R1campr

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?

oldognewtrick

If you can provide positive slope so that water drains away from the structure, avoiding ponding water, rubber membrane might be an option. How are you planning on making a water tight transition against the wall? Use the accessories for making lap seams available where you buy the material. You will need cleaner, primmer and seam tape.

Remember,asphalt products are not compatible with rubber membrane. If you do put down sleepers, cut some slip sheets from left over material to layer in between the sleeper and the rubber. Rubber will shrink as it ages, so allow for contraction of the material.

R1campr

There is good slope on the slab, will probably cut the sleeper joists to level the decking. I am planning to use galvanized flashing under the siding, and under and over the sleeper joists (2 pieces), same as a ledger would have. The rubber membrane under the flashing. The rubber would be one piece (10 x 10). Does the flashing (normally used for windows and doors) that has adhesive on one side have asphalt in it? I think the name was protecto wrap. I was thinking of using that to wrap the sleepers with the overlap on the bottom. I think the non sticky side is plastic. The reason I'm doing all this is because I'll never be able to seal the concrete again.
Is there an easier approach to this project?
What happens when asphalt comes in contact with rubber?

joecaption

Nothing, but galvinized flashing will corrode if it's in contact with pressure treated limber or concrete. Use copper, vinyl, EPDM or rubber instead.

BridgeMan

I have to think that wrapping the sleepers in rubber will cause them to rot and deteriorate sooner. Sooner or later, you know they will be taking water, and never have a chance to dry out.