Wick, thanks for the info! I am in Indiana and I suspect it is at least or even deeper than 30". I will have to check that. I do have landscaping next to the patio currently, so it should be easy for me to verify the depth of the concrete. Also, after posting my first message, I did fail to mention that the patio is cracked right down the center. The gap is between 1/8" to 1/4" and the slab is still level. I don't know if they will destroy the integrity of the slab as a foundation to anchor posts, or if reinforcing with more concrete will cure any problems that might cause. I do have a lot of depth to play with as the doorway is a little over 1' above the slab. I do want the deck flush with the door threshold. Thanks!!!