What is the purpose of a DPM sandwich?

The ultimate purpose of a Damp proof membrane (DPM) sandwich is to prevent/suppress moisture coming from a concrete slab damaging a final floor covering. However, the term ‘sandwich’ refers to a specific method. We’ll be looking at the purpose or even several purposes of using a DPM sandwich in this article…You can find a brief description of a DPM sandwich by clicking here… 

Firstly, I think it’s worth explaining the difference between a DPM sandwich and shall we say, the alternatives. The term sandwich, in this context, simply means the liquid DPM is sandwiched between two layers of smoothing compound (The first layer being applied to a foundation concrete slab, followed by a layer of DPM, and finally a top coat of smoothing compound. The alternatives being the concrete slab would be suitably prepared, then the liquid DPM applied straight to it, with a final smoothing compound layer on top or in some instances, with no final smoothing compound layer.

Of course one could say, “Well if the liquid DPM was just applied directly to the concrete slab and then the floor covering installed straight over the top, that is a DPM sandwich.”. Of course you could look at it that way, but again, in the context of this process, we are using the term ‘sandwich’ to explain a very precise and reasoned method of applying a DPM.

Now we’ve butted heads, let’s look at the instances where a DPM sandwich would really make sense. I’ll firstly outline some situations, then sum up at the end..

A very porous and/or pitted concrete slab 

To prepare a concrete slab, there is often the need to grind/sandblast/scabble the surface. This is done to remove any contaminants and loose particles. The surface would then be cleaned with a brush/vacuum and clean water. The grinding/sand blasting/scabbling process does not tend to leave a highly smooth surface.

We are talking about a preparation process, not a fine finishing process, which would take a lot longer and require a graduated finer blade or medium to create a perfectly smooth surface. The grade of concrete paste originally used and age is also a factor.

PinholingAfter a basic preparation of the concrete surface, one can often find pitting (small shallow holes) in the surface. These holes are often due to fine aggregate (stone chippings added to the concrete paste for strength) becoming loose and being removed during the preparation process. If a liquid DPM product were to be applied directly to the concrete with pitting present, you will often trap small pockets of air beneath the liquid DPM in these holes. Some DPM’s handle this better than others, but should the DPM being used have a thick consistency, as the DPM is drying (some dry extremely quickly), the weight of the liquid slowly pushes out the air (a type of hydraulic action). As the DPM liquid starts to dry and become thicker, the DPM is left in a suspended state. The trapped air working its way through the liquid DPM can now begin to create ‘pin holes’ that look similar to minute volcano’s or craters. As a liquid DPM requires a consistent unbroken film to do its job of suppressing the moisture, pin holes are an obvious issue. An example of pin holing can be seen in the picture above (This is merely a visual example to help explain pin holing – It is not a pin holed liquid DPM – Credit to www.Skutt.com for the picture).

Porous concrete is a very similar thing to pitting. However, many microscopic holes are present in the concrete (the concrete is very dry at that time). Think of a sponge. The same action above can apply. Where the porosity of the concrete pulls (sucks/suction) on the liquid and as it dries, any air that’s released as it swaps places with the liquid, if it doesn’t make it into the atmosphere in time, will become caught in a partial state of animation. This will show itself as a kind of crater with a very thin lid of dpm, a bubble in essence, or a full open hole straight through the DPM – both descriptions in the latter paragraphs are not exclusive to either pitting or very porous concrete. Either can happen in both circumstances.

Any of the effects above will typically require remedial work to rectify.

Tampered concreteRough concrete surfaces

Should the contractor deem the concrete to be in an acceptable condition to proceed with a liquid DPM, without grinding or preparing the concrete in any way (Which is rarely advised), it’s most likely the concrete will have many small peaks and troughs often created in the tamping stage. As clearly shown in this excellent picture (Credit to www.concrete.org.uk).

Snowy gullyIf a liquid DPM is applied directly to this type of surface (regardless of the issue of contaminants), what we can have is a situation where the DPM will settle thickly in the troughs, and extremely thinly on the peaks, due to general gravity (the weight of the liquid). A good analogy of this is to think of snow that has fallen on a jagged mountain ridge or likewise on a bigger scale mountain range. In the mountain picture on the left we can see where the snow (Liquid DPM) has collected within the troughs, but has not collected on several of the peaks.

In the context of this article, this effect on the peaks would be termed as potential week points that may let moisture through. Of course, several layers of DPM can be applied, and there’s no saying that for a particular circumstance that would not have the desired effect. However, the idea of applying a liquid DPM is to greatly minimize any failure of a floor covering.

Will a floor covering be guaranteed to fail should any of the issues we’ve look at in this article happen. No! There are other factors involved i.e. the type of floor covering installed (some will allow small amounts of moisture through without damage), the necessity of a liquid DPM in the first place (some companies will install a liquid DPM as a matter of course even when one isn’t required) etc…This article has not been written to strengthen anyone’s arguments not to pay a contractor. Each project has it’s own story.

DPM sandwich

The idea with the first layer of smoothing compound in the DPM sandwich is to create a smooth pitted free surface for the DPM liquid to bond and allow for a consistent film of liquid. After preparation (grinding etc) of the original concrete, if the surface is smooth, pit free, and suction is low, it’s fair to say a DPM sandwich is not required. Indeed, it is generally the preferred method to apply a liquid DPM direct to a concrete substrate as there are less layers to go wrong, less moving parts so to speak. However, a first well installed layer of compound can often prove to be advantageous to prevent any of the issues presented above from compromising a liquid DPM.

The top layer of compound typically gives two benefits. It can provide the installer with a second chance to make the surface flat ready for a floor covering such as Luxury Vinyl Tile. A process that can sometimes be difficult with one coat of smoothing compound, particularly with extremely uneven problematic sub-floors.

The second benefit is to the protection of the DPM. Although, floor covering adhesives in some cases can be applied directly to a liquid DPM, there is often a risk the troweling process and generally working directly on top of a DPM can damage it. We are talking about a very thin layer of material (DPM films can be 250 microns – or less – in thickness). Some adhesives also require the sub-floor to have a certain amount of suction to work. Something a second layer of smoothing compound can provide.

This is clearly not a ‘how to’ guide. Methods of applying liquid DPMs and smoothing compounds should be sort through manufacturers guidelines and your flooring professional.


© Copyright 2016 Wes, All rights Reserved. Written For: Fitmywoodfloor

I am a Pergo trained professional installer of 20 years. I've been up close and personal with lots of floors and have the knobbly knees to show for it...Should you have any questions or comments please feel free to add them below. Thanks for taking the time to call by and I hope the information you've found has given you some insight!........................................................................................................................................................................“When you click on links to various merchants on this site and make a purchase, this can result in the site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network, amazon.com, amazon.co.uk, amazon.fr, amazon.de, amazon.it, amazon.ca, affiliate window network.” This statement is to comply with current internet regulations regarding transparency to consumers.

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