What would be the potential pitfalls in pouring a concrete patio over deck footings and then securing the posts to the slab?

Question

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?

HoneyDo · Accepted Answer

I think your approach is right on target. Follow FreeMan's suggestion to embed bolts in the footings to attach your base plate for each post. It's unlikely that you'll have serious frost issues as long as you:

Make sure your support piers are below frostline - in my area it's 36".
Be certain to properly lay the patio base with a solid, self-draining material such as sand or fine granite.

If you're particularly concerned about frost heave you might want to consider going to 6" depth on the patio.

Brian Kalski · Answer

I am building something similar. My plan is to first pour the footings (sonotubes) and embed brackets. The top of the tubes will be at the height that I want the final slab height to be. The next step would be to put some kind of joint expansion material around the tubes to isolate them from the rest of the slab. Once the rest of the slab is poured it will not be rigidly attached to the post footings allowing for different heave rates.

Answered by Brian Kalski on June 30, 2021

What would be the potential pitfalls in pouring a concrete patio over deck footings and then securing the posts to the slab?

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