The Complete Florida Homeowner's Guide to Sinkholes, Sunken Concrete & Slab Leveling

Most of Florida sits on top of a very old, very thick bed of limestone and dolomite — soft, porous rock formed millions of years ago from compressed marine sediment. In a lot of the state that rock is surprisingly close to the surface, sometimes just a handful of feet under the sand, clay and topsoil we actually build on. This kind of landscape, where soluble bedrock sits under a thin soil cover, has a name: karst terrain, and it’s the single biggest reason Florida behaves so differently underground than most of the rest of the country.

Limestone and dolomite aren’t inert. Rainwater picks up carbon dioxide as it falls and moves through soil, which makes it mildly acidic — not dangerous to touch, but just acidic enough to slowly dissolve calcium carbonate rock over time. That slightly acidic groundwater works its way into the natural cracks, joints and pore spaces already present in the limestone, widening them year after year, decade after decade. Over a long enough timeline, small fractures become channels, channels become passages, and passages become open voids and cavities in the rock, sometimes well below where anyone would ever dig or drill.

How an underground void eventually shows up at the surface

A void opening up in bedrock forty or sixty feet down doesn’t announce itself immediately. The sand and clay soil sitting above the limestone can bridge a void for a surprisingly long time, arching over the open space almost like a shallow dome. But that soil bridge isn’t infinite — as groundwater keeps moving through it and individual sand grains keep raveling down into the opening below, the bridge slowly thins. Eventually it can no longer support the weight of the soil, and whatever is sitting on top of it, above.

Two different modes: gradual sinking versus sudden collapse

Geologists generally describe two patterns. Cover-collapse happens when that soil bridge fails all at once, producing a sudden, sometimes dramatic depression or hole — rare, but the version everyone pictures when they hear the word “sinkhole.” Far more common, especially in a residential setting, is cover-subsidence: a slow, gradual settling as sand continues raveling into a void over months or years, showing up as a slab that drops unevenly, a yard that develops a shallow low spot, or cracks that widen a little more every season. The overwhelming majority of the settlement we’re called out to look at in Florida is this slower, subsidence pattern — not a sudden collapse.

Settlement rarely announces itself with one dramatic event. It usually shows up as a handful of small, easy-to-dismiss changes around the property that, taken together, point to soil movement underneath. Here are the seven we see most often, in roughly the order homeowners tend to notice them.

1. Cracks running diagonally from door and window corners

A single hairline crack in concrete or drywall is often just ordinary curing shrinkage and not worth losing sleep over. A crack that runs at a diagonal from the corner of a door or window frame, especially one that’s widening over a period of weeks or months, is a different signal — it usually means the structure above it is twisting slightly as the ground underneath settles unevenly.

2. Doors and windows that stick, bind, or won’t latch

When a slab or foundation shifts even a small amount, door and window frames rack out of square. A door that used to close easily and now drags on the frame, or a window that won’t slide or latch the way it used to, is often the first thing a homeowner physically feels rather than sees.

3. A widening gap between the slab and the foundation wall

Walk the perimeter of a patio, driveway or garage slab and look at where it meets the foundation wall or the house itself. A visible, growing gap there — sometimes wide enough to fit a finger or a coin — usually means the slab has dropped relative to the structure it used to sit flush against.

4. New depressions or low spots in the yard

A shallow bowl-shaped low spot that wasn’t there a year ago, especially one that collects standing water after a normal afternoon rain, is one of the more reliable outdoor signs of soil settling into a void or loosened area below. It doesn’t need to be dramatic to be meaningful — even a subtle dip worth noticing on a mower pass is worth a second look.

5. Fence lines, posts or mailbox footings leaning or shifting

Fence posts and mailbox footings are set shallow and are sensitive to even modest soil movement. A section of fence that has started leaning, a post that’s visibly tilted relative to its neighbors, or a run of fence that no longer sits level along the ground can indicate the same subsurface movement showing up nearby.

6. A slab or floor that’s no longer level

Set a level, or even just a marble or golf ball, on a driveway, garage floor or patio and watch what it does. A slab that visibly slopes toward one corner, or a ball that consistently rolls to the same spot, means that section has dropped relative to the rest of the pour — often before the difference is obvious just by eye.

7. Interior cracks at ceiling corners or along drywall seams

Settlement outside doesn’t always stay outside. Cracking at the corners of ceilings, along drywall seams, or where an interior wall meets the ceiling can reflect the same structural racking that’s showing up as exterior slab movement, especially when it appears on the same side of the house as a sinking driveway or patio.

Any one of these on its own can have an innocent explanation. Two or three showing up together, especially on the same side of the property, is the pattern worth getting a professional set of eyes on.

Once a slab has been diagnosed as structurally sound but sitting on soil that has settled, eroded or lost support, polyurethane injection is the most direct way to put it back where it belongs — without breaking the slab out.

Step 1 — Small entry holes

A pattern of holes roughly the size of a dime is drilled through the slab at the locations where support is needed. That’s a fraction of the size mudjacking requires, and it’s easy to patch flush once the work is done.

Step 2 — Expanding foam injection

A two-part polyurethane material is pumped through each hole under controlled pressure. Once it contacts the soil, it begins expanding almost immediately, flowing into voids, loose pockets and gaps that are actually causing the lack of support.

Step 3 — The slab rises within minutes

As the foam expands, it exerts gentle, even lifting pressure from below. The technician monitors elevation continuously during this stage, working the injection points in sequence so the slab rises evenly rather than tilting or overcorrecting in one spot.

Step 4 — Same-day return to use

The material reaches enough strength to support the slab in about fifteen minutes. Entry holes are patched, the area is cleaned, and the driveway, garage floor or patio is back in normal use the same day — no waiting days for a slurry to firm up.

Polyurethane injection is an excellent tool for a specific problem: a structurally sound slab sitting on soil that has lost support underneath it. It is not the right tool for every problem that looks similar on the surface, and a contractor who tells you otherwise isn’t doing you any favors.

If a slab has cracked into separate pieces that no longer move together as one unit, injecting foam underneath it can lift the pieces unevenly rather than solving anything — replacement is the more honest path in that case. If probing and inspection point to a void that’s unusually large, deep, or still actively developing, filling it with injection foam alone may not address what's happening well below the reach of a slab-lifting crew. In those situations, the right call is a referral to a licensed structural engineer or geotechnical firm, who can evaluate whether deeper structural fill, underpinning, or another engineered solution is needed before any surface repair makes sense.

We tell homeowners this upfront rather than after the fact: a same-day lift is the right answer for the majority of settlement we see, but not for all of it, and knowing which situation you’re actually in is the entire point of a real inspection.

A properly diagnosed and executed lift should hold for years without further work. That said, a little ongoing attention goes a long way toward catching anything early if the ground around the repair ever moves again.

Take dated photos of the repaired area right after the work is finished, including a level or straightedge laid across the slab — that baseline makes it easy to spot even small changes later. Recheck the same spot after Florida's rainy season each year, since heavy, sustained rainfall is when soil movement is most likely to show up again if it's going to. Keep the drainage and grading improvements from the prevention step in place, since a repair sitting in the same wet conditions that caused the original problem is working against itself. If a new crack, gap or dip appears anywhere near the repaired area, get it looked at while it's still small — it's far easier to address early settlement than to wait until it's advanced.