Polystyrene Floor Supports

I recently began replacing my bathroom (ground floor) in my flat. The floor was carpeted and where I was careless in jumping out of the bath without a bath mat, the floorboard (chipboard) absorbed the water and so had to be replaced. Wasn’t a big deal – I was replacing the positions of the toilet and sinks and replacing the bath with a shower anyway, so had to rip up the floor to get to the pipes anyway.

What I found was that the entire bathroom floor was supported by sheets of 50mm polystyrene – not something I had encountered before. I’ve since been told the likely justification for this is that polystyrene is employed to prevent problems associated with ground heave. Ok – no problem. Now I didn’t have any problems before (at least when it was carpeted) with the floor so I’m compelled to just lay the polystyrene again (like for like).

My question is, knowing that polystyrene does have a bit of “give” in it. Will it be safe, once the new floor has been put down, to lay floor tiles? I’m just nervous that, while before I never noticed any slight movements in the floor that as soon as I lay tiles the grouting will crack or I’ll get creaky tiles or something else untoward.

Just after reassurance or advice (novice DIYer) from anyone who has seen polystyrene used as floor supports…

Your post is a little confusing on what floor you have.. youve a chipboard floorboarding which i assume is on a timber joist floor?

Assuming that is true the floor is then suspended and these joists will be built into or hung from masonry walls.. if there is any polystyrene near the support locations for the joist it may well be there for clay heave as someone told you.. but the joists will go through it into something solid..

If your boarding is ontop of a concrete slab then polystyrene under that then that is insulation and care should be made not to pierce the DPM below the slab/insulation..

Course there is also a slight chance that the polystyrene is a void former for clay heave under a ground bearing concrete slab but it would be alot more than 50mm thick, unless you just poked through a pocket.. photos maybe...

The entire floor was supported on polystyrene sheets - no timber joists at all. Beneath the polystyrene is the concrete slab.

As I say - this was surprising to me but as far as I could tell has done the job for 20 years...

I'll see if I can get some photos...

Ahha.. so just the boarding is over the insulation.. yer thats a little uncommon but its more a durability issue..

Normally the insulation would be protected by a screed creating a more solid floor.

Yeah - this is what I would have expected but there's no screed.

With that being the case, would you anticipate I would have any problem laying tiles?

hhrr said:

Yeah - this is what I would have expected but there's no screed.

With that being the case, would you anticipate I would have any problem laying tiles?

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Only so much as how your boarding is fixed down.. you dont want the boards to move once they have tiles on them, normally you would screw decent sized boardin into whatever supports it below, so in your case you could add some timber battens.. aterntively maybe consider screeding the floor properly..

The flooring is tongue and groove and appears to be pretty well constrained (though i'm still suspicious). I had considered putting a tile skirt around the edge of the room. It was suggested to perhaps lay the tiles and then fix the tile skirt with silicon instead of grout as it may have more "give" and cope with any (up/down) movement better.

WBP ply is a much better material for a floor (especially in a bathroom) than chipboard

JohnD said:

WBP ply is a much better material for a floor (especially in a bathroom) than chipboard

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Have heard WBP suggested but only since I have laid the chipboard (as I said before, I just replaced like-for-like) and i'm reluctant to pull it up and start over. Is it desirable and just best-practice or is it going to prevent a disaster?

if or when the chipboard gets damp, it will not dry out (being tiled) so will swell up and break or dislodge the tiles, then crumble into mush.