Tiling onto plaster backgrounds
Plaster is a weak material compared to brick and cement. When applying tile to a plastered surface it is important not to overload the weight of what the plaster will take before failure occurs, plaster will support about 20kg/m2 or about a 9mm thickness of ceramic tile.
New plasterwork should be fully dried before tiling work is commenced, this drying time varies upon temperatures but can take up to 4 weeks before fully cured.
Plaster is not ideally the best surface especially in a bathroom as moisture and water can get in causing failure.
When using a cement adhesive on your tiling projects you have to be careful when tiling direct onto neat plaster. Gypsum plaster based products and cement products do not get on with each other and can cause de-bonding, this is due to the chemical reaction that goes on when the two come together. This reaction forms a weak layer of ettringite, which can cause the failure.
To stop the ettringite reaction, plaster surfaces must be primed with a diluted acrylic primer or SBR, this primer acts like a barrier and stops the 2 products from contacting each other. PVA must not be used
Another problem when tiling on plaster is to be careful that the plaster has not been over trowelled which results in a smooth shiny finish, this finish will have to be roughed up a bit with a wire brush prior to priming.
Good quality tiling courses can give you a good understanding of how to tile and understand the backgrounds. UK Pro Tiling Training offer fast track courses teaching you all you need to know about tiling, for more information visit www.tiling-courses.co.uk