Substrate for ceramic tiles

Ceramic tiles should be laid on a substrate with appropriate features. The substrate should meet four basic criteria:
- appropriate dryness
- no impurities
- firmness
- appropriate adhesiveness
All four criteria are rarely met simultaneously, and so the surface needs to be properly prepared – cleaned, levelled, the gaps filled, and cohesion improved.
Ceramic tiles can be laid both on old surfaces which were previously covered with other materials and on new concrete or plasterboard surfaces.
Old substrates
Wall covered with oil or emulsion paint
Such substrates are problematic for tiling. To prevent problems remove the paint layer. You can do this mechanically (tirelessly scraping off the paint) or using chemical agents facilitating the removal of coatings. Such agents make the coating softer and swollen, which greatly facilitates its removal, e.g. with a putty knife. If the paint is very sticky and difficult to remove, you can try another method – puncture the wall thus piercing the coat layer. The surface of punctured points should be at least equal to one-third of the tile surface.
Substrate made of old tiles

If the old tiles stick too well to the wall it means the substrate is firm and you can lay down another layer of tiles. The adhesive power of the old tiles is best checked by tapping – if the tiles produce no hollow sound, it means they stick well. If it turns out that some old tiles do not stick well (they make hollow sound), they must be removed and the space left filled with levelling compound.
Before laying down new tiles we should carefully clean the surface of the old ones – first, they need to be degreased by washing their layer with caustic soda and then rinsed with water. Pay particular attention to cleaning the joints.
Substrates after removing old tiles

If after tearing off old tiles, the old plaster sticks too hard to the wall, we can leave it, using only levelling compound to fill the remaining gaps.
If new tiles cannot be laid down on the old material surfaces, we have to hammer the old plaster off the wall. However, before laying down a new layer of tiles, such a wall must be plastered again, with consideration for preserving the verticality of the wall and creating a level surface of plaster. The fresh plaster layer should be left to dry, usually for one week per each cm of its thickness. If we lay down new tiles on plaster that is not fully dry, tiny cracks and crevices may appear due to tensions in the drying plaster.
New substrates
A new substrate should be dry and "cooled off", i.e. at least a twelve or so days up to several months should have passed since its application.
Tiles can be laid down on concrete substrate screeded at least three months before. Otherwise, the tensions in the concrete during its drying might cause cracking of the hastily installed tiles. For concrete surfaces, it is recommended to use priming emulsion before tiling.
Plasterboard substrate
If you plan to lay down tiles on plasterboard surface, the substrate must be first primed with appropriate priming emulsion.
Such an emulsion will reduce the absorbability of the substrate and protect adhesive against quick drying. Although the manufacturers of plasterboard claim they are already primed at the production stage, they should be additionally protected by applying appropriate agents. Such protection prevents swelling of the substrate, as a result of humidity, and prevents tiles from falling off.