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A BDPM Or No BDPM, That Is The Question?

ONE of the main problems encountered on refurbishment projects is determining if the building is constructed with a structural base damp proofmembrane (BDPM).

Lack of knowledge or appreciation of this can have a flooring system laid that is immediately at risk of failure. In this article I will explore how to avoid this, by helping you to identify the risks before carrying out the project. Everybody knows that a newly screeded floor needs to be dry (75%RH or less) to be suitable for laying a flooring system. On new build projects there is the opportunity to gather information on how the subfloor is constructed i.e. is there a BDPM where required, is there any under floor heating, how thick is the screed, how long it has
been laid, is there any insulation etc.

All this information, along with moisture testing where necessary, enables us to make recommendations with some certainty.

With refurbishment projects, more often than not there is little information available on how the floor was constructed, so we have to use our experience and visual evidence as the primary investigation tool.

Consider the following points to determine whether the property has a BDPM, thus helping us recommend a suitable preparation and surface DPM, where required.

1. Is the refurbishment on a ground floor or more precisely a floor level direct to ground?

(Floors levels with rooms or voids underneath are not considered direct to earth). A ground bearing floormay not have had a BDPM incorporated at the construction stage. As a consequence moisture canmigrate into the
building and, like on new build projects, cause problems with adhesives and floorcoverings.

Upper floor do not pose the same risk.

2.What is the property’s approximate age? It is generally thought that properties built pre 1970 had no prerequisite for a BDPM in thei construction design. If the building seems of this era or older assume a BDPM is absent. It is also not fair to assume that more modern buildings will have BDPMs. I have seen domestic properties constructed as recent as mid 1980’s where the kitchen has had ceramic or quarry tiled floors with no BDPM beneath the concrete. Also, buildings such as warehouses and commercial properties don’t necessarily
have BDPM incorporated as its end use is not affected by subfloor moisture.

3. Are there modern resilient floorcoverings? Fully resilient flooring in a building, provided it is not failing, is a good clue that there is an effective BDPMor that a surface DPM has been used in the installation. However, older vinyls, particularly thermoplastic (crunchy) tiles laid into bitumen adhesive, were very tolerant of moisture so were often used even if a BDPM was absent.

4. Are any coverings present susceptible to problems with subfloor moisture? An overview of floorcoverings presently installed is also helpful information. Quarry tiles (or other stone materials), woodblock or parquet flooring (laid into bitumen), Marley type tiles (as mentioned above), carpet tiles, needle punch carpets can
all indicate an absence of BDPM. Modern sheet vinyl or rubber flooring on the project is a good indicator that moisture is not a problem unless they are fixed with a glass like very hard adhesive (i.e. epoxy type adhesives).
Addressing these points is, in my opinion, far better than blindly carrying out a moisture test to determine if a BDPMis present. A moisture test will only mirror the state of the subground at the time of the test. A relativity dry spell can result in ground that does not have a high moisture level so subsequently moisture tests will not indicate an issue.

Re-test the same floor in the midst of winter or after several days of rain and you may suddenly find a subfloor with moisture levels nearing 100%RH due to the ground becoming saturated andmoisture being pulled through into the building above.

Sadly we know that with our UK climate, at times in the year the subground becomes sodden putting our modern flooring systems at risk, more so than on a new build where the moisture levels in the subfloor should only ever decrease.

In summary, moisture tolerant floorcoverings and bitumen based or epoxy based adhesives on ground floor properties built before 1970 are very unlikely to have a BDPM. Any one or a combination of the above may indicate absence of a BDPM, so assume this in the first instance and consider appropriate preparation and use
of suitable surface DPMS, smoothing compounds and adhesives.

Clients will often argue that there aren’t any moisture problems as they have never had a failure before. But explain to them that a change in nature of floorcovering will highlight moisture as a problem and point out why none of the present flooring has failed.

Finally, don’t be fooled by a dry moisture reading regardless of what test kit you are using as this only mirrors the moisture level in the ground at the moment of test.

The worse case will be mid winter when the ground is full ofmoisture and the room above is nice and warm. The warm air can hold a significant amount of moisture and will attempt to ‘pull’ it from the subfloor with only our
levelling compounds, adhesives and floorcoverings in the way…. a recipe for disaster.

Martin Cummins, Ultra Floor technical sales manager

T: 01827 871871

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