If you tile to your walls or floor without giving thought to preparation, you’re lucky if your tiles don’t fall off.
You have to consider tile weights. Plasterboard can carry 32kg per square meter. So including tiles, adhesive and grout, that’s your weight limit. It isn’t much when you consider the weight of some stone, or even porcelain tiles.
Skimmed plaster, can carry even less! That’s only 20KG per square meter, rendering most skimmed bathroom walls suitable for only 6×6 whites, or a little bigger. And only ceramics.
If you’re tiling on top of existing tiles, you’re almost certainly going over the weight limits.
…. working on this page.