Tiling over celotex without laying down a screed. - Tile Advice

[space refurbs]

What would you all recommend for a wetroom install over celotex insulation, other than a screed which is not possible due to height limitations?

The Celotex is level and secure as the slab it sits on has been levelled and I was going to float WBP ply over it, then some 6mm Wedi or Marmox boards on top of that, but the WBP is slightly warped and rocks about slightly. Celotex do not recommend an adhesive to bond the ply to their insulation boards so how am I going to get a secure surface to tile onto?

T&G chipboard had been recommended by Celotex but I'd rather avoid chipboard so any other suggestions?

 

[bugs183]

Sorry Space but it wouldn't be a good idea to tile over any floating floor, especially as this is going to be a wet room, you must have no movement whatso ever or you will have leaks. A floating ply floor too will move and leak.
How thick is the cellotex??? Maybe you could remove it and build up the entire floor with Marmox, i've not seen this done but i've no reason to see why it couldn't be done.

 

[nybor62]

im with bugs floating floor on celotex , not a good idea surly will fail, as for building up with marmox maybe , any pics of floor please

 

[space refurbs]

How thick is the cellotex???

In the main area it's 80mm thick, in the shower area it's 180mm (120mm plus 60mm sandwiched) - I had to have a step up into the shower due to waste pipe heights.

I'm now thinking it might be best to lay some timber joists over the concrete slab with celotex between them, then screw 22mm ply to the joists and go from there. Would this render the insulation useless though as the area beneath is unheated (it's the communal bin store!)

Funnily enough Celotex do say you can float ply over their insulation boards and tile onto it, but they seem to assume that ply comes perfectly flat and don't have any recommendations as to how to get it all nice and rigid.

They also flog a ply-backed insulation board but don't divulge how they glue it on!

 

[bugs183]

The good thing is if you go down the Marmox route they make a pre formed shower tray, so this will make your life easier. I can't say i like the look of the waste trap that they use. I think the Schluter Kerdi Drain is much better designed waste as it's been well thought out for secure fixing and height adjustment, yet i don't feel the Schluter Kerdi Board feels as solid as the Marmox board.
Next wet room i do i will use Kerdi Drain with Marmox Boarding.

 

[space refurbs]

The good thing is if you go down the Marmox route they make a pre formed shower tray, so this will make your life easier.

I tend not to bother with preformed trays as I prefer to use linear drains which only require a one-way fall that I just build into the subfloor. Much easier and it looks better than having a lot of diagonal cuts in your tiles in my opinion.

Worth a thought though - anybody know what the insulation properties of 80mm of Marmox are compared to 80mm of Celotex?

 

[bugs183]

Using joists would work fine, you can build you fall in with furring strips and as you say have just one fall, which will give neater results.
The Schluter Kerdi Line drain would be ideal for that application.
I can't see any problem with insulating between the joists.
As for cellotex's spec of tiling over floated floors, there's quite a few manufacturers spec this system and is is very very dodgy, i don't care what they say.

 

[jonnyc]

could you not do your timber joists over slab . fill in with celotex . ply over and then fix wedi/marmox dukka or similar .if you have linear drain then set joits to fall or bed insualtion baord to fall ove level plywood.
still need to tank

 

[space refurbs]

...anybody know what the insulation properties of 80mm of Marmox are compared to 80mm of Celotex?

Look like they're both around 2.4 so could be an option but all that Marmox won't come cheap.

I think I'll just build a timber frame over the subfloor with Celotex between the joists, then screw down the ply.

Cheers all.

 

[jonnyc]

i think you are missing the point unless i am.
you can get 50mm marmox for not too much.
last time i bought that thickness was from insulation express and they were a pretty good price i thought.
i think you said that you will not be offering decent insulation by simply setting between joists but you are now prepared to do this.

 

[space refurbs]

i think you said that you will not be offering decent insulation by simply setting between joists but you are now prepared to do this.

I didn't know whether setting between the joists would significantly compromise the insulation, hence why I asked. Bugs said he saw no problem and nobody else said it was a no-no so I concluded that it was the most economical way forward, given that I have already bought and cut down the Celotex.

Are you saying the insulation won't meet BC requirements if placed between the joists?

 

[bugs183]

I think Johnny means that plying the floor then putting Marmox on top is a much more stable and better surface to tile than tanking the plywood and then tiling.