A very common decision to make when re-tiling is whether to tile over existing tile or remove the old tiles first.
It is quite common to tile over existing tile, and should not be a problem, but there are a few points to bear in mind, or worth considering.
If tiling over tileThe existing tiles must be soundly bonded to the walls. Obviously, no matter how well fixed the new tiles are, if the old tiles de-bond from the wall and fall off, then so will the new ones. Make sure the old tiles are well fixed.
One way to check is to gently tap them. If it sounds very hollow then they are likely not well bonded. Pay particular attention to wet areas which are more likely to have problems.
The existing tiles need to be clean and free of grease or other contaminants which could affect the bond of the adhesive used for the new tiles.
Some backgrounds such as plaster and plasterboard have a recommended weight limit for a square unit of measurement. That means that for instance plasterboard will have a maximum weight of tiles or stone you can fix to it, which if surpassed may cause a structural failure.
This is unlikely to be a problem if both old and new tiles are smaller format, thinner tiles, but could be an issue if using large format, heavier tiles on top. It especially likely to be a problem with natural stone tiles which tend to be heavier due to their thickness.
Only certain tile adhesives are suitable for tile on tile. Check manufacturers instructions or with your tiler.
One last thing to consider is the aesthetics. Tile over tile can be noticeable around the perimeters of a room such as coving, door jambs, and especially if the room is only half tiled.
Why bother tiling over tile ?The reason this might be a good option is mostly time and money. It can be time consuming to remove old tiles and also the wall surface may be damaged by removing the tiles and need subsequent making good before re-tiling. This is obviously going to cost more if you are employing someone.
Another reason is that if you have say a good quality bath suite, then there is obviously a risk of damaging it whilst removing the old tiles. Of course, this should be avoidable but nevertheless, it is a potential problem.