I am about to cork tile my bathroom but the floor board surface is in
poor condition (curled up edges due to CH drying and also past plumbing
work). Following getting them screwed down properly etc I need to board
out to provide a smooth flat surface to tile on to. The dimensions are
approx 2x2m.

In the past I have used hardboard (on top of chipboard) but I think
plywood might be better in this application.

I have a few questions:

1) What's an appropriate thickness for the board, I want it to be thick
enough to disguise the board imperfections (under constant footfall) but
not so thick to loose too much of the skirting (which I'd rather not
refit) and introduce significant level differences with the landing.

2) I have heard talk of wetting down the boards to get them to shrink a
little following fitting ... does this work / is this necessary, as
plywood has long grain running both ways which might the effect minimal?

3) Are expansion gaps necessary around the edge (again the plywood
should be reasonably dimensionally stable)? If so what is a suitable size?

Any other tips/info would be greatly appreciated.

TIA,

Alex.