Hi Stebed
My situation is a bit weird - I'm doing a church conversion and I have a whole new floor that is a couple of feet above an existing floor, giving me the floor void. I built short studwork off the old floor, under each joint in the ply, and laid firrings ontop of the stud to make the fall.
If the existing floor is sound, I was just suggesting that firrings under the ply, maybe on top of the existing floor boards, was an easier way to get the fall than using a screed...
Wonderdust - you're quite right about the fibreglass - the actual glass fibres make tiny holes in the resin when you sand it off smooth, so water can leak through. because I was sticking mosaic tiles on top of the fibreglass, rather than gel coat, I used flo coat to seal it all - its pretty much the same stuff, but gel coat has wax in to help make a smooth shiny surface; the flo coat remains everso slightly tacky, but the adhesive sticks better.