Hi i normally tile wooden floors by overlaying with 18 mm marine ply and using a flexi adhesive normally N&C as this is normally supplied.
I have now started doing wet rooms using a tuff form former.

I would normally lay the former then lay 18 mm ply around the the tray then lay the water proof membrane (tanking rubber)over the whole floor sealing all joints and then tiling on top using a flex adh and then flex grout.

From reading a lot of posts and getting a bit confused is it best to secure all existing floor boards then use a 6mm cement board then overlay this with water proof membrane.

Or is it best to remove floor boards lay new 18 mm ply then cement boards etc.

I was always told to use 18mm to strenghen the floor and stop it moving.

Last thing how do you test if the floor is flexing too much.

Hope you can follow this and any help would be appreciated.