If a plastered substrate must be tiled, their limitations must be respected, such as their sensitivity to water. Gypsum plasters should not be used where they may become wet or in humid conditions, unless they are fully protected, such as by a tanking system such as PCI’s Lastogum X.
Plaster must be allowed to dry fully and to develop sufficient strength to support the weight of the tiling. British Standard BS 5385 states that plaster should be at least 4 weeks old and dry.The recommended maximum weight for tiling fixed to gypsum plaster is 20kg/m?.Tiling must not be carried out to undercoat plaster; only to finish plasters which have been left with a wood-float finish. Some relatively recently introduced multi-purpose plasters have proved to be unreliable substrates for tiling - please contact the PCI Advice Line if you are unsure.
Two common faults, from a tiling point of view, which may affect plaster are over-polishing and overtempering, as described here:
Over-polishing is the result of an over-enthusiastic use of the steel trowel, resulting in a slick, shiny surface which means that it‘s difficult for the adhesive to bond on to. However, it can be corrected by abrading or wire brushing, and removing the dust with a damp sponge, before fixing the tiles.
Over-tempering results from excessive use of water after the plaster has set in an attempt to achieve a good finish. This tends to bring the fat to the face and results in a dusty surface when dry.This can usually be corrected by using a damp sponge to remove the dust.
Neither of the above problems are likely to be solved by the application of PVA as a primer because priming is rarely a good substitute for thorough preparation.
Some of the substrates faced by tilers are perfectly ready for tiling - many are not. Specifying and carrying out the necessary preparatory work is crucial. If cementitious tile adhesives are to be used in conjunction with plaster / plasterboard backgrounds then it is essential that PCI Primer G is applied first, and allowed to dry.
“Excessive trowelling of the plaster should be discouraged as this practice may result in a dusty surface that is unsatisfactory for tiling and it may also create a denser finish than normal, thus reducing suction” BS 5385 : Part 1:199
Existing gypsum skimmed walls
Preparation technique
Ensure plasterwork is sound and strong.
Tiling weight should not exceed 20kg/m? (generally this translates to using 8mm thick ceramic tiles)
Do not fix ceramic tiles to an undercoat or bonding coat as they are too weak. Only use a finishing coat.
Plaster is an unsatisfactory background for tiling in continuously damp areas. In occasional wet areas (such as domestic showers), gypsum plaster must be protected from water ingress. It is recommended that, in this situation, the installation is tanked out with PCI’s Lastogum X.
All plasterwork must be thoroughly dry in all situations.
Roughen a smooth gypsum plaster surface using a wire brush to remove the surface laitance thus ensuring a strong bond.
If using cement based adhesives on plastered surfaces, the plaster should be first sealed with a product such as PCI’s Primer G (dilute 1:1,water:Primer G). If a paste adhesive is to be used then the plastered surface will not generally require priming.
Adhesive application Paste adhesive
Paste adhesives, such as PCI’s Tilefast Non –Slip, are the preferred adhesives for tiling onto gypsum plaster and in normal, dry conditions, there will be no associated problems as those found when using cementitious materials with a plaster background.
Cement-based powder adhesive
Cementitious adhesives, such as PCI Tilefast 6 have specific preparation guidelines, so carefully follow the manufacturer’s guidelines on the packaging.
Gypsum plaster can only hold a certain weight of tiling (about 20kg/m2 - this translates to using 8mm thick ceramic tiles).
Adhesive drying and curing times will vary depending on the type of adhesive, the porosity of the plaster and tile, as well as site conditions and grout joint widths.
Grouting
Choose a grout to suit the situation.
Grouting may commence once the adhesive bed has firmed up.
Drying times will vary according to the site conditions.
Movement Joints
Please refer to Special Situations Bulletin 1 due to the complexity of this subject.
New gypsum skimmed walls
Preparation technique
It is much better to tile directly onto a cement based plaster rather than a gypsum plaster. However, in some situations, it may be unavoidable, so:
Ensure plasterwork is sound and strong.
Tiling weight should not exceed 20kg/m2 (generally this translates to using 8mm thick ceramic tiles)
Do not fix ceramic tiles to an undercoat or bonding coat as they are too weak. Only use a finishing coat.
Plaster is an unsatisfactory background for tiling in continuously damp areas unless its porous suface has been treated. In wet areas (such as domestic showers), gypsum plaster must be prevented from water ingress. It is recommended that the installation, in this situation, is tanked out with PCI’s Lastogum X.
All plasterwork must be thoroughly dry in all situations - drying time will depend upon many factors such as plaster thickness, background absorbency as well as humidity and the site conditions.
Do not apply tiles to plastered walls which are showing fine, random cracks (map cracks). These cracks appear when the plaster has been allowed to dry too quickly or has been subject to excessive background movement.
Roughen a smooth gypsum plaster surface using a wire brush to remove surface laitance to ensure a strong bond. Over trowelling of new plaster may produce a dusty surface with the finer particles being brought to the surface and second it could make the surface denser, which can reduce suction. (Ready mixed adhesives need suction).
If using cement based adhesives on plastered surfaces, first seal the plaster with a product such as PCI’s Primer G. If a paste adhesive is to be used, then the plastered surface could be wiped down with a damp cloth.
Adhesive application
Paste adhesive
Cement-based powder adhesive
Cementitious adhesives, such as PCI Tilefast 6 Flex have specific preparation guidelines. The surface of the plaster should be primed with PCI primer G diluted 1:1 water - Primer G.
Gypsum plaster can only hold a certain weight of tiling ( about 20kg/m2 - this translates to using 8mm thick ceramic tiles).
Drying times of the adhesives will vary depending on the type of adhesive, the porosity of the plaster and tile as well as site conditions and grout joint widths.
Grouting
Any grout may be used to suit the situation.
Grouting may commence once the adhesive bed has firmed up.
Drying times 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