Knock on your driveway near the garage. A solid thud means the slab is bedded on soil. A hollow drum means there's an air pocket under there, and in Wheat Ridge, on concrete this old, hollow is common. A void isn't an emergency the day you find it. It's the stage before the emergency, the gap a slab hangs over until enough weight or one more wet winter drops it. Filling that gap is the cheapest work we do, and it's the repair that spares you the expensive one.
Where the ground goes in a seventy-year-old neighborhood
Three exits, mostly. First, the backfill placed around these foundations in the 40s, 50s, and 60s is still compressing, and as it compacts downward it leaves space up top, right under the slabs poured over it. Second, water moves soil: short original downspouts and roof water concentrated at slab edges have been carrying fines away, a coffee can's worth per storm, for decades. Third, plumbing from the galvanized era develops slow leaks that wash out bedding around the lines, which is why voids so often trace old trench paths. None of these announce themselves. The slab bridges the gap silently until it can't.
Shade keeps the secret
On open, sunny lots, a developing void often telegraphs itself: soil pulls back from the slab edge in a dry August and you can see daylight under the corner. Wheat Ridge lots don't offer that courtesy. The big trees keep the ground damp and covered, the slow spring dry-out keeps the clay swollen against the slab edges, and the void grows out of sight in the middle where the old fill is compressing. That's why the tap test matters more here than in the newer suburbs. Sound finds what sight can't.
The thirty-second test we run free
During any estimate, we drag a steel rod across the slab and tap our way along it, listening for the tone change from solid to hollow. It costs you nothing and it regularly saves people from buying repairs they don't need. Some homeowners call convinced the whole driveway has to be lifted; the tap test shows one bedded slab with a single hungry void under the apron, and the ticket drops accordingly. The reverse happens too, and finding a void early is the good version of that news. Curious how we map them? Here's how contractors find voids under concrete.
What filling costs next to what sinking costs
Void fill is quoted by volume, and most Wheat Ridge jobs land between $400 and $1,500. Compare the timeline: fill the void now and the slab never moves; skip it, and in a few winters you're paying for a full lift, or, if the slab cracks while it's hanging unsupported, for new concrete at many times the price. On concrete as old as this city's, keeping the original slab supported is almost always the better bet than gambling on how long it can span a growing gap. The spots where we fill most often in this city:
- Under the driveway apron where it meets the garage slab.
- Along foundation walls, over that ever-compressing original backfill.
- Beneath slab corners that sat under a downspout for decades.
- Around sewer and water line paths on homes with older plumbing.
Half a day, and the ground is solid again
The fill itself is quick work. We drill 1.5" holes over the void, pump our Portland cement, sand, and topsoil slurry until it stops taking material, and patch. The slurry flows to the far edges of the pocket and cures harder than the clay around it. Half a day on site, back in service the same day, no lifting unless you want lifting. Unlike leveling, the goal here is for nothing to move, ever. Because the material is mineral through and through, it doesn't rot, shrink, or wash out the way the soil it replaced did, and a filled pocket stays filled through every freeze-thaw season after.
Find out what's under there
If a slab sounds hollow, if the soil has pulled away somewhere, or if you just want old concrete checked before it becomes a project, call (303) 526-2222. Trevor runs every estimate in person, the tap test rides along free, and we've been keeping metro slabs off the ground-floor of settlement since 1988.