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I am going to build a pavilion over a concrete slab and I am wondering what would be the best way to set up the footings. I was planning on using steel column bases set in 12 inch sonotubes. There will also be a concrete slab under the pavilion.

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I am exploring three options.

The first option would be to first pour the footings (12 inch sonotube, 4 ft deep) at the final height of the slab. Expansion joint material would be placed around the footings before the slab (4 inch thick) is poured around them so they are independent of each other.

The second option would be to bury the sonotubes (4 ft deep) and pour the concrete for the tubes and the slab (4 inch thick) at the same time with no separation.

The third option would be to pour a thicker slab (5 to 6 inches thick) with no footings and set in the steel column bases when the slab is poured.

The second and third options would require less work but to me it seems like the first option would yield the best result.

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  • Is there rock underground that would support footings? Do you plan to compact the area under the slab? Is lime stabilization used where this will be built? What are the dimensions of the slab and what will be constructed over it?
    – Jim Stewart
    Commented Apr 23, 2021 at 10:12
  • I was planning on putting 6 inches of gravel under each of the footings before they are poured. Rebar in each footing. I was planning on putting 2 inches of compacted stone under the slab before pouring. The slab would also have rebar throughout. A pavilion will be going over the slab. Basically a square 6 x 6 structure with a roof.
    – Brian Kalski
    Commented Apr 23, 2021 at 14:18
  • I would probably have a 12” thick 24” square footing and pour the slab over the footing 4’ sounds deep to me for such a small structure maintaining the reenforcing metal as you have planned. That size would require a permit in my area, it’s just over the requirement with a solid roof.
    – Ed Beal
    Commented Apr 23, 2021 at 15:06
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    It seem to be that you would also have rebar going out of the footing at right angle into the slab. The tie-in to the slab would assist in keeping the footing level and help prevent any independent sinking of any footing. Yes- I would tie the footing to the slab with the use of Rebar - This was not an option you listed. Pouring the concrete footing with slab without rebar tie-in would not be a strong connection.
    – Programmer66
    Commented Apr 23, 2021 at 18:38
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    There are no walls. All the weight will be put on the footings through the posts. The ground is mostly level. I will remove some soil to have enough room for the gravel under the slab. I figured I would go with 4 ft footings so that I wouldn't have to worry about frost heave. My main question is whether or not having the footings connected to the slab is a good idea.
    – Brian Kalski
    Commented Apr 24, 2021 at 0:43

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I am not going to comment on the sonotube piers, since I don't know where the project is located, and the loadings the structure is subjected to. But I do want you to make sure the bottom is set below the local frost depth, and the soil bearing is adequate with applicable safety factor.

The next important thing is the subgrade preparation. The subgrade shall have a layer of well-graded, compacted, coarse gravelly material. In humid and wet locations, a layer of sand can be added to form the bedding for the vapor barrier, or retarder.

Now to the slab. I suggest the second approach with a 6" thick slab. The first approach is undesirable because it is prone to having an uneven settlement problem, as the piers and the slab will settle at a different rate. The third approach is simple and cheap but the most problematic. Your structure will experience horizontal force from the wind, which can cause lateral movement of the unsupported slab, and tilting due to the overturning effect. The remedy will be costly.

For strength and durability, I suggest the piers and the slab shall be reinforced. Also, unless you want to use post-installed anchors to fasten the columns, don't forget to set the anchor bolts securely before pouring the concrete.

Your project involves quite a few engineering uncertainties - loadings (dead, live, seismic, and environmental loads), soil strength and capacity (that affects pier sizing and length), and the building structure; also the permitting requirement. Therefore, it is strongly recommended to consult with a structural engineer, so you won't have any regret later. Good luck.

Another approach you may consider is to have a slab-on-grade with turndowns at the perimeter. But it may not be cheaper than the pier approach though.

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  • I live in southern Michigan. It gets pretty cold in the winter. That is why I was planning on putting the sonotubes down 4 ft.
    – Brian Kalski
    Commented Apr 27, 2021 at 17:39
  • @Brian Kalski The frost depth looks right. Are you close to the lake? Are you going to do the building design yourself?
    – r13
    Commented Apr 27, 2021 at 19:58
  • I am not close to a lake. 5 acres of flat land. I have a set of pre-designed plans for the pavilion. It's basically just a roof supported by four 6 x 6s, there are no walls. I was trying to overbuild the footings to prevent trouble in the future.
    – Brian Kalski
    Commented Apr 27, 2021 at 20:31
  • Is the plan issued by the City? Are you sure you don't need an engineer to stamp the permit application? Your roof will subject to significant uplift from the wind, do you have enough weight to counter it?
    – r13
    Commented Apr 27, 2021 at 20:59

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