"The porch is 10 ft up with plenty of ventilation underneath. In the
conversion, we removed the original (6 x 1-1/4 in round-nose) decking
laid 3/4 in OSB T&G plywood. Then erected open stud walls, built a roof
(lean-to off the house roof), and screened all around.
Now the floor is delaminating & rotting, and the base plates for the
are rotting as well. This is due to collection of rainwater on the
RicodJour's NexWood suggestion is intriguing, but there don't seem to
NexWood dealers in my neighborhood (Georgia). I have seen similar
locally, though - this is worth a look-see.
Still open to other suggestions, though. Is there any way I can put
concrete subfloor over wooden framing? "
I'd suggest instead of throwing good money after bad, you get some
estimates from professionals for straightening this out. Particularly
since you're now thinking about putting a concrete floor 10' up in the
air, supported by God knows what. This is starting to sound like one
of those DIY decks that collapses every so often with 10 people on it,
sending them into the street. I'll bet you didn't even have a
building permit or inspection when you did the original conversion that
lasted all of 7 years, did you?
Forget concrete unless you dont mind puting in all new and extra
supports of steel. Go back to planking treated, cedar, or one of the new
composits. But many composits flex and will need additional supports.