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This month Stephen Boulton, Technical Service Manager at F. Ball and Co. Ltd., analyses the challenge of working with calcium sulphate screeds.

While calcium sulphate subfloors may have been perceived as difficult to work with in the past, this need no longer be the case, thanks to modern advances in smoothing underlayment technology.

Technological developments in subfloor preparation have helped to smooth over many of the difficulties that can be faced by modern flooring contractors. From new, professional smoothing underlayments that can accommodate the needs of adhesive residues that would otherwise need to be mechanically removed, to fast-track solutions that enable contractors to complete projects within increasingly tight deadlines, the manufacturers of subfloor preparation products are always working to anticipate the needs of contractors and provide products that will minimise preparation time and maximise reliability and quality.

The popularity of calcium sulphate

screeds Calcium sulphate screeds

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are becoming increasingly popular, because of their perceived faster drying times, speed of application over large areas and their ability to be force dried. These are all qualities that recommend calcium sulphate screeds for installations where contractors are under pressure to complete projects faster than traditional screeds would allow. However, such speed of application comes with a risk for contractors – calcium sulphate screeds, if not properly prepared, can suffer from compatibility problems with some smoothing underlayments, and if not adjusted for, this can cause new floors to fail.

Dryness levels of the subfloor should always be noted by contractors working with calcium sulphate screeds. If cement-based levelling compounds are applied over calcium sulphate screeds in a damp environment, a reaction can take place that leads to the formation of an expansive called ettringite, which has been known to cause failure of the floor. For this reason, F. Ball recommends using a calcium sulphate based

self-levelling compound over calcium sulphate screeds, to avoid the risk of such costly chemical reactions taking place.


solutions New calcium sulphate based smoothing underlayments, such as F. Ball’s Stopgap 100 Gypsum, have been designed specifically to offer excellent compatibility with calcium sulphate screeds. Unlike most cement-based products, calcium sulphate based smoothing underlayments do not shrink during the drying process and are virtually tension free, limiting the amount of stress that takes place at the interface between subfloor and smoothing underlayment. Calcium sulphate smoothing underlayments are not only effective over calcium sulphate screeds – most can also be used over a wide range of surfaces, such as waterproof surface membranes, concrete and sand/cement screeds.

Calcium sulphate screeds must be suitably smooth, dry and sound before they are considered ready to receive a new floorcovering. Laitance and contaminants

must be removed by mechanical or other means, and F. Ball recommends using a Digital Hygrometer to conduct a moisture measurement test before continuing. Only once a subfloor is suitably dry can the surface be primed to receive a smoothing underlayment. The priming of calcium sulphate screeds is essential to the appearance and integrity of the finished floor. Priming not only promotes the adhesion of the smoothing underlayment to the subfloor, it also prevents rapid drying of the underlayment.

Many flooring projects that might once have been considered challenging can now be easily accommodated, by taking advantage of new developments in smoothing underlayment technology. Professional contractors faced with calcium sulphate screeds should specify calcium sulphate based smoothing underlayments to ensure compatibility and guarantee a lasting, high quality finish.

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