I am going to have a 4" concrete patio poured in my backyard, and over that I want to build a 2nd story deck. All of this essentially occupies the same space, meaning the posts either need to be on or through the patio slab.
In the interest of making things look at neat as possible, I wondered about setting sonotube footings to a height that would equal the expected level of the concrete base, so about 4-6" of anticipated gravel base depth. That would have the footings and the base at the same level. Then, simply pour a single monolithic slab of concrete over the whole thing.
That would just leave needing to re-locate the footing locations using the same triangulation used to site them in the first place.
I am assuming that frost heave of the footings themselves would not be an issue as they will be done deep enough. Could the rest of it heave around them and cause a problem? Could settling of the concrete slab/base material when the footings do not be an issue?
Anything else you can think of?