I am in the process of renevating my master bathroom. There is 3/4" tongued and groove subfloor with 1/2" ply ontop joist are 16" centers.
Because the 1/2" ply is so dirty do I need to add glue before laying down 1/2" cement board? Do I also need to add a water proof membrain between the tiles and the cement board?
Thanks

You ALWAYS MUST install cement boards into fresh thin set mortar as the directions tell you. Has nothing to do with dirty or clean, but you should get it fairly clean. You also do not have to use 1/2" on floors unless you need the extra height. Tell us in more detail what your plans are & what materials you're using.

Waterproof membranes are not required on bathroom floor. There's no good way to make the room waterproof anyway, unless you build it like a shower with a curb etc.

Jaz

Thanks, I will take up the cement board and apply a coat of thin set then put the cement board back down.
The 1/2" ply was already on top of the original subfloor; after gutting the walls and installing a new Max tub/shower surround the floor got pretty messy.
Do I need to provide expansion gap for the cement board?
I was told I don't need cement board at all as the floor is strong enough but I was also told I do as the ply wood moves during expansion / contraction which could cause tiles to become loose over time.
Better safe than sorry.
Thanks again

A 1/4" or 1/2" cement board type underlayment or Schluter Ditra can be placed over the ply underlayment. The cement board needs to be set in thinset with a 1/4"x1/4" trowel and fastened per the manufacturers instructions.

I didn't say anything about the already installed 1/2" ply, I was referring to the 1/2" backer Alan was thinking of using instead of 1/4" backer.

The backer needs 1/4" space around the perimeter and all solid objects. Sheets are also spaced (usually about 1/8") per directions and of course taped etc.

The person who told you the floor is strong enough and therefore you do not need the backer, isn't a tile setter is he? #1 Backers add no structural strength to the floor and #2 it's not a good practice to install tile direct to ply, esp. in areas that will likely see some dampness.

Jaz

Sorry, my error.