Rotten floor in new build, how to replace

Hi all

We have discovered that the ply under the tiles in our utility is rotten, it looks like a leak from a drain. Below the tiles appears to be two layers of ply, below which is insulation and then concrete. The house is a relative new build and the room is 4m2.

My plan is to remove all the remaining tiles in the room and cut the ply to 'square' it off, replace it and then tile on top. My questions are:
If the ply is being put onto insulation foam how can I attach it?
Do I need to leave an expansion gap between the the new ply and the old ply at the boundary? or should it be attached in some way?

Any help would be appreciated.

SA

 

Ghost yes if is definitely a floater.

barnjules, thats an idea. I had a dig around and the bottom ply is tongue and groove so I think at the moment I will work back and see if I can find a tongue and groove join in the good wood that I can reuse and then screw the second layer of ply onto that.

My fall back position if I can't find a reusable T&G join is to straight cut the top layer back further to expose the bottom layer of T&G play. Then straight cut the bottom layer, glue some new ply against it. The use the upper layer of play as a baton and screw it down (a lot) either side of the join the in bottom ply. Does this make sense?

Ideally I'll be able to resurrect the old T&G join, but if not then I am thinking the other option may be good enough.

Any thoughts would be appreciated

SA