What carries the load, and what does not
A slab on grade sits on compacted subgrade and a stone subbase, and that base is doing the work a foundation wall would do on a different kind of floor. Driveways, patios, walkways and plenty of garage floors are built this way, poured flat on prepared ground with nothing structural underneath except the base itself.
That is the opposite arrangement from a floor over a crawl space or a basement, where a foundation wall carries the floor and the ground underneath the floor is doing nothing at all. On a slab on grade, the ground under the concrete is not incidental. It is the foundation, and it gets prepared like one, cleared, compacted and built up in stone before concrete ever gets ordered.
What goes wrong when the base underneath it is wrong
The base is where most slab on grade problems start, and they show up months or years later rather than on pour day. Soft, wet, or loosely dumped fill compacts unevenly once weight sits on it, and a slab on that kind of ground settles the same way, unevenly. The crack that follows usually shows up wherever the base was weakest, not on the joint line that was cut to control it.
A few signs point back to the base rather than to the concrete itself:
- One panel or corner sitting lower than the panel beside it
- A crack running through the middle of a panel instead of along a joint
- Standing water collecting in the same low spot after every rain
- A section that rocks or sounds hollow underfoot near an edge
Where a slab on grade needs a footing at its edge, and where it does not
Flatwork that only carries foot traffic, a patio, a walkway, most of a driveway, sits on the base alone and needs nothing more underneath it. The moment a slab is going to carry a wall or a building above it, a garage, an addition, a covered porch with posts, the perimeter has to bear below the frost line the way any footing does, either as a separate footer poured first or as a thickened turned down edge built into the slab itself.
That is also where a permit and a footing inspection come into the picture, since work that supports a structure needs both before concrete goes in. Whether your county treats a particular slab that way is a five minute phone call, and our Virginia permit guide has the details.