Bathroom floor for tiles

[JohnBishop]

Hi All,
I have floor boards in the 1st floor bathroom. I have screwed the loose ones properly and hammered the existing nails.
I don't see any movement or hear any squeaking.
I have 12mm STS insulation boards but not sure if this is enough to make this floor.

I was going to paint the floor boards with EVOSTICK primer then glue the STS boards + use STS screws and washers on top.

How viable is this?
Should I do this or add 5mm or 12mm plywood?
In that case should I just screw ply and STS together?

Thanks

[aeromech3]

You don't say what tiles and when you say floor boards do you mean T&G or chipboard?
My story: I have used marine ply and 9mm OSB prior vinyl small bathroom, OSB 3 I prefer and only ever screwed down and opposite floor joints; my last project I had checked out the pipe runs and sketched with the joist lines, when it came to screwing down the toilet bowl I was confident, an hour later not so as a damp patch appeared, the nylon bowl washer had split and allowing the screw to just penetrate the flooring whence it found a CH pipe which was hard up against it, no clearance; I had never moved so fast with tooling, nor since, but I would not have succeeded with minor damage had I used a contact adhesive too.
Unless your boarding is so uneven, I should have expected the 12mm STS to be adequate.
Where the vinyl did not reach in high risk areas such as the toilet bowl outlet, also I try to leave a plastic tray to catch any slop when aligning and checking, I water proofed the original chipboard floor.

[dewaltdisney]

I do not know why anyone would want ceramic tiles (if that was your intent) in a bathroom. They are cold, easy to crack if you drop something, and the grouting always gets stained. You should consider vinyl sheet or vinyl plank type flooring. Hardboard tacked down if necessary as a substrate and lay your floor. Sheet vinyl is easy to lay and you can tack it with spray adhesive around the edges if needed.

DWD