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The second floor of an old 3000sq.ft. barn has shifted and sunk. There is a 15" difference between the highest and lowest point of the floor. I want to poor a new concrete layer, but I'd rather it be as thin as possible. Can I put a base of gravel first to level the ground below the concrete? Are there any special consideration in comparison to a slab poured at ground level?

Thank you for all replies

Edit following comments:

The structure is made of thick stone walls (ca. 2ft) and the second floor is supported from below by stone columns connected with arches and spaced at every 10 ft.

Because of the hilly terrain, the second floor is also at ground level on one side, so vehicles can enter the second floor directly.

The floor of the second floor is 3ft deep, made of stones packed with some type of cement (not concrete) and supports 5 ton tractors going in and out.

The sinking that took place seems to be the result of earth movements. I'm waiting for an expert report on that.

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    What kind of structure is underneath this proposed pour and does it have structural integrity since it has already sunk 15" on one side?
    – RetiredATC
    Aug 14, 2022 at 9:35
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    You need to consult a structural engineer. "Old barn", "shifted and sunk", "second story", and "concrete pour" do not belong in the same idea. If a structure has subsided 15" under its own weight, I can envision almost no circumstance where pouring concrete on the second story would be acceptable and especially not with an underlayerment of gravel. Assuming an average gravel depth of 7.5" (zero at high side, 15" at subsided side) you're talking about adding ninety tons of gravel onto a second story. (Also how are you going to get ninety tons of gravel up there?)
    – user278411
    Aug 14, 2022 at 11:14
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    Probably won't have to elevate all the 90 tons of gravel once enough gets up there to make the barn fall down...
    – Ecnerwal
    Aug 14, 2022 at 14:11
  • please see added comments.
    – gash
    Aug 15, 2022 at 22:25
  • 100% agree that you need to address the subsidence issue before adding tons of gravel and concrete. If left alone, eventually your 5 ton tractors on the 2nd floor will end up in a heap on the 1st floor and that will cost far more than addressing the problem up front. Especially if people happen to be in the barn when it happens.
    – FreeMan
    Aug 16, 2022 at 14:31

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As folks have commented, this is absurdly wrong-headed and more likely to cause complete failure than to correct your barn floor.

There are folks that specialize in jacking barns back into shape. This is a process that seems simple, but has enough complexities that can fall down and kill someone to be considered "best hired out to an experienced firm" as opposed to a great DIY opportunity. You want someone who has done this before, many times, who has the right tools and judgement. Doing it wrong can break beams catastrophically.

In a general sense, they will provide some temporary cribbing or shoring and gradually jack up the sunken parts, then provide new foundations to hold those parts in place. Upside, it's much less likely to destroy your barn (and if it's too far gone, they'll most likely tell you that and refuse the job) and it will probably cost a lot less than your proposal to fill the second floor with concrete & gravel, as sunken (much less the probable cost of breaking your barn but good in the process, and possibly killing someone in the process as well.)

Pouring concrete on upper floors is exceedingly common in properly designed and supported steel frame structures, and is usually done on a base of corrugated steel attached to the framing. But that depends on the building framing being stable, properly supported, and designed for the load - none of which appear to apply to your barn from the limited description in the question.

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Yes. Yes you can put gravel under concrete, even over other concrete. In fact, that's probably the right way to do it as you add some degree of movement tolerance by doing so. Note that "as thin as possible" is probably no less than 6" for your loads.

As to why you probably shouldn't, see Ecnerwal's answer. Your question updates don't ease my concerns about adding many tons of weight to an already questionable structure. 15" of slope in a fairly small barn indicates severe instability.

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