I don't think drainage should be a problem after thinking about it. The concrete slap is not level, so after I shim up the low side, any water would be able to run off the slab.
I had another question. The slab is about 36" wide. I am framing the perimeter of the slab with treated 4x4 and then laying the decking over that. To keep my heighth at a minimum, could I just screw into the 2 4x4 on the outside of the slab and then use some sort of exterior adhesive in the middle, so that I won't have to drill my decking into the concrete?
I wasn't sure if having tapcons or something else would look all that great in the decking.