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Impervious tiles or substrate

When tiling an area that is already tiled or painted, it is often necessary to invest considerable time in stripping and then repairing the substrate. In certain situations it is more convenient to tile over the existing layer. However, this requires careful consideration and an adhesive with exceptional properties.


Standard cement-based tile adhesives won’t stick to a completely smooth surface

Standard cement-based tile adhesives rely on a mechanical keying action to bond onto the substrate. The wet cement engages physically with small irregularities, pores etc in the surface and uses this to form a strong bond.

When the surface is very smooth this grip is reduced resulting in tiles de-bonding. Existing ceramic tiles and paint are examples of such surfaces.

The back of very low porosity tiles, such as porcelain, is another example.

These types of surfaces are largely impervious to water and this results in a longer time delay

Ready-mixed adhesives are dispersions of polymers and inert fillers in water and do not gain strength until the majority of the water has dried out. With large tiles on impervious surfaces, this can take weeks.

At lower temperatures, even standard cement-based products can take several days to set and dry. The other complication from slow extended drying, is that if the joints are grouted before the drying is complete, the water will be trapped. As it subsequently permeats through the grout, it will bring dissolved salts from the adhesive or grout and cause a white deposit on the surface of the grout, known as efflorescence. As much as there is always a possibility of this happening, the chances are increased the more water that is trapped.

Tile directly onto well-adhered ceramic tiles or paint or with low porosity tiles.


Highly polymer-modified adhesives achieve a sound chemical bond onto impervious surfaces overcoming the need for suction and mechanical keying.

Stage 1: Assess and prepare the floor/wall

It is essential that the layer that is to be tiled over is completely clean and able to withstand the extra weight of another layer of tiles and adhesive:

  • Gypsum plaster can only support 20 kg/m2 and plasterboard 32 kg/m2 – the weight of both tiling layers must be considered.
  • The bond strength of any paint may well be the weakest link. Epoxy paints on floors and gloss paints on walls are usually strong enough but emulsion paint is not and should be removed mechanically.
  • If the existing layer is over something flexible, such as wood or a heated screed, then this level of flexibility will be required of the new tile layer and the adhesive specification may need to be upgraded.
  • If possible, lightly abrade the surface with aluminium oxide or carborundum paper to improve the mechanical key.
  • Ensure that any traces wax, polish, grease, dirt, dust etc are completely removed from the surface.

Stage 2: Fix the tiles

Fix the tiles with a polymer-modified adhesive:

  • For solid and floors use: weber.set Ultra-flex
  • For heated floors use: weber.set Ultra-flex
  • For wooden floors use: weber.set Ultra-flex

A solid bed of adhesive is recommended to maximize the strength of the bond.

Stage 3: Grout

Leave the adhesive longer than normal before grouting to allow the water to escape the system. This will be longer for larger tiles. Grout with a grout that is suitable for the area concerned. Use a suitable flexible sealant for the perimeter movement joints.

Solution with the following weber products

Solution 1

weber.set Ultra-flex

A highly deformable, fast setting, high strength cement-based tile adhesive for all types of tiles.