Floating floor subfloor

Howdy folk.

Part way through my floor construction and have myself a question.

Currently i've levelled my slab. Installed 120mm of PIR and then "attempted" to lay a ply sheet over the top.

Issue? A 2 cm hump in the sheet that makes the sheet feel like a trampoline.

I've introduced moisture to the cupped side, put weights on it (250kg) and even tried to reverse the bend to find a nice neutral. None of that worked so I cut a lot of groove 4mm into the 22m sheet... Actually made it worse. Went back down Wickes, bought another one... It's got a 4 inch bow across the board. Facepalm.

So in a nut shell. I don't like floating ply floors very much! Should I just resort to chipboard as i've got no means to fix it down? Alternatively are some of the cement boards an option? - Just looking to put laminate plank ontop (it's a downstairs WC which is why I went for ply vs chipboard originally.)

Cheers

 

Really interesting idea. I'd already fixed the PIR down sadly so was not an option. Opted for chipboard instead - much less headache and perfectly flat. Bit of a pain though as I was hoping to have somethign better to fix a toilet to.