Tiling over floorboarding

Providing the floor is set and level you shouldn't have any issues with tiling over although a lot of people would advise removing the tongue and groove and laying ply before tiling.

Overboard with 6mm cement board glue and screwed which is better than ply as tiles are prone to crack if you use ply,do it once do it right.

Hi tomjones_59, If the structural base is done and you say it is sound, double check on mechanical fixing and screw it well to timber joists, apply Ditra Matt to the entire surface, use only flexible powder adhesive on fixing the matt and the tiles on top, none of that already mixed stuff ... on a timber joist structure you are always prone to cracks due to vibration, but following the steps you have done all you can do...:) Best, Dan P.

I am a professional tiler and although the base may seem rigid there is a still a chance of flex in the floor I personally wouldn't tile over it directly or even if was overplayed with plywood. I have walked away from jobs where this has been the prep work as there is no way with the lack quality of orders I would dare to offer any form of guarantee. I lay 6 or 12mm cement board firstly bedded on a base of flexible tile adhesive spread with a 6mm notched trowel then I screw it down about every 150mm with corrosion resistant screws. Make sure you leave a gap between each board edge of about 5mm then tape with reinforced mesh tape and then given a very thin skim of flexible adhesive ensuring you don't leave a ridge. Lay the boards brick fashion so that no 4 corners meet in the same place. As for Ditra matting this will offer no help with deflection (bounce) in the timber floor, it is designed to be a decoupling layer to stop any cracks which form in a concrete floor transferring through to the tiles.

You don't mention the area to be tiled, but if as you say the existing floor is sound you could use BAL Single Part Fastflex adhesive. This is a one part elastomeric tile adhesive suitable for most tile types that are subject to movement and vibration. It allows tiling directly onto sound timber floors without the use of decoupling matting. In addition it helps to towards sound reduction.