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Raising the level

John Green explains how to raise floor levels quickly and easily with the aid of a real-life example.

THERE’s a wide range of levelling compounds on the market that can typically be applied up to a maximum thickness of 20mm ahead of the installation of floorcoverings. However, sometimes flooring contractors need to raise the level of a subfloor by more than a couple of centimetres before new floorcoverings can be installed – after removing thick ceramic tiles and adhesives, for example, where underfloor heating needs to be encased, or simply for aesthetic reasons.

This was the case when F Ball’s technical service department provided a cost-effective alternative to completely removing and replacing a weakened screed that would be unsuitable for the application of subfloor preparation products as part of a flooring refurbishment at the Aberystwyth Arts Centre, part of Aberystwyth University. Prior to the installation of floorcoverings over two floors, a total area of approximately 1,400sq m, it was recommended contractors mechanically remove weak sections of screed, leaving about 75mm thickness to be made up in these areas.

The challenge
This is when problems can occur using standard levelling compounds, which are primarily made up of cement and are often combined with water, allowing them to flow easily but potentially leading to a separation of the components in the compound (eg sand, aggregate and cement) when applied thickly. This separation can lead to layers forming, some weaker than others, with the densest materials at the bottom, compromising the strength of the levelling compound when dry.

Also, when cement reacts with water it results in an exothermic reaction, which can lead to a build-up of heat and thermal expansion of the product when applied at thicker depths, increasing the risk of thermal cracking when the heat dissipates and the material contracts.

Moisture tests conducted as part of a site inspection at the Aberystwyth Arts Centre also indicated subfloor relative humidity (RH) levels in the concrete subfloors were above 75% in some areas, necessitating a moisture management solution to prevent excess subfloor moisture attacking adhesives and floorcoverings, potentially causing complete floor failure.

The solution
After the weak spots were removed along with surface contaminants, dust and debris had to be vacuumed away to create a clean, sound micro-textured finish.

Contractors then needed to apply a single coat of F Ball’s Stopgap F77 solvent-free epoxy waterproof surface membrane and use a roller pre-coated with the product to obtain a continuous, pinhole-free barrier against subfloor moisture over the entire area.

Once the waterproof surface membrane had cured, the surface was primed using F Ball’s Stopgap P141 primer, which is designed to create a textured finish to enhance bond performance between levelling compounds and non-absorbent surfaces, such as epoxy waterproof surface membranes. It’s particularly recommended where levelling compounds will be applied at a thickness greater than 20mm; the strong bond created when using Stopgap P141 is better able to withstand the increased stress that thicker depths of levelling compound exert when drying.

Deep section base
For situations where floors need raising by more than 20mm, levelling compound manufacturers have developed deep section base compounds, such as F Ball’s Stopgap 600 Base, which can be applied at thicknesses between 5-50mm in one application, meaning only two applications would be needed to achieve the required 75mm thickness where weak patches of screed had been removed.

To minimise the capacity for thermal expansion, the amount of cement in these products has been reduced. As decreasing the amount of cement would weaken the material, the water content in the mix is also reduced. This helps limit the flow of the levelling compound and minimise the separation of components when the compound is applied in a thick section.

Stopgap 600 Base is also formulated to chemically bind all the remaining water in the mixture when it cures so that flooring contractors are not forced to wait an extended amount of time for all of the water to evaporate naturally as they would if applying a regular levelling compound. Consequently, the drying time of Stopgap 600 is dramatically reduced, and it sets in as little as three hours after application.

The product can also be pump applied, as the contractors opted to do, saving further time. Once the first layer had dried, it was primed using F Ball’s Stopgap P131 general-purpose primer, diluted with four parts water to one part primer, before the second application.

Final steps
The whole subfloor area could then be primed using Stopgap P131 prior to being capped with a layer of a suitable floor levelling compound to create a perfectly smooth base for the receipt of new floorcoverings.

The final phase of the refurbishment was to install Nora rubber sheet floorcoverings using F Ball’s Styccobond F48 PLUS in accordance with F Ball’s Recommended Adhesives Guide (RAG). The adhesive develops the high bond strength required to hold floorcoverings in place when exposed to high temperatures, which are expected in heavily glazed areas of the university building.

It’s also fibre-reinforced to provide high initial grab to prevent floorcoverings moving when working.
John Green is technical service officer at
F Ball and Co

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