How to Level a Sinking Concrete Slab: 3 Repair Methods Compared

TL;DR: A sinking concrete slab can be leveled three ways: mudjacking (pumping a cement-based slurry underneath for $3-6/sq ft), polyurethane foam injection ($5-25/sq ft, lighter and faster-curing), or full replacement ($6-15/sq ft when the slab is too cracked). Mudjacking works for most driveways and garage floors; foam is better when weight matters or you need same-day use; replace only when cracks exceed ¾ inch or the subgrade has washed out.

_Last reviewed: July 2026 · 5 min read_

A driveway that slopes toward your garage, a patio with a 2-inch step where there wasn't one, or a garage floor that pools water after rain — all point to a sinking concrete slab. The slab hasn't failed; the soil underneath has compacted, eroded, or shifted. Catching it early keeps the repair under $1,000 in many cases. Waiting until the slab cracks into pieces pushes you toward a $5,000+ replacement.

Okoniq Property Hub keeps a timestamped photo log of every slab, crack width, and repair date — helpful when you're deciding whether to mudjack now or wait another season.

What causes a concrete slab to sink?

Soil settlement is the primary culprit. Clay soils expand when wet and shrink when dry, creating voids beneath the slab. Sandy or poorly compacted fill settles over time, especially under heavy loads like a parked RV or a water-filled hot tub. Broken downspouts or clogged gutters can wash away subgrade soil; if you've had basement waterproofing work done, check that grading and drainage improvements extend to slab edges. Tree roots can also displace soil as they grow, leaving gaps when they die and decay.

How much a slab sinks matters: a ¼-inch drop across 10 feet is cosmetic; a 2-inch drop at a single joint creates a trip hazard and shifts water flow. Measure with a 10-foot straightedge and a tape measure at the deepest point. If the gap is under 1 inch and the slab is still in one or two large pieces, leveling is straightforward. Beyond 2 inches or when you see cracks wider than ¾ inch with offset edges, the structural integrity is compromised and replacement may be the only durable fix.

How does mudjacking (slabjacking) work?

Mudjacking — also called slabjacking — pumps a cement-sand-water slurry through 1½ to 2-inch holes drilled in the slab. The slurry fills voids and lifts the concrete until it's level. A crew drills holes every 3-4 feet in a grid pattern, pumps the mix, monitors the lift with a laser level, then patches the holes with non-shrink grout. The entire process takes 2-4 hours for a typical two-car driveway, and you can walk on it the same day (drive on it after 24 hours).

Cost runs $3-6 per square foot, making a 400 sq ft driveway section $1,200-2,400. The slurry cures to about 100 pounds per cubic foot — heavier than the polyurethane alternative — so it's not ideal for slabs over crawl spaces or above basements where load matters. Mudjacking works best on driveways, garage floors, patios, and sidewalks that rest on stable subgrade. It won't fix the underlying drainage issue; pair it with downspout extensions or French drains if water is the cause, just as you would when closing a seasonal home to prevent freeze-thaw heaving.

When is polyurethane foam injection the better choice?

Polyurethane foam injection — marketed under names like PolyLevel or similar — uses a two-part expanding foam that cures in 15 minutes and weighs about 4 pounds per cubic foot. Technicians drill ⅝-inch holes (much smaller than mudjacking), inject the foam, and monitor the lift in real time. You can use the slab within an hour, and the foam won't add significant load to the subgrade.

Cost is $5-25 per square foot, so that same 400 sq ft section runs $2,000-10,000 depending on depth of void and regional pricing. The high end applies when voids are 6+ inches deep or access is difficult (underneath a deck or between a slab and a foundation wall). Foam is the better pick when the slab is over a crawl space, when you need immediate use (a rental turnover day, for example), or when the slab has been mudjacked once before and the slurry's weight contributed to re-settling. The foam is also hydrophobic, so it won't wash away if groundwater is intermittently present.

What are the signs you should replace the slab instead?

Replace when the slab has multiple cracks wider than ¾ inch, when edges are offset by more than 1 inch vertically, or when the subgrade has eroded so extensively that leveling compound would just fill a pit with no stable base. If tree roots have heaved the slab into separate fragments or if the concrete surface is spalling (flaking off in sheets due to freeze-thaw cycles and deicing salts), leveling won't restore a smooth, safe surface.

Full replacement costs $6-15 per square foot, including demo, haul-away, subgrade compaction, 4 inches of new concrete, and finishing. For that 400 sq ft section, expect $2,400-6,000. The upside: you get a 4-6 inch engineered base (crushed stone compacted in lifts), a vapor barrier if needed, and 3,000+ PSI concrete with control joints cut to minimize future cracking. If you're replacing, address drainage at the same time — install a basement dehumidifier if the slab is a basement floor, or regrade and add surface drains if it's exterior.

Use Okoniq to log the replacement date, contractor name, mix design (the concrete ticket often notes PSI and slump), and any warranty terms. A well-documented repair history helps when you sell or if you need to file an insurance claim for subsequent settlement.

How do you prevent slabs from sinking again?

Prevent re-settlement by fixing the water source. Extend downspouts 6-10 feet from the foundation so roof runoff doesn't saturate the soil under slabs. Regrade so surface water flows away at a 2% slope (¼ inch per foot). If the soil is expansive clay, consider a French drain along the slab edge to intercept groundwater before it reaches the subgrade. For interior slabs like basement or garage floors, a perimeter drain and sump pump (see basement waterproofing) can keep the soil moisture stable year-round.

Limit point loads: don't park heavy equipment or stack firewood on thin residential slabs (typically 4 inches). If you're adding a hot tub or outdoor kitchen, pour a separate reinforced pad with a thicker base. Seal cracks annually with a flexible polyurethane or epoxy sealant so water can't infiltrate and erode the base. Inspect quarterly — take a photo of any new crack or settlement and note the date in Okoniq so you can track progression and decide when to act.

FAQ

How long does mudjacking last?

Mudjacking typically lasts 5-10 years if the underlying drainage issue is corrected; without drainage fixes, re-settlement can occur within 2-3 years as water continues to wash away or soften the subgrade.

Can you mudjack a cracked slab?

Yes, if cracks are hairline to ½ inch wide and the slab is still structurally sound; fill cracks with epoxy or polyurethane sealant after leveling to prevent water intrusion and further movement.

Is polyurethane foam injection permanent?

Polyurethane foam itself is stable and won't degrade, but the slab can sink again if soil erosion or poor drainage continues; the foam will not compact or dissolve, so re-lifting is only needed if new voids form underneath.

Can I level a slab myself?

DIY leveling with self-leveling concrete overlay works for indoor slabs with minor dips (under ¼ inch) but won't lift a sunk slab; mudjacking and foam injection require specialized pumps and pressure gauges, so hire a licensed contractor for outdoor or structural repairs.

Does homeowners insurance cover sinking slabs?

Standard policies typically exclude earth movement and settling unless it's sudden and accidental (like a burst pipe washing out soil); review your policy and document the timeline — gradual settlement over months is almost never covered, but subsidence from a single storm event may qualify.

This is educational information, not engineering or construction advice. Consult a licensed structural engineer if you see offset cracks, foundation movement, or suspect soil instability beyond normal settlement.