The joists if they re 2x8 newer code and not 2x10 older code must be braced every four feet with cross braces which minimizes independent flexing of each joist and the subfloor above.
Glue tile to plywood.
It gives perfect bonding to porous substrates like wood ceramic as it is a thick adhesive.
It is a cyanoacrylate adhesive which is single component gives instant strong pasting.
Remove any obstacles from the installation area.
Try using polyfix instant wood glue.
There may be times when you want to bond tile directly to plywood or another wood surface rather than screwing down backerboard first.
Plywood will expand and deform if subjected to moisture or humidity.
Unscrew switch plate and outlet covers and set aside.
Unscrew register covers and save the screws.
You can bond tile directly to wood with a flexible adhesive.
If you are working in an interior setting in a dry area where the tile will never come into contact with water you can install the tile directly onto the face of the plywood so long as you use the proper type of thinset.
You can bond tile directly to a plywood subfloor and expect it to last.
Level the surface using a wood filler.
Fill in all large gaps and holes as these will prevent proper adhesion of the tiles.
Backerboard can double the thickness of an installation which can be troublesome in some locations.
How to glue tile to wood.
However the floor must be properly reinforced to minimize flexing.