The gap under the slab you can't see yet
Of all the cities we work in, Aurora might be the best argument for void fill as its own service. The recipe is right there in the local conditions: open plains clay, almost no tree cover to shade the ground, and drought cycles that bake the soil until it shrinks. When clay shrinks under a slab, it drops away from the concrete and leaves a pocket of air where support used to be. The slab doesn't move at first. It bridges the gap, quietly, sometimes for years, until enough weight or enough water calls its bluff.
Void fill ends that standoff early. Instead of waiting for the slab to drop and paying for a full lift, we pump a cement-sand-topsoil slurry into the empty pocket at low pressure until it's packed solid. The slab never moves. It just goes back to resting on something.
Two Aurora weather patterns, one empty pocket
Drought does the slow version. Dry year after dry year pulls moisture out of the clay, the soil contracts down and away from slab bottoms, and the pocket opens a little wider each season. Storms do the fast version. Aurora catches some of the metro's most violent summer weather, and when a cloudburst drops water faster than hardened clay can absorb it, the runoff moves with force: under slab edges, along old utility trenches, out through any path it can carve. Moving water takes soil with it. One bad storm can flush out more base material than three dry summers.
Most Aurora voids we find are both at once. Shrink opened the pocket, and storms have been enlarging it ever since.
How to check your own slabs this weekend
You don't need us for the first test. Take a screwdriver and tap the handle along your garage floor, porch slab, or patio every couple of feet. Bedded concrete answers with a dull, solid thud. Concrete over a void rings hollow, and once you've heard the difference you can't miss it. Other signals worth trusting:
- A slab corner that flexes or clicks when a car rolls over it
- Soil washing out at a slab edge after storms, or a gap under the edge you can see into
- A recently repaired plumbing leak under or near concrete; the pipe is fixed, but nobody put back the soil the water carried off
- An inspection report on a home sale that flagged hollow-sounding flatwork
Any of those in an Aurora drought year is worth a call while the fix is still small.
Cheap now or expensive later: the actual numbers
Most void fill jobs run $400 to $1,500, priced by the volume we pump, with the crew on site about half a day and the area back in service the same day. Compare that against what the same pocket becomes if it's ignored: a settled driveway panel, a cracked garage slab, or a low basement floor, each a bigger repair with its own bill. Filling the space before the concrete moves is the cheapest work we sell, and the version of this trade we'd buy for our own houses. The material cures harder than the surrounding clay, so a filled pocket doesn't reopen, and the five-year written warranty applies here like everywhere else.
The estimate includes the answer, not just a price
Trevor brings a steel rod to every estimate and taps the slab with you standing next to him, so you hear what he hears. Hollow ring: you get a written number for the fill on the spot. Solid thud: he'll tell you there's no void, and the visit cost you nothing but twenty minutes. If the slab has already started to drop, he'll walk you through whether a fill, a lift, or both makes sense for it. There's no season this work can't happen in; a void found in a January inspection gets filled in January.
Storm season feeds this problem from one side and drought feeds it from the other, so there's no wrong month to check. Call (303) 526-2222 or book the free tap test. Curious how the pockets form in the first place? We wrote up how voids form under concrete on the blog.