Take a screwdriver and tap the handle along your driveway apron or the slab by the front step. Solid concrete answers with a dull thud. Concrete with nothing under it rings hollow, and that hollow note is the sound of the cheapest repair you'll ever decline getting more expensive. Void fill is the one service we sell that's pure prevention: the slab hasn't moved yet, and the whole point is that it never does.
What a hollow slab means in this climate
Concrete is strong in compression and weak in bending. A slab bedded on soil is supported everywhere; a slab bridging a void is a beam that was never designed to be one. It holds until a tire, a piano dolly, or a season of freeze-thaw finds the unsupported span, and then it cracks and drops. Filling the gap while the slab still sits at grade costs a fraction of lifting it after it falls, and it skips the cracking entirely.
The fill itself is the same material we lift with: a slurry of Portland cement, sand, and topsoil pumped through 1.5" holes until the cavity takes no more. It flows into spaces a shovel could never reach, then cures harder than the clay around it. The slab goes from bridging a gap to bearing on solid ground, usually inside half a day.
How Broomfield hollows out its slabs
The clay along the turnpike corridor swells when wet and shrinks when dry, and Broomfield's summers are windy and wind-dried enough to pull moisture from under slab edges faster than almost anywhere in the metro. The clay contracts, and a gap opens between soil and slab with no visible change up top. Add the wet-season half of the cycle, spring melt and roof runoff carving fines out from under aprons and downspout corners, and you get the two void-makers we find on nearly every estimate. The third is plumbing: a slow leak under or beside a slab quietly floats soil away for years, something we run into most in the older 1960s-era parts of town, where the pipes have had the longest to age.
Where we find them in Broomfield homes
- Under driveway aprons, where street water and gutter flow undercut the edge
- At downspout corners of patios and walkways
- Under garage aprons and the first slab inside the door
- Along utility trench lines under 2000s Anthem-era flatwork, where deep backfill keeps consolidating
- Beside old supply and sewer lines in 1960s Broomfield Heights lots
The cost logic: fill now or lift later
Void fill is quoted by volume, and most jobs land between $400 and $1,500. Waiting converts the job into a leveling project, or worse a replacement, at several times the price, plus whatever the cracking took with it. Home inspectors around Broomfield flag hollow-sounding slabs on pre-sale reports too, so sellers often call us to close out an inspection objection for a few hundred dollars instead of renegotiating thousands off the price. One distinction matters when you call: filling stabilizes, it doesn't raise. If a slab has already dropped, that's leveling, and if you need both we can pressure-pump until the slab moves. We'll tell you plainly which service your situation calls for, since void fill and leveling get mixed up constantly and only one of them is what you need.
The tap test rides along free
Every estimate we run in Broomfield includes a tap survey of the slabs you're worried about, and usually a few you weren't. It takes minutes with a steel rod and it has saved plenty of homeowners from booking a full lift when a modest fill was the real need. Curious how the detection works? We explain it in our post on finding voids under concrete.
Book the look before the drop
If something under your concrete sounds hollow, or a corner of a slab feels springy underfoot, get it checked this season rather than after the next freeze. Trevor comes out in person, maps what's under there, and quotes the fill on the spot, free. Call (303) 526-2222. Prevention is the one repair that gets cheaper the earlier you buy it.