Bathroom underfloor: Plywood? OSB? ...?

[andre_xs]

We would like to re-do our bathroom floor with Vinyl. It's a mid 1950s house, bathroom in the first floor. The original floorboards are old, a bit uneven, some smaller gaps, so we'd like to put in a thin underfloor first, and only then the Vinyl on top.

Theoretically, we plan of course that the vinyl floor will be 'waterproof' (sealing around toilet, wash basin pedestal, skirting boards,...), but there's always a chance that some humidity goes into the floor as well.

What would be good options for an underfloor layer? Should ideally be rather thin, 5mm or so.

 

[RandomGrinch]

I think you may struggle with anything 5mm thick. We had a thin layer of marine ply laid on a relatively flat floor, but it was too thin and squeaks when walked over!
Is it too much of an upheaval to repair, or renew the original floor?

 

[mikeey84]

I'm in a similar situation, and plan to rip out the old floor entirely and replace.

 

[crazydaze]

If you are fitting sheet vinyl, then this only requires 4mm pre-wet hardboard to cover most eventualities with the subfloor.

If you are looking at LVT or a stick down product, you will need to lay on 6mm Ply, leaving gaps around all edges so the ply won't creak, then microcoat/featheredge the gaps in the joints and the counter sunk screwheads to leave a flat surface to glue the LVT onto.