Gypsum Based Screeds
Gypsum based (anhydrite) screeds can be pumped quickly and smoothly onto a wide area of flooring and for this reason is often used in fast track construction.Drying rates, however, are the same as standard floor screeds at approximately 1mm per day for the first 50mm. Gypsum based screeds are not recommended for use in wet areas.
Preparation technique
A primer should always be used in order to seal the screed and to separate any cement-based product from the gypsum screed.
Ensure that the screed is completely dry (with a moisture content of less than 1 per cent if a permeable covering (such as a carpet) is to be applied and less than 0.5 per cent for all other coverings such as tiles.
Always use a primer on the surface prior to tiling. An unprimed gypsum based screed will suck all the moisture from an adhesive which will significantly reduce the bond achievable. A suitable primerwould be Primer G (dilute 1:1,water: Primer G), followed by one coat of neat Primer G.
Underfloor heating
Gypsum screeds are suitable for use around underfloor heating pipes because they can be easily pumped and will flow around awkward areas.
Once the screed is laid and dry, switch on the heating and increase the temperature at 5°C per day until at full temperature, then switch-off and allow to cool down.
After grouting,wait for a further 48 hours and then increase the temperature at no more than 5°C every 24 hours.Do not use the heat to speed-up the drying process of the screed.
N.B. PCI Novoment Screeds and Periplan range of levellers are suitable for use with underfloor heating pipes and can be tiled upon after 3 days. Instead of waiting a further 20+ days for an anhydrite screed to dry depending on thickness.
An uneven surface
Some screeds may require some remedial leveling prior to the application of the final covering with a self levelling compound, such as PCI’s Periplan.
Before laying the leveller, use a primer to seal the gypsum based screed.
Adhesive application
Grouting
Once the adhesive bed has set, grouting can be undertaken.
Any grout may be used to suit the situation.
For vitrified or porcelain tiles use PCI’s Groutfast 20 Rapidflex to promote bond strength at the tile/edge interface.
Drying will vary according to the site conditions.
Movement Joints
Please refer to Special Situations Bulletin 1 due to the complexity of this subject.
For more information, contact PCI’s Techincal Services Department on 0161 794 7411