Lin Twidale, Technical Service Advisor of Saint-Gobain Weber, has prepared a helpful check list when tiling onto floors.
What the doctor ordered
Before you get started on applying tiles to a floor substrate the techies at Weber have prepared an eight-point check list to help you get it right first time. Remember good tile adhesion depends on the correct diagnosis of the substrate you are working on, various factors of which will require treatment and preparation.
How to check for flatness
This may seem an entirely unnecessary reminder, but don’t confuse flatness with horizontality; a substrate can be flat without being completely horizontal. To avoid unsightly appearance, or any defects that may affect the behaviour of the tiles after you have fixed them, it’s wise to check.
To do this use a 2m straight edge mounted on 3mm thick spacers.
In the case of direct bonding of the tiles, any defects should not exceed +/- 3mm, that is high points not touching and low points no more than 6mm below, under the 2m straight edge. If the area is not suitable you may want to consider a levelling screed such as weberfloor flex.
Check the substrate for hardness
To avoid cracking, or debonding, of the tiles the substrate must be both hard and resistant. Check the surface hardness by scratching it with a pointed tool in several places, just a superficial scratch. If the substrate is not hard enough, it must be removed until sound material is reached.
This is important to avoid deterioration of the tiling at a later stage. This check mainly concerns wooden floors laid on joists or battens, wooden panels and, less often, partitions. Make sure there is no movement of the floor when stepped on and partitions must not flex when pressed by hand. If this is the case reinforce the floor with noggins between the joists and replace the boards. Brace any unstable partitions.
Check the porosity of a cement-based substrate
This type of substrate must have normal absorption in order to avoid premature water loss from the cement-based adhesive and to ensure that the bond can develop correctly. Simply pour a little water onto the substrate and if it is absorbed in less than 1 minute the substrate is excessively porous and must be primed with weber PR360.
The substrate must be cohesive to ensure a proper bond. First of all check the adhesion of existing tiles or rigid floor tiles by tapping with a hammer. Any hollow sounding tiles or tiles with poor adhesion must be removed and replaced, or repaired
Then check the adhesion of existing paint, carry out a cross-hatch test using a suitable knife or cutter. Remember emulsion paint is not suitable for tiling over.
Score the paint in small 2 x 2mm squares over a total area of 10 x 10cm. The paint is considered suitable for tiling if 80% of the area of the small squares remain bonded. If not, the paint must be removed mechanically before work can start.
Cleanliness of the substrate
Time for some good housekeeping to ensure the adhesive bonds properly and the method you use depends on the substrate you are tiling.
Eliminate any traces of contamination with a scraper. Carefully vac any dust and then apply weber PR360 primer
If the existing floor covering has been removed, eliminate any traces of adhesive so that no film residues remain, only residual coloration of the substrate; apply weber PR360 primer.
If the existing covering is to be retained, remove any traces of varnish or wax with an emulsifying pad and sugar soap. Wash existing paints, vinyl or ceramic tiles. On concrete, remove any residues that may affect the adhesion, such as superficial free lime or traces of oil, using high pressure cleaning, sanding, abrasive cleaning etc.
On anhydrite screeds there must be no more than 0.5 per cent residual moisture before being covered, and cement/sand screeds must be left for 3 weeks prior to tiling unless special fixing methods are employed.
Finally, priming correctly immediately prior to tiling is essential. Remove any laitance from anhydrite screeds and seal with weber PR360 primer before applying any cement-based product, levelling compound or tile adhesive.