It is an important point to know that when tiling over any floor that is considered flexible i.e. timber, you should use a flexible tile adhesive and a flexible grout. These products are usually powder based to which you add water but are very worthwhile because without them you are more likely to get cracks in the grouting and possibly the tiles themselves when the floor underneath flexes and moves, expands, contracts etc. Another important point is the size of the grout joint between the tiles on a flexible floor; we almost always use 5mm tile spacers.
Grumpy Old Git