SCREEDS & RESINS
WELL PREPARED
F. Ball provides expert guidance on preparing calcium sulphate screeds ahead of the installation of floorcoverings.
Calcium sulphate screeds have become an increasingly popular choice of subfloor, especially where underfloor heating is incorporated within. 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.
However, these screeds necessitate a few special considerations when it comes to subfloor preparation, and not undertaking certain procedures when working over them is a common cause of floor failure.
FIRST STEPS
When installing 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 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. If present, it can cause a subsequently applied smoothing compound to debond from the substrate. Laitance should be removed using a rotary sanding or abrading machine. This should be undertaken by the screed supplier, but it is frequently left to the flooring contractor. 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 levels are above 75% (or 65% if bonded 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 adversely affect floorcoverings and adhesives, potentially resulting in floor failure.
A moisture test may deliver a positive reading right up until the point the subfloor is completely dry. Therefore, contractors should be patient when waiting for this to happen. 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 18
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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 Gypsum, is used to create a base for floorcoverings 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.
Once the smoothing compound had cured, contractors could proceed with installing floorcoverings using a compatible flooring adhesive.
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