Or you could tile it.

No height problem, no water problem, no lost keys, no stuff growing underneath, no mould or rot if the drainage isn't right, no weathering (well, that depends on the tile). Insulation? Probably negligible. Put some batts in the ceiling of the room below. Don't think a timber deck is going to help much there anyway.

Probably even be cheaper too. Last time I looked you could buy tiles for under $60 a metre.