We need a bit more to go on here. What is the ply going over (if
joists are they spaced at 400 ot 600mm). It may be that you are asking
your building merchant the wrong question. ie I'm not sure if you
really mean 'structural'??? Also, write your merchant off since you
are not comparing like with like... get a quote for chineese ply from
them too.

It may also be that floor grade chipboard is what you need... though I
dont like the stuff it has its place and this may well be it.

Good luck
Calum Sabey
01556 690544