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Tiling onto plaster

Plaster is a very common interior substrate that generally provides a good, flat surface for tiling. There can be, however, certain problems associated with plaster and these need to be overcome with correct preparation and application.

Problem

Plaster can only support relatively small / light tiles

Plaster is a relatively weak material (in comparison to wood, brickwork, concrete etc). The maximum weight of tiling which can be supported by a dry, well-adhered plaster background, is 20 kg/m2.

New plaster work should have been completed at least 4 weeks prior to tiling commencing. Even if the surface appears to be dry in less than 4 weeks, the layers underneath will not be.

Plaster is water sensitive

Plaster is water sensitive and is therefore not an ideal substrate in areas subject to wetting such as showers or wet rooms.

The finish quality can be very variable

Plaster finishes, when prepared for tiling, should be finished in accordance with the recommendations given in SANS 50197 and SANS 50413 Class MC12,5 X*. Very often the finished surface of plaster can be very variable, depending on the conditions and the plasterer. The surface can be very dusty or, if overtrowelled, it can be very dense and shiny.

Cement adhesives react with plaster

When plaster comes into contact with cement it forms a weak layer of laitance (cement calcium surface residue ettringite) that can cause the adhesive and plaster to not adhere to one another. Careful preparation is needed to stop this happening.

Solution

Plaster finishes, when prepared for tiling, should be prepared as recommended in SANS 50197 SANS 50413*. Quite often though, the finished surface can be very shiny or dusty and this needs to be prepared correctly prior to tiling, otherwise failures could occur.

Tile size and thickness

The maximum weight of tiling which can be supported by a dry wall-adhered plaster background is 20 kg/m2. This is including the adhesive and grout. A good guideline is that tiles should be no thicker than 8 mm if they are ceramic, or 7 mm if natural stone. Where thicker/heavier tiles are to be used, plasterboard (up to 32 kg/m2) or tile backer (see manufacturer’s recommendations) should be installed.

Protecting plaster substrates when subjected to wetting

Plaster loses nearly all its cohesive strength when wet so it needs to be protected in areas where it could be subjected to wetting such as showers or wet rooms. An easy to use tanking system such as weber.prim Plaskey WB250 with weber.ad Key-it WB117 will protect the plaster and provide a suitable substrate to tile onto.

Preparing the surface prior to tiling

Sometimes the finished surface of a plaster substrate is not ideal for tiling. If the surface of the plaster has been over-trowelled it will be very dense and shiny and this will make the bond with the adhesive very weak. It is recommended that the surface is abraded with a stiff wire brush and then all dust removed prior to tiling. If the surface of the plaster is dusty, it should be wiped with a damp cloth until all dust has been removed.

Using cement-based adhesives on plaster

If cement-based adhesive is being used, the surface of the plaster must be sealed to stop the cement reacting with the plaster and forming a weak layer of ettringite which could cause the tiles to de-bond. It is recommended that weber.prim Plaskey WB250 or weber.ad Key-it WB117 is applied to the surface until the plaster stops absorbing. 

Solution with the following weber products

Solution 1

weber.prim Plaskey WB250

Cement-based primer and surface preparation.

weber.ad Key-It WB117

A latex-based liquid primer and keying agent specifically formulated for use with weber.prim Plaskey WB250 onto gypsum and cement surfaces.