22mm OSB3 vs Ply bathroom subfloor

[DIYWarrior]

Just sanity checking if 22mm OSB3 T&G is fine for a subfloor 1st floor bathroom instead of Ply? The joists are 400mm centres and I plan to stiffing up with noggings. I'm replacing the existing 22mm floorboards which are in bits and have been cut in lots of places by previous owners. It's an ensuite so room is small enough (~2.4M x 2.4M) The floor will be tiled and I was planning on the 22mm T&G OSB3 with a decoupling mat on top.

A plumber who I had doing some other work reckons lots of the ply supplied nowadays has cheaper glue used in it and is more prone to delamination. He prefers to overboard the existing floor where possible with a 6mm magnesium board.

 

[Deluks]

You can get structural cement boards that won't swell or delaminate. Use a decoupling mat over this.
Otherwise I would go 18mm ply with 6 mm cement board on top.

 

[endecotp]

I would go 18mm ply with 6 mm cement board on top.

Or 18mm T&G OSB with cement board on top.

 

[Deluks]

Or 18mm T&G OSB with cement board on top.

It's true that there is some cheapie ply out there with rubbish glues. But the t&g flooring ply is no worse than osb, if it's wet enough that it's delaminating then the osb equivalent will be swollen up in the same conditions.
In some ways softwood floorboards are better than sheet materials.

 

[endecotp]

if it's wet enough that it's delaminating then the osb equivalent will be swollen up in the same conditions.

I mentioned it mainly because OSB is likely to be cheaper.

 

[DIYWarrior]

You can get structural cement boards that won't swell or delaminate. Use a decoupling mat over this.
Otherwise I would go 18mm ply with 6 mm cement board on top.

yeah my ideal would have been this NoMorePly 22mm T&G Cement Board but unfortunately they don't supply this in the Rep. of Ireland and the stores I checked up North don't stock it either.

Do you reckon 18mm Ply is sufficient then for the structural subfloor and 22mm not needed? (as my understanding is the extra 6mm cement board doesn't really give any structural support to the subfloor and is more as a waterproofing layer above the ply and something that can be directly tiled on to).

 

[Deluks]

18mm is ok if the floor joists are sensibly spaced and the floor isn't bouncy. Use plenty of screws.

One thing to note, cement board isn't waterproof, unless you have some fancy stuff, it will get damp and it will let moisture through, so don't use it thinking it will save your wood floor if there's a leak. It's more to decouple for movement, and to save the subfloor from getting trashed if the tiles ever get replaced, or need to be removed for access (if a cable or pipe needs to be repaired for example)