Plywood - is 12+9mm as good as 18mm?

[ChrisKingston]

I need to replace large chunks of plywood that are about 20-22mm thick, which is between the standard plywood sizes of 18mm and 25mm. 18mm should be strong enough (it'll have a wooden floor over top as well), but it's wrong height-wise. Few questions about plywoods:

Would a sheet of 12mm with another 9mm sheet over the top be ok structurally? If so, can I just screw through both together, or do I need to glue them too?

Also, does the answer change if I'm talking about WBP plywood for a shower, or do I have to buy another whole sheet of 18mm?

Is WBP also ok to support a floor, or is it not structural?

Thanks,
Chris

 

[Monkeh]

Screw and glue it, overlapping any joins, and it should be fine.

You can get 22mm ply if you look around, though.