I have a 10 x 10 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 that is further away and was thinking of continuing the deck over the slab. The decking would need to be 3.5 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 rubber/plastic flashing ( adhesive on one side) made for windows and doors. It will be attached on the outside of the slab to posts and rim joists. Will this plan work? Anyone have ideas to make this easier?