Protecting against moisture when installing floors

One of the most common failures within the flooring industry is a lack of preparation and testing, when it comes to mitigating moisture in the substrate. It happens in all flooring disciplines, with floating chipboard floors over underfloor heating, rising in the rankings of common failures.

Although it is best practice for the estimator to take readings at this stage, it is also essential to test the subfloor before starting any installation. Many factors can happen between estimating and starting that could retarded the drying process. Commissioning of the UFH (Under Floor Heating) can also force more moisture upwards.

So with any job, it’s important to glean certain information before commencing. The following should be identified:

  • What is the substrate?
  • How old it is?
  • How thick it is?
  • Is there any knowledge or evidence of a structural damp proof membrane?

Often it is possible to tell the substance of a substrate, just from looking at it. Similarly, it is often possible to recognise the age of a building from looking at it. So that’s two out of the four main pieces of information. The problems arise in the latter two as it is often impossible to see the thickness of a substrate and see evidence of a structural DPM (damp proof membrane). It is important that this information is known. If the customer can’t answer them, ask if they have any way to find out. Main contractors and surveyors should hold the answers where appropriate. Domestic customers may not have any idea, so you will have to evaluate. Remember though, better safe than sorry! Thankfully there are indicators, devices and tests which can help ascertain the right way to move forward in safeguarding the subsequent floor covering installation. We’ll cover these at a later date.

The British Standards states that for solid substrates, a level of 75% rh (relative humidity) or below does not need to be protected against moisture before installing the floor covering (apart from wooden floors which must be 65% rh or below). However, if the property is pre 1970, there is a very high chance that there is no structural DPM, and thus no protection from moisture entering the slab when the water table rises.

If you can identify that there is no structural DPM due to the age of the property, then a suitable DPM must be used which not only protects against high levels of moisture, but also the possible mild effects of hydrostatic pressure. This comes into play when the property is at, or near the bottom of a hill, in a valley, near a natural water source, or where there is knowledge that it exists. Two part epoxy resins are usually used as retrospective damp proof membrane such as UZIN PE 480. For safety and to avoid all element of doubt, a two coat system is recommended. However a one coat system can be used on the proviso that a sufficient quantity is applied to the floor to safeguard against pin holes, any breaches and making sure a continuous barrier is formed.

Such epoxy resins DPMs offer a superior level of protection, and in so doing, require a little more attention in the installation process. Whilst there is no real upper limit in the application of an epoxy, there is usually a lower limit of 400-500 grams / per m2 for single coat applications. The benefits of a one coat is drying times and of course application, this is a little easier with the use of a notched trowel and a pre-soaked nylon roller to ensure the correct coverage. Such products need time to penetrate the substrate and capillaries, whilst sealing the surface to prevent moisture permeating through. Once fully cured, an epoxy should be impenetrable to moisture, leaving the only reason for product failure due to a high level of hydrostatic pressure which may cause the substrate to fracture. In turn this causes a breach in the epoxy membrane, leaving the moisture free to pass through.

For substrates where there is definitive knowledge / evidence of a structural DPM sheet being part of the structure, steps need to be taken to protect against residual moisture. It’s important to point out at this stage, that a slab will never be 100% dry. Figures less than 50% rh can sometimes suggest that something is not quite right with the test equipment, or that particular substrate. As mentioned earlier, it is entirely correct to prepare and install most floor coverings on substrates up to 75% rh with the knowledge of a structural DPM. With an upper limit of around 95-98%, many MVS (Moisture Vapour Suppressant) systems are applied on a two coat basis to eliminate the chances of micro pin holing in the first coat. Many of these are PU products such as UZIN PE 404 which should be applied thinly and evenly with a nylon roller in one direction for the first coat, and the second applied at right angles to the first, ensuring good coverage. Two part epoxy products can also be used in most circumstances, and will invariably offer a higher level of protection.

Wooden Substrates

There seems to have been a shift in construction practice of late, with instances of underfloor heating systems being covered with floating chipboard, mostly 18mm-22mm thick. This causes concern when considering overlaying with ply and the subsequent fixing. Many contractors will use a suitable primer and smoothing compound such as UZIN PE 630 and UZIN NC 175 over the chipboard providing it meets certain criteria P4-P7. However, whilst this is a safer system to use verses taking a risk with ply & the mechanical fixings, it is imperative that the chipboard has been fitted correctly, acclimatised and the moisture content is generally between 8%-12% WME. This depends on a number of factors, temperature and air humidity levels will determine what the moisture content should be within the timber, these should always be checked before any installation. So often, pallets of chipboard are stored outside on construct sites with little protection, fitted to buildings that aren’t sealed against the elements, and then soak up moisture. Fitting over this traps the moisture and with the addition of heat from the UFH system, the chipboard will soon start to cup and curl which shows through to the final floor covering. Not only does this causes aesthetic issues, but also creates a potential trip hazard.

Projects over damp wooden substrates is fraught with complexity. That’s why it is critical to evaluate the risk of moisture and use preventative measures. The use of plastic sheet membranes will prevent the moisture from passing from the subfloor into the timber; however, it would not stop any deformation of the final floor covering if the moisture in the timber was too high in the first instance. It is simply a case of letting the wood dry.

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