Whether you're fixing natural stone Bianco Carrara marble tiles, or concrete effect porcelain tiles such as Cementone Light, and considering fixing new tiles onto existing tiles you need assess the condition of the existing tiles, and suitability of the substrate underneath to ensure your project is a success. Failure to assess and prepare the surface correctly may result in the floor tiles cracking, coming loose, or the tile adhesive de-bonding from the substrate.
There is no technical reason why, assuming the existing tiles and substrate are sound and solid, that new tiles can't be fixed on top of existing tiles, however, whenever possible I'd always recommend removing old tiles and fitting a new substrate as installation of new tiles will be easier, the overall finish should be better, and any guarantee for the work is subject to unknown undetectable deterioration in the substrate.
One scenario where you may wish to tile over tiles is in an older property where the base is flagstones or natural limestone tiles. In this situation, lifting the tiles may damage the substrate below and add additional time and costs to project. Ensure the tiles are firmly bonded to the original substrate, any loose tiles are made good, and the surface is free from dust, dirt and contaminants, and that any sealer treatments have been removed. If you're happy the tiles are even, flat, and smooth, then simply apply a coat of bonding agent such as Kerakoll Primer A which will help the floor tile adhesive form a strong bond with the new floor tile.
If the existing floor tiles are sound but are uneven, have wide grout joints, or have tumbled edges, such as Dijon tumbled limestone tiles, an option is to apply a self-levelling compound to give a flat and smooth surface to tile onto.
One scenario where we would recommend not tiling on top of tiles is when installing new bathroom wall tiles. The main reason for this is that it's impossible to detect the quality of the substrate. The weight of new wall tile and adhesive may exceed the load-bearing capacity of the substrate. As a guide, our suggested maximum weight of tiles and adhesive per square meter is:
- gypsum plaster: 20kg
- plasterboard (without skim): approx 35kg
- tilebacker boards: up to 60kg
If you're happy that the substrate is sound and can take the combined weight of the existing bathroom wall tiles, the new tiles and the adhesive, it’s time to clean the tiles, making sure they’re free of dirt, grime and grease. Finally, a coat of bonding agent such as Kerakoll Primer A will help the bathroom wall tile adhesive form a strong bond with the new floor tiles.