Tiling on plywood

[Spang]

Hi, I just took out all floorboards from my bathroom and screwed 18mm interior ply to the joists instead. The old boards were rotten, but not the joists. I now want to tile, but it's first floor in an 1880s terrace and, while the floor is now stiff and level, it isn't totally rigid. There is 2-3 mm of bounce if you take a good jump. This is partly due to the way the joists are cut short to the old fireplace and braced at 90 degrees onto other joists. I'd rather not add any more depth to the floor before tiling, unless I have to.

Do I need more ply? Cement backboard? Must I unibond the ply first? Is 'flexible' wood tile adhesive going to do the job alone?

(P.S. the old fireplace fell out on Sunday afternoon while laying the new floor. Had 200kg of rubble behind it. Nice.)

 

[masona]

Have you check the floor joists condition ? You can't tell just by looking at it. If the floorboards is rotted the chances are the floor joists could be. I know it's a pain but remove 1 sheet and bang a nail in on the sleeper wall joists on the side ( sleeper wall joist is where the floor joists rest on ) A very common problem regarding rotted timber which is in the middle which you can't see. The nail should be hard to bang in to start with but when the nail goes in faster that's where it's rotted.