You're sweeping the patio and one section sounds different under the broom. Or the corner of the driveway gives a little clunk every time you pull in.
Concrete is only as good as the dirt underneath it. When that dirt leaves, the slab bridges the gap for a while. Then it doesn't.
What a Void Actually Is
A driveway, walk, or patio slab isn't built like a bridge. It's usually only about 4" thick, and it handles weight by spreading it into firm, even ground below.
Concrete is very strong when it's squeezed and much weaker when it's bent. Over a void, part of the slab has nothing under it, so every footstep or tire bends that section a little. Bending is exactly what concrete is bad at.
A void can be a gap you'd barely fit a finger into or a hollow you could lose your arm in. Most start small and grow, because water uses any open path under the slab.
What Creates Voids in Northeast Wisconsin
Almost every void comes back to soil leaving, shrinking, or rotting away. These are the usual suspects here:
- Downspout washout. A downspout that dumps at the edge of a walk or patio sends all the water from a big section of roof to one spot. Over a few seasons, that stream carries fine soil out from under the slab.
- Sump pump discharge. In a wet spring, a sump pump can cycle over and over. If the discharge lands next to a patio or driveway, it does the same damage as a downspout, just more often.
- Loose foundation backfill. When your house was built, the hole was dug wider than the basement walls and the gap was backfilled, often without much compaction. That soil keeps settling for years. Stoops, garage aprons, AC pads, and walks along the house all sit on it.
- Rotting tree roots. When a tree comes down, its roots rot in place and leave tunnels behind. Plenty of ash trees have been removed in this area, and nearby slabs often start sinking a few years later. More on that in our tree roots article.
- Burrowing animals. Chipmunks love digging along the edges of patios and walks. Woodchucks dig under stoops and slabs. The hole you see at the edge is often much smaller than the tunnel underneath.
- Frost cycles. Wet soil under a slab freezes, expands, and pushes the concrete up. In spring it thaws loose and settles unevenly, and the gaps add up year after year.
- Broken drain tile and buried pipes. A cracked underground downspout line, a separated sump pipe, old clay tile, or a leaking water or sewer line can wash soil out from below with no sign on the surface. These are the sneaky ones.
- An open joint at the house. An open gap where a slab meets the foundation lets runoff pour straight down and under it.
Homeowner Self-Check: Do You Have a Void?
No special tools needed. Run through these on any slab you're suspicious of:
| Check | How to do it | What it tells you |
|---|---|---|
| Tap test | Tap the slab every foot or so with a hammer (gently) or the end of a broom handle. Compare spots. | Solid concrete sounds sharp and dense. A dull, hollow, drum-like sound means there's likely air under that section. |
| Edge gap | Look along the slab edges. Slide a tape measure or yardstick underneath. | If it slides in more than an inch or two, soil has washed out from under the edge. |
| Hose test | Run a garden hose at the slab edge for a minute or two. Not longer. | Water that disappears under the slab, instead of running off or soaking into the grass, has an open path to go. |
| Step test | Step on corners and edges. Have someone drive slowly over the joints while you watch. | A slab that rocks, clunks, or tips has lost support underneath. |
| Crack pattern | Look for cracks that cut off a corner or run diagonally from an edge. | Corner cracks often form right over a void, where the unsupported concrete broke under load. |
| Joint lip | Lay a straight 2x4 across the joint between two slabs. | If one slab is lower than its neighbor, soil compressed or washed out under it. |
| Holes and dirt piles | Check slab edges for round holes or fresh mounds of soil. | Animals are tunneling under the concrete. |
| Gap at the house | Look where the slab meets the foundation. | A widening gap, or a slab tipping toward the house, usually means the backfill below is settling. |
One caveat on the tap test: an old slab with a thin topping or patch layer can sound hollow because that layer has separated, not because there's a void below.
What Happens If You Leave It
Not every void is an emergency. A small gap under a backyard walk can sit for years. But voids rarely stay the same size:
- 1Water finds itEvery rain and spring melt runs into the open space and carries a little more soil out.
- 2Winter makes it worseWater in the void freezes and heaves the slab, then drops it again in spring.
- 3The slab cracks under loadThe unsupported section bends every time it's loaded until it cracks. Driveway corners and garage aprons are the classic spots, because that's where tires roll over the edge.
- 4Sometimes it breaks all at onceA slab can bridge a void for years, then snap under a delivery truck, loaded trailer, or dumpster drop-off. Once it's in pieces, you're looking at replacement instead of a lift.
And a slab sinking next to the house often tips toward it, steering every rain at your basement wall.
How Foam Fills Voids While It Lifts
This is where foam lifting earns its keep. The same injection that raises the slab fills the empty space under it first.
- 1Find the voidWe tap the slab, check the edges, and see how it sits compared to the slabs around it.
- 2Drill a few 5/8" holesAbout the size of a dime, placed over the low or hollow areas.
- 3Inject the foamIt goes in as a liquid and follows the open space, filling gaps and hollows before anything else.
- 4Lift in small, controlled incrementsOnce the void is full, the expanding foam starts pushing up on the slab. We watch it the whole time and stop when it's where it belongs.
- 5Patch and clean upThe holes get patched. You can walk on it right away and drive on it the same day.
If a slab is still sitting at the right height but sounds hollow, foam can fill the void and stop there, firming it up without raising it.
Two things make foam a good match for voids specifically:
- It's light. Lifting foam weighs a few pounds per cubic foot. Mudjacking slurry is in the neighborhood of 100. Piling heavy fill onto soil that already couldn't hold its ground is how slabs end up sinking again.
- It won't wash out. Cured foam is waterproof. Since water created most of these voids in the first place, that matters.
When Foam Isn't the Fix (Yet)
We'd rather tell you this up front than fill the same void twice:
- There's an active leak. If a buried pipe or drain line is leaking under the slab, foam fills today's void and the water digs a new one. Get the leak found and fixed first. That's a plumber or drain contractor, not us.
- Animals are still living there. Get the woodchuck or chipmunks evicted before anyone fills the tunnel.
- The slab is in pieces. Foam can lift a cracked slab, but a slab broken into several loose chunks is usually a replacement job.
- The void is under your foundation. Basement walls and footings are a job for a foundation repair specialist. We lift slabs, not houses.
Stop the Next Void Before It Starts
Lifting fixes the slab. Keeping water out from under it keeps it fixed:
- Extend downspouts several feet past the edge of slabs and the foundation, onto ground that slopes away.
- Point the sump discharge somewhere the water can run off, not next to a patio or driveway.
- Backfill eroded edges with soil and get grass growing so water runs off instead of under.
- Seal the joints. The gap where concrete meets the house, and the joints between slabs, are open doors for water. We caulk these, often right after a lift.
- Check for burrows every spring before a small hole becomes a big tunnel.