It's possible your slab is still 'moving', or will move over time. Consider intalling a Ledger Board to the house. (This process may be best left to a Pro who's looking for a little side-work ...)
From there, use Joist Hangers and angle-cut your Joists properly so they run DOWNHILL to keep the water off your deck. Maybe the Pro will give you some 'free advice' on site. Use Pressure-Treated wood for the Ledger AND Joists.
Perhaps, before you do any work, seal up whatever cracks and crevices you can on the slab, including where it meets the house. Again, talk to the Pro on what to use. Of course, have this done BEFORE he shows up for work (so be sure to ask him when he's bidding the job.)
Beyond that, you didn't say what kind of porch-flooring you're using. Oh, go 16" OC for your joists. Another 'oh' - be sure to grade the landscape around the slab so it slopes AWAY from the slab and house! This way, if you decide to use the slab at all for 'structural support', then over time, its chances of 'moving' down on you are slim to none. There's nothing like seeing your fine work 'move' with the slab (which was the ORIGINAL problem!)