My threshold from the house to the concrete patio slab is 3". I want to put a Trex like deck over the slab. The slab is approximatly 16'x16'. I have read that adding a deck with such low cleareance is a bad idea due to mold. I live in Orange county, Ca and it is pretty dry. Little rain and less humidity. I was thinking of running treated 2x4's flat for and inch and a half height then adding the 7/8" Compostite on top with hidden t-fasteners. What do you think? Tearing up the concrete is really not an option.
Could I put some kind of 1/2" or 1/4" blocking under the 2x4 joists if you will to help with air movement?

With the 1.5" 2x4 flat and the 7/8" composite deck I only have about 1/2" left over before I get to the level of the house threshold.