted pins into centre of the boards which gave readings of 15%; eventually I got into the plywood producing readings of over 20% MC. This clearly indicated that the moisture source was below.

The contractor was still confused. He insisted he had installed a dpm. We decided that we would cut out part of the solid wood flooring to see exactly what he had installed and how.

On eventually removing the plywood, I found that he had used Sisalkraft he had purchased from the wood flooring supplier. I asked why he thought this was a dpm; he said the supplier had recommended it and that it was better than applying a surface epoxy dpm and much quicker.

I immediately suspected that he was making this up, and once I exposed the bitumen paper he realised that he had been found out. However he insisted that his story was true.

I then took readings of the concrete subfloor which were quite high and explained to the installer that this was the cause of the moisture problems. It would require further testing with a hygrometer to confirm the RH. After hearing my explanation the installer was quite upset. He genuinely did not fully understand how cupping occurs or how to prevent it.

I asked my usual question on whether he had ever attended any courses on subfloors and installation. He admitted that he hadn’t because never has the time. However, he had served an apprenticeship as a carpenter and gained his NVQ in carpentr y.

Out of curiosity I asked what he was taught about working with wood. He revealed that the college he attended did not teach about water and wood.

Other qualified carpenters also tell me that their training did not deal in depth with moisture and how it affects wood. So it’s not really surprising that there are problems when carpenters fit wood floors.

Returning to the cupping problem, we popped into the supplier based only 15 miles from where the complaint occurred. I had briefed the installer to make sure to talk to the person who told him that Sisalkraft was a dpm. In fact, the person in question greeted us on our arrival. I told him I was installing a wood floor onto a wet concrete subfloor and asked what he would recommend?

Sisalkraft, he said immediately. Was this really suitable, I asked. Oh yes, he said, no problem. I then asked if there was anything else I should do to sort the problem. No, he replied, ‘this is brilliant stuff. It will stop anything’.
It was at this stage that I introduced myself and explained the problem. He was quite shocked and said I would have to speak to his boss.

It so happens that the boss is someone I know well; I told him what had happened. He then turned on the salesman, ‘who on earth told you that Sisalkraft is best to deal with the situation we had?’

The embarrassed salesman said he had heard it from customers who are installers, so he thought it was OK.
The boss immediately explained what was required to help this installer solve his problem. I have to take my hat off to the boss who agreed to cover the cost of the flooring, which was a very fair gesture as his employee should have had the knowledge to correctly advise customers.

The installer in question then attended a British Wood Flooring Association (BWFA) installation course and was amazed at how much it taught him. He kept saying how embarrassed he was after he found out the right way to approach wood flooring jobs and how to correctly check subfloors.

Sisalkraft, as I regularly explain, is not a dpm. It is a moisture suppressant which aids in helping prevent minor moisture issues. But will not work as a dpm if your subfloor is wet regardless of type.

I tell installers they can use Sisalkraft when installing over suspended floors, but first they must confirm that there are no moisture issues and that the air bricks are clear and functional.

As always, I get permission from the company or installer before writing any of my ar ticles because this is a ser vice to help educate other installers.

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