Installing Floorcoverings Over Calcium Sulphate Screeds

Calcium sulphate screeds have become an increasingly common choice of subfloor to install in new builds, especially where underfloor heating is incorporated within the subfloor. This is because of the speed with which they can be applied over large areas and other advantages over concrete bases, including less shrinkage and fewer joints required within the subfloor. Neil Sanders, Technical Director at F. Ball and Co., gives his advice on achieving a long-lasting, professional flooring finish when working over this type of surface.


When installing new floorcoverings 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 associated with this type of substrate.

Just as when preparing to install new floorcoverings over other subfloor types, 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) using a rotary sanding or abrading machine. This process should be undertaken by the screed supplier, seven to 10 days after the screed is installed, when the laitance is easier to remove. However, the process is frequently left to the flooring contractor, allowing the laitance time to harden up and prolonging the drying time of the subfloor.

F. Ball recommends that a levelling compound be applied to a subfloor to create a perfectly-smooth base for floorcoverings and enhance the aesthetic appearance of the finished installation. If not removed, laitance can cause a subsequently applied levelling 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 measurement test should then be conducted to ascertain if the subfloor is dry enough to proceed directly to installing floorcoverings. If a subfloor has a relative humidity (RH) reading above 75% (or 65% if wood floorcoverings 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 attack floorcoverings, 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

The application of a primer is also required in almost all circumstances to promote the adhesion of subsequently applied subfloor preparation products and stop the rapid drying of levelling compounds when applied over absorbent subfloors. It also prevents pinholing, caused by tiny bubbles resulting from the slow escape of air from the subfloor, which burst on reaching the surface, leaving small pinholes or craters as the levelling compound cures.

F. Ball’s Stopgap P121 primer is specially formulated to promote the application characteristics of compatible levelling compounds when applied over calcium sulphate screeds, which are denser than concrete or sand/cement screeds – it contains finer particles than other primers in the F. Ball range. The product should be applied in two coats; the first diluted with one part water to one part primer, followed by a second coat applied neat. It contains finer particles than others in the F. Ball range, helping it to bond with calcium sulphate screeds.

Compatible levelling compounds

Calcium sulphate screed manufacturers advise that a calcium sulphate-based self-levelling compound, such as F. Ball’s Stopgap 1100 Gypsum, is used to create a base for floorcoverings when dealing with this type of subfloor. Unlike most cement-based products, Stopgap 1100 is virtually tension free and does not shrink during the drying process, which limits the amount of stress at the bond interface.

Another reason for the use of a calcium sulphate-based levelling compound is that the application of cement-based products onto calcium sulphate screeds can, under certain circumstances, result in a chemical reaction that causes the formation of ettringite, a crystalline material that can cause floor failure, where the two materials meet.

Final thoughts

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

Once the levelling compound has cured, specified floorcoverings can be installed using a suitable adhesive. To check the compatibility of particular floorcoverings and adhesives, contractors can consult F. Ball’s industry-leading Recommended Adhesives Guide (RAG), which lists adhesives recommended for use with over 6000 floorcoverings, 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 is on hand to answer your questions about F. Ball products and how to use them.

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