I am replacing a front porch, it currently is an elevated porch with your typical dirt and debris underneath, and it is an open porch. I intend to excavate a new basement room underneath it, cutting a hole in the current exterior basement wall to access the new room. The porch will remain open (not enclosed) - so the choice of floor for the porch is pretty important since it will also serve as a waterproof barrier for the new basement room. My chief concern is making sure this porch is waterproof, ideally with a redundant layer of protection.
It seems a fairly typical choice is wooden joists, steel or pan decking, and then concrete. Very similar to this article except the area below will be a room attached to the house, and I'll use wooden joists instead of metal C-channels.
An alternative my architect proposed is a fiberglass deck. There fairly common down the New Jersey shore, here's a couple of examples. We do get a wide temperature range here though - ranging from 10 degrees F to up to 100, with this porch in direct sunlight for half the day. (My architect isn't favoring one approach or the other, he just let me know of the option.) The fact that these decks are used down the shore - where you deal with not just water but salt - seems pretty good. Some random FAQs I'm finding online is that we could expect it to last 25-30 years (similar to concrete it seems?), another says get a new gelcoat every 3-7 years. Cracks could form from settling, and repairing them doesn't seem too difficult but a large crack would compromise the waterproofness until it was repaired. I think the same is mostly true for concrete - except I think the consequence of a crack is worse, and repairing it is not as reliable?
So my questions are more on the waterproofing and concrete side (although anyone with experience with fiberglass is also helpful!) How waterproof could I expect a concrete porch floor to be? What I'm reading seems to say that concrete by itself is not waterproof, but there is some sort of coating that would make it so? How reliable is that? Also - completely separate from waterproofing the concrete or the waterproof fiberglass layer - what could I put on top of the metal decking or beneath it as a separate waterproofing layer to act as a redundancy?