Just idly wondering really, but this occurred to me, so I though it was worth asking.
When I do the bathroom, I'll probably replace the rather old floorboards with 18mm ply (they'll all have to come up anyway to deal with a few dodgy joists so why not end up with a nice flat floor?).
In my experience, the thickness of ply is 'nominal', ie each sheet is a bit different. My instinct would be to cut the sheets to ensure that joins fall on a joist (or niggin) but this will, of course, show any difference in thickness. I haven't really settled on the floor finish yet, but it's likely that it won't be tiles (to avoid the pain of problems with tiles over timber floors), so this would be an issue if it's thin enough.
Two solutions come to mind: Either biscuit joint the ply edges and somehow 'shim' the joist under the thinner one or sand off the difference to 'lose' it over a few inches. Is there a preferred way of dealing with this?
Of course it's inevitable that a bathroom floor will see some water and even exterior ply won't like that too much. Is there any point in sealing the top face (and maybe the edges) with something before putting the finish down?