While anhydrite screeds are growing in popularity throughout the UK, they are not without their critics, especially as a number of flooring installations over anhydrite screeds have failed.
There are also many installations, of course, which have been completely successful. Like with fixing any flooring surface onto any type of substrate, long-lasting integrity can only be ensured by proper and adequate preparation.
While the Contract Flooring Association has published a technical paper on the generic installation of floor coverings onto these calcium sulfate-based screeds, Schlüter-Systems Ltd., is increasingly being asked about the specific requirements for a successful ceramic or stone installation on an anhydrite screed. And the tiling industry's trade body, The Tile Association, has also published a 21-page paper entitled "Tiling to Calcium Sulfate Based Screeds" which looks in detail at all considerations required for the perfect tiling installation.
As with all screeds, as long as appropriate steps are taken, there is no reason why tiled surfaces over anhydrite screeds cannot stay looking good for many years.
However, the choice of a ceramic or natural stone floor on anhydrite screeds needs careful consideration at the design stage - and contractors should consider bringing in tiling contractors who are familiar with fixing to calcium sulfate screeds, or, at the very least, seek advice from the tile manufacturers or suppliers - as, indeed, they should for all types of substrate.
One of the key considerations for a sound tiling installation on anhydrite screeds, is the preparation before tiling. The Tile Association recommends that where a calcium sulfate screed has been used - particularly where the finished floor may be exposed to moisture - a watertight membrane is installed.
According to the applicable standards, the residual moisture of such gypsum-based screeds should not exceed 0.5% prior to the tiles being installed. However, The Tile Association points out that with the use of a special separating - or uncoupling - system, tiling can commence on an anhydrite screed with a residual moisture content of 2% (by volume). It states: "One speciality separating system is a pressure stable polyethylene membrane, manufactured in a configuration that allows the tile adhesive to mechanically bond into the surface of the system. The separating function allows any stresses, such as drying shrinkage, that occur between the substrate and tiled surface, to be accommodated.
"Interconnected air channels on the underside of the separating system remain open allowing moisture from the substrate to evaporate, thus neutralising the vapour pressure in the calcium sulfate screed."
If required, the screed surface may need to be pretreated (sanding or priming) in accordance with industry standards and manufacturers' recommendations. A typical polyethylene membrane, as recommended by The Tile Association, is Schlüter-DITRA, which can be applied using a suitable adhesive that is suitable for the substrate.
It protects the screed against moisture penetration from the surface; especially important, as gypsum-based screeds are particularly sensitive to moisture and must be protected from additional moisture penetration.
Anhydrite screeds are a mixture of screeding sand and binder. If tilers are unsure of whether the screed they're to work on is, or isn't, anhydrite, they should always ask. Information about the screed should be kept in the building owner's Operation and Maintenance Manual.
Provision should be made at the design stage for the thermal expansion of the screed and surface covering. Industry guidelines for ceramic and stone coverings require movement joints to be placed at all perimeters, and where tiling meets restraining surfaces such as steps, kerbs, columns and fixed plant etc.
Intermediate movement joints should be placed in accordance with the covering requirements, which, for ceramic tile and natural stone flooring is between 8-10 metres in each direction. Ideally the tile fields should be kept as square as possible.
Where underfloor heating is installed within the screed layer, movement joints should be used to divide the tile fields into areas of no more than 40 square metres, with the longest side no more than eight metres.
The movement joint should pass through the covering, adhesive bed and screed.
Movement joints should be placed in the screed in line with industry guidelines, however, because of the difficulty of forming such joints in the required precise location while the screed is being laid, it is advisable to form intermediate joints in non-heated screeds by using a dry cut floor saw to cut through the screed when the tiling is being installed.
The structural movement joints in the bed and tiling should be sited immediately over, and be continuous with, the joints in the base.
This is just a brief overview of what is needed for a successful ceramic or stone installation on an anhydrite screed. For contractors who are in any doubt whatsoever about any aspect of tiling onto these calcium sulfate-based screeds - Schlüter-Systems Ltd (firstname.lastname@example.org), who are members of both The Tile Association and the Contract Flooring Association - will be happy to advise on the suitability.