Why it’s essential to prepare floors properly when a surface DPM is specified

Martin Cummins

Published by Martin Cummins

Martin has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Martin is an active member of the CFA manufacturers committee and he sits on the CFA council. He aims to keep abreast of all the changes in the industry and feeds this knowledge back to installers to make sure when floors are laid they perform to their maximum potential. UK Technical Support Manager, Bostik

It is regularly quoted, accurately, that moisture is the main cause of floor failures, and as a consequence, specifications offered by manufacturers will often incorporate the use of a surface damp-proof membrane (DPM). These have been designed over the decades to combat the rapid release of moisture from a subfloor and therefore protect adhesives, smoothing compounds and floor coverings when the moisture level is above the prescribed 75% relative humidity (RH). However, I believe the industry can be a bit blasé when it comes to understanding what is really needed from a surface DPM, and what the key aspects are to ensure the product does what it says on the packaging and datasheet.

The first thing to appreciate is how a DPM works. The principle is to create a continuous film across the floor in question at a given thickness (often quoted in microns, which are one thousandth of a mm) to afford the protection for the given combination of moisture level in the floor and floor coverings to be bonded above. Once this has been determined, it’s down to the flooring contractor to make sure the product is applied in the correct manner. This means bonding to the selected substrate correctly and making sure that the subsequent application of smoothing compounds and primers etc. is carried out appropriately.

Traditional DPMs such as epoxy and PU are curing products, so the amount of product applied is normally the same as the amount of product left after curing as it is a 100% active ingredient. The correct choice of notch trowel to give the coverage is therefore of utmost importance. Typically a 2 x 6mm or 1.5 x 5mm trowel will give you 3m2 and 4m2 per litre of product respectively, which will equate to between 350 microns and 250 microns approximately. Once rollered out to give an even film, this should do the job. Make sure though that the notches are kept in good order; a worn trowel will apply significantly less and may result in insufficient protection. Also remember that the specification offered will depend on how much moisture is present and how sensitive the system is to moisture, so be sure to follow the recommended coverage rates.

Another issue to be aware of is the state of the subfloor itself. I have experienced a significant increase over recent years of subfloors that are not really fit for an application of DPM. This is often due to one or more of the following issues, which will affect the control of the DPM film thickness:

• Lots of undulations with shadowing of the rebars within
• Damage from water spotting
• Exposed aggregate protruding from the concrete
• Exposed fibres
• Very open textured

The resolution to the majority of the problems above is to propose a pre-smooth of the subfloor prior to DPM application. The correct moisture tolerant, high bonding, pre-smooth smoothing compound systems will give a flat surface to enable you to apply a DPM to at the desired coating. Trying to DPM a pitted, absorbent or undulating floor results in significantly excessive DPM use as it fills in all the voids and hollows. It also leaves concern as to whether there is a continuous, even thickness where the subfloor overly absorbs the products applied. DPM costs can be significant, so pre-smoothing can offer both a technical recovery as well as a controlled cost.

When fibres are protruding it is difficult to get the DPM continuous. The fibres, particularly glass/plastic fibres, can pop back up and remove the DPM from the subfloor. The resolution here can again be a pre-smooth or, if the main contractor is in agreement, the fibres can be burnt off.

With protruding aggregate it may be most cost effective to get the screeders back to grind away these areas. If this isn’t the case then a pre-smooth can be considered. Alternatively a double DPM application, by brush, onto the areas of exposed aggregate can be considered if the aggregate is well bound and strongly fixed within the concrete.

Very open textured but level subfloors simply need a reduced coverage rate to ensure that there is sufficient product both to soak into the surface and also to leave sufficient surface material to give you the required film thickness.

Film thickness equates to performance. Control the film thickness and you will be protecting the materials being applied above and give a functional, long lasting floor.

Leave a Reply

Your email address will not be published. Required fields are marked *

Join our mailing list for quarterly updates!

Sign up to receive regular updates and the latest news. We also send out a quarterly email newsletter so you can stay up to date.



For more information on how your data will be processed please view our Privacy & Cookies Policy.