i need to tile on floor boards cant ply on top will loose levels, floor bords are screwed down with no movement and will be useing rapid flexible floor tile adhesive
If you can't or won't overply?????????? Then Floorboards up Replace with 25mm WPB Ply Tile using flexi cement and flexi grout
floor bords are screwed down with no movement They will move, no matter how you fix down wood it will find a way to move, if you over fix wood, it will split there is no way around this. A good solid good quality oak floor incorrectly fitted can move the brick work in the wall before the centre bows out. Tiling straight onto floorboards is a bad idea, Pine moves more then most other woods, its a softwood so no real strength to stay in one piece around fixings. Subfloor or remove boards and fix alternative like as advised above. Ether way as you have a wood base don't cut corners and use the best adhesive like Bal etc.