The gap under the slab that hasn't cost you anything yet
Void fill is the repair you make while the concrete still looks fine. Somewhere under the slab, the soil has pulled away, and the concrete is spanning air. Nothing has dropped, nothing has cracked, and the whole problem announces itself only as a hollow sound underfoot or a drummy note when you tap the surface. Fill it now and it's a modest job. Wait, and you're paying for a lift, or a new slab, later.
In Castle Rock we find more of these hidden gaps than the age of the concrete would suggest, and the reason is under the concrete, not in it.
Young town, restless ground
Most of Castle Rock was built in waves from the 1990s through the 2020s, which means enormous amounts of moved earth: overlot grading, foundation overdigs, utility trenches down every street and into every house. All that placed soil keeps consolidating for years after the builder leaves, and when it consolidates under a slab that can't follow it down, the space between becomes a void. Layer the Dawson Formation on top of that story, or rather underneath it. Its expansive clay bands swell in wet years and shrink in dry ones, and each shrink cycle can leave a slab bedded on less soil than the season before.
Where the water goes in a Castle Rock spring
The town's late spring snows are heavy, wet, and quick to melt, and at 6,220 feet the melt often lands on ground still frozen a few inches down, so it runs sideways instead of soaking in. Running water is what carves voids: along foundation edges, under garage aprons, beside porch pads, anywhere a downspout concentrates flow at a slab edge. The big day-night temperature swings here keep that melt-freeze-melt engine running from October well into May. The common void sites we check:
- Under the garage apron, where door-line melt slips through the joint
- Along the foundation side of walks and patios, following the old excavation line
- Beside porch and stoop pads, where downspouts discharge
- Over utility trenches, where trench backfill settles on its own schedule
The tap test, and what we do when it sounds hollow
You can screen for this yourself with a screwdriver handle: solid support answers with a dead thud, a void answers with a hollow ring. We do the same check with a steel rod during every estimate, at no charge, and map where the support ends. Then we drill 1.5 inch holes, pump our Portland cement, sand, and topsoil slurry until the cavity is packed solid, and patch the holes. Filling is not lifting; the slab stays exactly where it is, just bedded on cured material harder than the surrounding clay instead of on air. If the slab has already dropped and you want it raised, that's a pressure-pumping decision we'll walk through with you on site.
What prevention costs here
We quote void work by volume, and most Castle Rock jobs run $400 to $1,500. We're on site about half a day and the slab is back in service the same day. Compare that with what the void becomes if it grows: a settled, cracked slab needing a full lift, or a replacement pour measured in five figures for a driveway. For a house still inside its first couple of decades, which describes most of this town, a fill today protects concrete that should have another forty years in it. The work carries our 5-year written warranty like everything else we pump.
Home inspections are the other way these jobs find us. Buyers' inspectors in the newer subdivisions flag hollow-sounding slabs regularly, and a documented fill before closing is cheaper and cleaner than a renegotiated price. Sellers call us for exactly that reason.
Book the free tap test
If a spot on your slab sounds wrong, or you've watched a downspout trench a channel along the concrete edge, have Howard out. The estimate is free, in person, and honest about which slabs need filling and which just need a longer downspout. Curious how these cavities form in the first place? Our explainer on how voids form under concrete goes deeper, and the void fill service page covers the method. Call (303) 526-2222.