On Aug 7, 9:17 pm, "D'Olier" wrote:
What do I need to tile an upstairs bathroom? I know you shouldn't tile
directly onto the plywood. I've heard you need another sheet of plywood and
then some mortar/backer?? board but then the floor will be about an inch
higher than the hall floor.

There is a scientific way to determine whether the floor is strong
enough, but 1 1/4" plywood and 1/4" backerboard is normal from what
I've been reading. What is there now?

A change in elevation between rooms is pretty normal. We're used to
picking up our feet as we walk through doorways. The bathrooms in my
house are 3/4" higher than the hallways and I didn't even realize that
until I retiled one of them.