Start where the slab ends
Go to the outside edge of the pad and dig a small hole against it with a shovel. You are looking for two things. How thick the concrete is right at the edge, and whether it holds that thickness or drops down into a wider foot of concrete running under the perimeter.
A patio or a parking pad usually reads about four inches all the way across and then stops. Concrete, then dirt. A slab poured to carry a building thickens at the edge into a reinforced beam that reaches down to soil that will hold weight. That turndown edge is the whole difference, and you can find out whether you have one in ten minutes with a shovel.
While you are down there, look at what the edge is sitting on. Stone you can see and feel under the concrete says somebody prepared the ground. Concrete straight onto soft clay tells you something as well.
Flatwork and a foundation are two different pours
Everything above a garage wall funnels down into a narrow line where the wall meets the concrete. That is a concentrated load, and it has to land on something wide enough for the dirt to hold and deep enough that winter cannot lift it. Water in the soil expands when it freezes, and it lifts whatever sits on it, one corner at a time, a little each year.
A parking pad was never asked to do either job. It is a slab on grade, spreading a car over a large area, sitting near the surface where frost still reaches it. Put walls, a roof and a load of wet snow on that and you have asked a pad to do work nobody designed it for. It may hold for a season or five. When it moves, the building moves with it, and doors stop latching.
What the slab has to pass before it is worth keeping
Even a slab that was poured as a foundation has to be in good enough shape to build on. This is the short list we look at when we come out.
- A thickened, reinforced edge under the whole perimeter, not just a fat corner where the truck sat
- That edge bearing on firm undisturbed ground below the local frost line, rather than on fill
- No crack wandering across the middle, and no corner sitting lower than the rest
- Square, and large enough that the walls land inside the concrete instead of hanging over an edge
- Sound concrete where the sill plate has to sit, not surface that is scaling or crumbling under a screwdriver
Four ways this usually goes
Once we know what is under the edge, the answer is normally one of four.
Which one fits comes off the building you want, how the ground sits under it, and what your plan and your building official will accept. On a pad that has already settled and cracked, the last one is the cheap answer rather than the expensive one, because it is the only one that does not build a new garage on top of a known problem.
- The slab was poured as a garage foundation and simply never got built on. Build on it.
- You go post frame instead. The posts stand on their own footers, dug outside the concrete or through it, and the old slab stays as the floor because it is not holding anything up.
- We cut the perimeter back, dig and pour a footer and a wall around the outside of what you have, and keep the middle of the old slab as your floor.
- It comes out. Tear out and haul off runs $2 to $3 per square foot on top of the new work.
The permit stops being optional once something stands on it
The moment concrete is carrying a structure it is no longer flatwork. Work that supports a building needs a permit and a footing inspection before concrete goes in the trench. That is standard code administration rather than a gray area, and it applies to a footer poured around an old slab exactly the way it applies to one dug in open ground.
Whether your county wants anything at all for the pad that is already there is a five minute phone call to your building department, and our Virginia permit guide walks through what they will ask you. Make that call before you order material, not after. Everything on our side, the scope, the mix and the workmanship warranty in writing, gets confirmed on your free written estimate.