I have a 100sf concrete slab for a porch, 6 inches thick, that is also a roof over a basement room (used to be for a cistern). It sets on the foundation walls, 8 inches thick. I am planning to add to an existing deck further away and was thinking of continuing the deck over the slab. The decking would need to be about 4 inches above the slab making it 1.5 inches below the door sill plate. I cannot drill into the slab with tapcon etc. I was thinking of laying down a rubber membrane for flat roofs first, then treated sleeper "joists" either heavily painted or wrapped with that stick on rubber/plastic flashing. It will be attached on the outside of the slab to posts, rim joists. Will this plan work?