How do I replace particle board on top of polythene layer?

[ChrisC2]

Some advice really appreciated.

I pulled the tiles up in our downstairs toilet to find the particle board beneath was soaking wet and literally fell apart. I've found and fixed the water leak but now I want to replace the floor. Removing all the old particle board reveals a polythene layer which wraps up into the wall, and underneath that is some polystyrene insulation and then I presume the concrete sub floor.

My question is how to replace the particle board and then retile. Do I just lay it on the polythene without fixing it? The original particleboard went a little way under the stud wall. It's going to be impossible to do that putting new boarding down. I could push it in on one side but I will have to leave a gap on the other to get it in. So I can't see how best to be sure it doesn't move? I don't want to puncture the polythene. Could anyone advise how to replace the particle board so that it's stable enough to retile afterwards? Thanks.

 

[Deleted member 174758]

The polythene is probably black and quite thick - which more than likely means it is a Visqueen DPM. You should not puncture it

Trim back the chipboard sticking out from the skirting carefully, then treat it with wood hardener. This will get rid of any moisture and seal it as well. The remaining ing floor can have chipboard just placed on it, but it should be a snug fit. If you do end up with gaps they can be filled with a gap filling foam tape such as Xpanda

 

[ChrisC2]

Thanks. The plastic is blue - not sure whether that’s relevant. So I lay the board as far under the wall as possible. It’s maybe 2cm under the stud wall, that’s all. All the old particle board is gone.

 

[Deleted member 174758]

Yes. If you are concerned about movement/gaps stuff some Fossa Caulk Saver or Xpanda or the like round the edges. TBH if the chipboard is big enough it won't move