DPM needed for behind marine ply for shower surround?

[Kester Ratcliff]

Hi, I've been getting conflicting advice about whether I should put a DPM behind the marine ply I've got to support the tiles for my shower surround.

The reason I'm using marine ply is because I'm making my own tiles- an old artist friend of mine has done this too, made all the floor tiles for the downstairs of her house, and she's helping me, so I'll be fine, although I expect it to take me probably a year of weekends..

Anyway, do I need to put a DPM behind the marine ply for the shower surround?

And if so, does it need to be one of the special, super-expensive DPMs designed for wetrooms, or would just thick polythene sheet like is used for underneath pouring concrete floors be okay?

I've got waterproofing PVA, would that be okay for waterproofing the edges of the marine ply?

I can get PU varnish/sealant to seal the edges of the marine ply if waterproofing PVA is not okay, but I'm also worried about whether the tile adhesive will stick to PU okay?

I'm only using marine ply on the shower surround, because that's the wettest area. The rest is WBP exterior-grade plywood.

Also, would bathroom vinyl paint on the ply be a problem for the tile adhesive later?

 

[Kester Ratcliff]

p.s. I can actually simplify the question-

Could it possibly cause any problem if I put polythene sheet behind the marine ply before putting the tiles up?

The polythene is cheap and easy to do, so if it's not going to cause any problem I'd rather err on the side of overkill on the waterproofing.

I think it's probably fine, but I wonder whether if the moisture can't get through the plywood into the wall, perhaps it might condense on the polythene at the back of the plywood and cause the plywood to rot? But it's marine ply so surely the point of marine grade is that it'd be okay in that situation?

Thanks very much!

 

[bugs183]

Hiya Kes.
If you have moisture passing into and through your plywood the chances are it will delaminate and fail before it gets to the polythene.
Using plywood for tiling showers is now becoming outdated as there are so many products out there that will do a much better job.
Instead of ply use Marmox boarding. All you'll need to do is fix this to the stud work and tape over the joints and the joint onto the shower tray with waterproof tape. No need to prime either! PVA isn't water proof, or indeed much good for anything so in priming cases use SBR.
As for the polythene, it's part of building code in the U.S but not in the UK, chances are you shouldn't be experiencing any vapour issues if you use a quality cement board.

 

[Kester Ratcliff]

Thanks Bugs, I'll look into it.