Product category:
Floors
News Release from: Schluter-Systems | Subject: Anhydrite Screeds - tiling queries
Edited by the Buildingtalk Editorial
Team on 14 July 2005
Anhydrite Screeds -- increase in tiling
queries
The Tile Association, has published a 21-page paper entitled "Tiling to Calcium Sulfate Based Screeds"
With the increased usage of anhydrite screeds in the UK - particularly in new-build projects, world market-leader in tiling solutions, Schluter-Systems, is often asked if it is possible to tile onto them successfully with ceramics and natural stone 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
This article was originally published on Buildingtalk on 11 Apr 2008 at 8.00am (UK)
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However, as Schluter Technical Manager Ian Knifton explains, the choice of a ceramic or natural stone floor on anhydrite screeds needs careful consideration at the design stage - and builders should consider bringing in specialist tiling contractors.
Or, at the very least, seek advice from the tile manufacturers or suppliers - as indeed, they should for all types of screed.
Also, the tiling industry's trade body, The Tile Association, has published a 21-page paper entitled "Tiling to Calcium Sulfate Based Screeds" (available from The Tile Association - www.tiles.org.uk) which looks in detail at all considerations required for the perfect tiling installation.
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By "anhydrite screeds" we mean pump-applied, self-levelling screeds based on a calcium sulfate binder, which is used primarily in new-build domestic and commercial projects to provide a flat, smooth surface to take thin floor coverings, including tiles.
The largest manufacturer of anhydrite screeds is Lafarge Gyvlon, which says that Anhydrous Calcium Sulfate - CaS04 - can be derived from gypsum, which comes from mines, power stations, and is a by-product of the production of hydrofluoric acid, which is common in the UK.
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 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 Schluter-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, this should be clarified prior to work commencing.
Information about the screed should be kept in the building owner's Operation and Maintenance Manual.
MOVEMENT JOINTS.
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 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 joints should pass through the covering, adhesive bed and screed.
Movement joints should be placed in the screed in accordance 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 - Schluter-Systems, who are members of both The Tile Association and the Contract Flooring Association - will be happy to advise on the suitability. Request a free brochure from Schluter-Systems ...
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