If you have a tiling job to do in your bathroom and the walls have been skimmed with plaster you have to follow a few guidelines when installing tile to make sure you do not have a failure resulting in all the tiles coming off the wall.Firstly check the condition of the plaster skim, it should be fully dried out and a light pink colour.
You must also check that the plaster does not have a shine on it, this is caused by over trowelling the plaster when applied, if it has then a wire brush is needed to roughen up the surface.
It is important to check the weight of your tiles as plaster skim has a 20kg per square meter weight ratio therefore your tiles and adhesive should not exceed these weight otherwise failure could occur due to the skim de bonding from the wall, cement adhesive will usually weigh about 4kg per meter squared plus the tile weight.
If using a cement powdered adhesive you must put a barrier between the gypsum based plaster and the cement adhesive otherwise a reaction can go on between the two products, when these two products meet a layer of ettringite crystal can form which causes the failure. To stop this happening a diluted mix of 2 parts water to 1 part acrylic primer or SBR must be applied to the plaster to create a barrier between the two products.
Once the tiles have been applied a suitable grout must be applied between the joints of the tile, this grout locks everything into place and stops water from getting between the tile joints into the backgrounds.
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