NGS FIRST
ults in delays to projects, major inconvenience and expense when remedial work is required. ver newer calcium sulphate-based screeds compared to traditional sand/cement varieties.
to their original state and offering a solution for temporary flooring installations.
CALCIUM SULPHATE SCREEDS With advantages over sand/cement screeds, including the speed with which they can be applied over larger areas less shrinkage and fewer joints required within the subfloor, calcium sulphate (otherwise known as gypsum or anhydrite) screeds have become an increasingly common choice of subfloor, especially in new builds and where underfloor heating is present.
However, it is not recommended to apply a liquid waterproof surface membrane over this type of substrate. If excess subfloor moisture is present and permanent floorcoverings are to be installed, contractors will need to allow further time for the screed to dry or risk floor failure.
“It is not recommended to apply a liquid waterproof surface membrane over a calcium sulphate (anhydrite) substrate.”
barrier against residual construction moisture where relative humidity values are up to 95%. The first coat cures in 15-20 minutes. A further 30 minutes curing time is required for the second coat.
AN ALTERNATIVE SOLUTION An alternative solution for dealing with damp is available for where a waterproof surface membrane cannot be applied. This may be the case where flooring installations are part of a refurbishment project or screeds are contaminated with oil, other chemicals or old adhesive residues.
In these situations, a loose-lay isolator membrane can be laid directly onto the subfloor, without the requirement for an adhesive, to provide a barrier to stop excess subfloor moisture causing floor failure.
These membranes typically have nodules on the underside, creating an airspace to allow water vapour to travel to the edge of a room, into a dry wall or ventilated area. A wide range of floorcoverings can be adhered to its upper surface. This enables durable new floors to be installed and easily removed at a later date, allowing buildings to be returned
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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 enough time 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.
NEXT STEPS Once relative humidity levels in the subfloor are sufficiently low or a suitable moisture management solution is in place, contractors can proceed with preparing the subfloor for the installation of new floorcoverings. Where an isolator membrane has not been used, this should include the application of a levelling compound to create a perfectly smooth base for the receipt of floorcoverings, remembering to prime beforehand. Specialist primers and levelling compounds are available that offer maximum compatibility with calcium sulphate screeds.
Once the levelling compound has cured, contractors can proceed to installing floorcoverings using an appropriate adhesive. At this stage, the compatibility of particular floorcoverings and adhesives should be checked to further ensure against floor failure.
www.f-ball.com SCREEDS, RESINS & COATINGS | 19
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