Is this marine plywood (B&Q)?

glubalub

The main problem with tiling on timber floors is that the timber flexes and moves and the tiles are rigid. If the floor moves underneath the tiles the adhesive fails. 18mm ply is fine to use directly on top of the joints as long as it can be fixed directly to the floor at 300 centres in both directions. As floor joists are typically set at 400mm centre to centre this does mean adding extra noggins (short lengths of timber fixed between the floor joists) to provide the fixing points. Noggins should also be used to provide support to any cut edges of the boards. This also assumes that the floor joists are large enough to stop the floor flexing. The longer the span the deeper the joists need to be. On modern properties this isn't likely to be a problem but on older houses the joists are often undersized by modern standards and so the deflection at mid span is too much and the flexing causes the tile adhesive to fail (the jump test is a quick and dirty way to test this). If there is too much movement in the joists they either need replacing with larger joists or additional joists fitted between the existing. If you get to this stage it might be worth thinking about tile effect vinyl floor coverings rather than ceramic tiles.

leveller2911

+1 for a good quality vinyl type floor covering. I'm not a fan of floors being tiled and never have been.Its also worth remembering that if you go down the 18mm ply over floorboards and then a 6-9mm tile you will end up with difference in floor level from bathroom-landing and a small 30mm difference in height is worse, neither level nor a full step.

There are some really great vinyl floor coverings now which are really thick and decent quality. In our hallway we have a good quality vinyl that replicates a medium Oak timber floor and at least 4 builders , 2 carpenters and a heating engineer mistook it for a solid timber floor..