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Sub-floor preparation for ceramic floor tiling
Cyril Potter, Chief Chemist at Ardex UK looks in detail at sub-floor preparation for ceramic floor tiling.
Any preparation work carried out on sub-floors is intended to provide a surface that is clean, dimensionally stable, rigid and sufficiently flat so that the ceramic floor tile can be well adhered and provide a hard wearing and durable floor finish. Cyril Potter, Chief Chemist at Ardex UK looks in detail at sub-floor preparation for ceramic floor tiling.
Some of these requirements may not be in the control of the tile fixer, especially the requirement for dimensional stability of concrete and cement sand screeds.Rigidity of the sub-floor can be improved by stiffening the construction of the sub-floor and steps can be taken to achieve the required flatness and cleanliness.
This article deals with tile fixing with adhesives and the Codes of Practice state that where adhesives are used to fix floor tiles the sub-floor should be sufficiently flat so that when checked with a 2 metre straight edge any gap under the straight edge does not exceed 3mm between points of contact with the sub-floor, i.e surface regularity 1 (SR1).SOLID SUB-FLOORS.In the case of new concrete or screeded sub-floors the required drying out times may not have elapsed when the tiler is expected to commence fixing.
At site temperatures of 20C and a relative humidity of 65% a new concrete slab requires at least 6 weeks drying after curing and a new screed requires at least 3 weeks drying after curing.These times should be extended under poor drying conditions, e.g cooler and poorly ventilated conditions.In most cases other trades install concrete and cement sand screeded sub-floors but in some areas the tile fixer may install the screed.
In the latter case the tile fixer should ensure that the freshly laid screed meets the surface regularity requirement and is covered with an impervious sheet for about 7 days to prevent drying and then allowed to air dry for at least a further 2 weeks. If these drying time are not available then the use of a rapid drying cement and sand screed will allow the ceramic floor tiles to be installed after 24 hours.
When new concrete floor slabs or cement and sand screeds are installed the surface will have surface laitance and lime bloom on the surface.In many cases this may not be visually evident but if surface laitance and lime bloom is not removed then the ceramic floor tiles will not be adhered to the concrete or screed surface but to these cohesively weak materials.
Surface residues, contamination and other barriers to adhesion should be removed using appropriate mechanized equipment that is available from plant hire firms and select the quietest and least dust generating equipment if working in occupied premises.
Primers do not replace the correct preparation of the concrete or screed surface! Calcium sulfate based screeds require special treatment and for guidance on testing and preparation, when fixing floor tiles on this type of screed a guidance document from the Tile Association is available.
Note that these gypsum plaster screeds should not be used in locations where they could subsequently become wet.
Where heated screeds are to be tiled it is necessary to pre-heat the screed before the ceramic floor tiles are fixed and there are requirements for movement joints at specified spacing in the screed and possible use of metal mesh reinforcement in the screed.Concrete and cement and sand screeds that previously had other types of floorings adhered to them can sometimes present problem since the old adhesive residues may be very well adhered and difficult to remove.
If it is possible, the best procedure is to remove all surface residues and expose a clean surface.If this is not possible then the tiler should check with the adhesive manufacturer as to the best procedure.In some instances an appropriate primer will adhere to the old adhesive residues, or the adhesive residues can be covered with an appropriate smoothing compound before the tiles are adhered.In these cases the tile fixer should use his past experience for assessing that the adhesion of the old adhesive residues will provide adequate restraint for the floor tiles.
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