I'm in the planning stages of building a 36" Pompeii oven in Minneapolis MN. The ideal location is on a concrete driveway next to our garage in the backyard and so I'm trying to figure out if the existing slab is a sufficient foundation. I drilled through and measured it, it's 4" thick. I know the plans call for 5.5" but this appears to be a very well-constructed slab, no cracks, and I've observed no frost heave issues through the two winters that we've owned the house. I've read that a 4" concrete slab can hold roughly 3,000 pounds per square inch, which seems far more than sufficient for the weight of the oven. I've talked the project through with our City building inspection staff and there are no code requirements (no footings required or anything like that) and they seemed to think the driveway would be sufficient.

Will this existing driveway be a sufficient foundation? If not, what could go wrong and what kind of risk am I running?

Any advice is much appreciated.