The gap under the slab you can't see yet
Some of the most valuable work we do in Denver leaves the concrete exactly where we found it. Void fill is for the slab still sitting at grade while the soil below has quietly left: shrunk away in a drought year, washed out by a downspout, or settled into an old trench. The concrete spans the empty pocket like a bridge deck, and slabs weren't designed to be bridges. One loaded truck, one saturated spring, and the span becomes a settled slab, priced accordingly.
Denver's dry years dig more voids than its wet ones
Most people assume running water causes every void. Under Denver's claystone, drought does at least as much. The Denver Formation clay beneath the city gives up volume as it dries, and a stretch of dry summers can drop the soil away from a slab's underside across its whole footprint. The space doesn't close when moisture returns; the swelling clay squeezes sideways instead of upward. Then the storm calendar goes to work on what drought started. Hail-season cloudbursts from June through August sheet water across ground too dry to drink it, and the wet snows of March and April melt into every slab edge at once. Moving water strips fines into the pocket and grows it season by season.
The city's older housing adds a third source. Under the early-1900s blocks, original sewer laterals and aging water lines fail slowly, and a slow leak beneath a slab flushes soil out for years before anyone notices. In Denver's oldest neighborhoods, plumbing history is the first thing we ask about.
Front porches and stoops on those same bungalows are frequent void carriers too. The stoop was poured against the foundation, the backfill under it was never compacted to modern standards, and a hundred years of settling leaves the steps sitting on a shell. If your front step booms when you stomp on it, that's not character. That's a void.
How you find out: knock on the floor
A slab with soil under it answers a rap with a dull thud. A slab spanning a void rings hollow, like a drum. Walk your garage floor, basement slab, or front porch tapping with a screwdriver handle and listen for the tone to change. It's crude, and it works. Trevor runs the same check with a steel rod on every estimate, and it's how more than one Denver homeowner braced for a big leveling bill learned they needed a few hundred dollars of prevention instead.
What filling costs, and what it saves
We quote void fill by volume, because material is most of the price.
- Most residential fills run $400 to $1,500, half a day on site, back in service the same day.
- Left alone, the same pocket matures into a settled slab, and lifting one of those runs anywhere from $500 to $2,500 depending on which slab dropped and how far.
- Bigger structural fills, under footings and commercial floors, take more volume, and our 10-yard truck handles them without pausing to reload.
The material is the same Portland cement, sand, and topsoil slurry we lift slabs with, pumped at lower pressure so the concrete stays put while the pocket packs solid. Once cured it's harder than the clay around it, and it doesn't wash out the way loose fill does. Fill work is quick enough that we often handle it the same visit we lift something else on the property, which trims the cost of both.
When to make the call
Call after a plumbing repair under or beside concrete, after sewer or utility excavation next to a slab, when an inspection flags voiding during a sale, or any time the tap sounds hollow. Season doesn't gate the work: the slurry cures in cold weather, so we fill voids in February as readily as July. What we won't do is guess from the phone. The free estimate happens at your slab, rod in hand, and you'll stand there while it finds the hollow spot, before anyone talks numbers. If you want the deeper method, read how contractors find voids under concrete, then call (303) 526-2222 or set up the visit.