Most houses in England are terraced or semi detached houses and most bathrooms are upstairs.
This means that in 9/10 cases, bathrooms have subfloors constructed from floorboards or chipboard screwed or nailed onto joists.
Both require specialist attention in order to prepare them for tiling.
Proper surface preparation is essential on wooden floors as they have the tendency to move, bend, bow, stretch and shrink. In contrast, tiles (even when fixed with flexible adhesives & grouts) are relatively solid.
Any stresses between the movement of the wood and the tiles can cause failure of the tiles to grip to the substrate, resulting in loose tiles or cracks forming in the grout or across the tiles.
You can do this by doing a combination of the following:
(Floorboards & chipboard should not be tiled onto directly)
Alternatively, you may wish to replace all of the floorboards at this time with plywood for reasons outlined in (part 2)
Please see Tiling on wooden floors (Part 2 – floor strengthening) for the next step.