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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?

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    I'm not sure if this is acceptable, but if it is, instead of attempting to relocate the footings after the slab pour hides them, embed anchor bolts in the footings long enough to show above the final slab grade. You can then use these bolts to hold down your post anchors to ensure the posts don't move, and no remeasuring necessary. – FreeMan Jun 23 '20 at 18:20
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    There's some chance that an inspector would object to the fact that you're interrupting the actual footing with something less stable, but it probably wouldn't be a problem, practically speaking. – isherwood Jun 23 '20 at 18:20
  • @FreeMan I thought about that. Just figured if I could avoid it, then the pouring/smoothing work would be easier. – DonBoitnott Jun 23 '20 at 19:28
  • In my experience, the concrete is poured & leveled, then the bolts are inserted and the surface is touched up around them. Check with some experts, YMMV. – FreeMan Jun 24 '20 at 10:36
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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:

  1. Make sure your support piers are below frostline - in my area it's 36".
  2. 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.

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    What does 6” concrete patio do for frost heave that 4” does not do? – Lee Sam Jun 23 '20 at 19:28
  • 6" certainly doesn't guarantee anything - frost heave can damage a 12" concrete pad as well. However, from my experience a thicker pad will resist minor shifts in the subsoil that a thinner pad (4") may not. If you're a structural engineer, I'm always anxious to learn and would like to hear your opinion - but that's my take. – HoneyDo Jun 23 '20 at 22:45
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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.

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  • Although I would say that if the footings are dug deep enough, heave should not be an issue. That's the point of digging beyond frost depth. – DonBoitnott Apr 22 at 14:55
  • Wouldn't the slab and the footings react differently to the ground freezing? The footings would be below the frostline but the slab would not be. – Brian Kalski Apr 23 at 3:35

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