Calcium sulphate screeds have become an increasingly common choice of subfloor, especially where underfloor heating is incorporated within the subfloor. This is because of advantages over concrete bases, such as greater thermal conductivity, the speed with which they can be applied over large areas, less shrinkage and fewer joints required within the subfloor. Neil Sanders, technical director at the UK’s leading manufacturer of subfloor preparation products, F. Ball and Co. Ltd., advises on the steps necessary to achieve a long-lasting, professional flooring finish when working over this type of subfloor. 

When installing new floor coverings over calcium sulphate screeds, it is essential to follow the basic principles of subfloor preparation, while considering a few critical points to avoid common causes of floor failure. 

First steps

Just as when preparing to install new floor coverings where other subfloor types are present, the first step in the process should be to ensure the subfloor is suitably sound, smooth and dry. This often means beginning by removing any laitance: the crust of fine particles formed on the surface of the screed as it dries.

Ball recommends that a smoothing compound is applied to a subfloor to create a perfectly smooth base for floor coverings and enhance the aesthetic appearance of the finished installation. If not removed, laitance can cause a subsequently applied smoothing compound to debond from the substrate, potentially causing floor failure. Any contaminants that may prevent adhesion, such as wet trade waste or debris, should also be cleaned off. 

Drying time

A moisture test should then be conducted with a digital hygrometer to determine if the subfloor is dry enough to proceed to the next step in the subfloor preparation process. If subfloor Relative Humidity (RH) levels are above 75% (or 65% if bonded wood floor coverings will be installed), contractors will need to allow further time for the screed to dry, otherwise there is the risk that excess subfloor moisture will adversely affect floor coverings and adhesives, potentially resulting in floor failure.

Calcium sulphate screeds dry from the bottom up, so a moisture test may deliver a positive reading right up until the point the subfloor is completely dry. Therefore, contractors should be patient when allowing for the subfloor to dry sufficiently. This process can be accelerated by ensuring areas are adequately ventilated, by opening windows or using a dehumidifier, and/or turning on underfloor heating, if incorporated within the subfloor, at a low temperature. 

Prime importance

If the RH level of the subfloor is sufficiently low, the contractor can proceed to the next stage of the flooring installation: priming. Priming prior to the application of a smoothing compound is critical for the finished appearance and performance of the floor; it stops pinholing, tiny bubbles formed by the slow escape of air from absorbent substrates as they dry, which burst on reaching the surface – leaving small pinholes as the smoothing compound is curing – compromising its integrity and aesthetic appearance. Priming also helps to promote the adhesion of the smoothing compound to the screed and prevents it drying too rapidly, which can result in poor strength build up.

Specialist primers, such as Stopgap P121 from F. Ball & Co. Ltd., are available that are formulated to promote the application characteristics of compatible smoothing compounds; such primers contain finer particles than other products, helping them to absorb into the surface of calcium sulphate screeds, which are denser than concrete or sand/cement screeds. Primers should be applied according to the manufacturer’s instructions.

Compatible smoothing compounds

Calcium sulphate screed manufacturers advise that a calcium sulphate-based self-smoothing compound, such as F. Ball’s Stopgap 1100, is used to create a base for floor coverings when dealing with this type of subfloor as they shrink at the same rate during the drying process, limiting the amount of stress at the bond interface.

Another reason for the use of a calcium sulphate-based smoothing compound is to avoid the potential formation of ettringite, a crystalline material that can form where cement-based products are applied over calcium sulphate screeds and there is a high concentration of moisture – this can cause failure of the bond.

Final thoughts 

It is worthwhile remembering that calcium sulphate-based smoothing compound typically take longer to dry than other, cement-based alternatives. Therefore, when a 3mm thickness of smoothing compound has been applied, contractors should be prepared to wait up to 24 hours before installing textile floor coverings, and up to 48 hours before installing resilient floor coverings. Underfloor heating must be turned off at least 48 hours prior to, and throughout, the application and drying of the smoothing compound, and the temperature of the subfloor should be kept above 10°C from start to finish. 

Once the smoothing compound has cured, specified floor coverings can be installed using a suitable adhesive. To check the compatibility of particular floor coverings and adhesives, contractors can consult F. Ball’s industry-leading Recommended Adhesives Guide (RAGⓇ), which lists adhesives recommended for use with over 6,000 floor coverings, produced by over 200 manufacturers. A continually updated version of which is available on F. Ball’s website and as a free app. It is also available as an A5 printed booklet. Alternatively, F. Ball’s technical service department are on hand Monday to Friday, 8.30am–5.00pm, to answer your questions about F. Ball products and how to use them.