www.ultimatehandyman.co.uk

For a bathroom floor, if I have added noggins to the floor joists, and then laid fresh P5 18mm chipboard flooring, do I really need to add 6mm ply before tiling (as recommended to me by a bathroom shop)?

Been researching this myself. Glass of water, jump up and down, if it ripples fix ply, if not then a good flexible adhesive and grout will do.Maybe. I wouldn't risk it. Even flex adhesive doesn't like bonding to that stuff it seems.
http://www.ultratileadhesives.co.uk/dow ... loadID=245
Water resistant isn't waterproof btw, but this is mainly about flex. 18mm is not best and we don't know how good your joists and nogs spacing are ...
Rip it up, use marine ply at 300 or less centres. Thats what I am going to do, we can compare notes againin 5 yrs!

Dont rip anything up. gently putting the weight down will tell you if theres any flex or bounce
6mm ply could rear up on you when wet with adhesive on one side. you could glue it down and screw at 100mm centres at the edge and 150mm centres in the field. use -18mm screws.
or you could overlay with 12mm ply the same way.

i've laid tile over a 3/4" subfloor and it worked out OK but there's always a risk of flexing

Don't use ply at all especially 6mm, if there's no deflection, glue and screw 6mm cement board (hardiebacker, no more ply are just a couple of makes) That gives a far better base for tiling on to.

If there is defection then you need to sort that first.

@darrenba
When using cement board, what glue to use?
Thanks

You need to check with the manufacturers instructions. Hardiebacker uses tile adhesive, No more Ply uses their own glue