Why concrete slabs sink
Outdoor slabs such as driveways, sidewalks, patios, pool decks and garage floors are usually thin and rest directly on soil. When the soil underneath settles, washes out or was never compacted properly, the slab loses support and sinks or tilts. Common causes are downspouts that empty onto the slab edge, leaking irrigation, erosion along the edges and clay soils that shrink in droughts. Sunken slabs create trip hazards and can direct water toward the house, which in turn affects the foundation.
Mudjacking (slabjacking)
Mudjacking, also called slabjacking, pumps a cement-based grout under the slab through holes drilled about the size of a coin or larger. The grout fills voids and lifts the slab back toward level. It is an old, well-proven method and usually the lower-cost option. The grout is heavy, though, which adds load to soil that already failed once, and the holes are larger than with foam.
Polyjacking (foam lifting)
Polyjacking injects expanding polyurethane foam through small holes. The foam expands, fills voids and lifts the slab. It is much lighter than cement grout, cures quickly, and the holes are small. Angi notes that the lighter weight puts less pressure on the underlying soil and lowers the risk of future sinking. It usually costs more per square foot.
What concrete lifting costs
| Item | Published range | Source |
|---|---|---|
| Mudjacking, per square foot | about $3 – $6 (up to about $8 in some markets) | Angi |
| Mudjacking, typical project | about $570 – $1,500 | Angi |
| Polyjacking, per square foot | can reach about $25 | Angi |
| Minimum charge for small jobs | often around $500 | Angi |
Published figures for orientation only, not quotes. The contractor you are connected with sets the price after an inspection.
Mudjacking vs. polyjacking at a glance
| Mudjacking | Polyjacking | |
|---|---|---|
| Material | Cement-based grout | Expanding polyurethane foam |
| Weight added to soil | Heavy | Light |
| Hole size | Larger | Small |
| Cure time | Longer | Fast |
| Typical cost | Lower | Higher |
| Typical use | Sidewalks, patios, driveways | Same, plus slabs where weight or quick use matters |
When lifting is not the right fix
- The slab is badly broken into many pieces; lifting fragments rarely gives a good result.
- The house foundation is moving. Lifting a patio does not fix a settling house; that needs an evaluation and, often, piers. See foundation piering.
- The cause is still active. If a downspout or leak keeps washing out the soil, the slab will sink again.
- Sinkhole activity is suspected; that needs investigation first. See sinkholes and foundations.
What happens on lifting day
- The crew checks the slab for cracks and marks where to drill.
- Small holes are drilled through the slab, larger for grout and smaller for foam.
- Material is injected in stages while the crew watches a level or string line so the slab rises evenly.
- Holes are patched, and joints may be sealed to keep water out.
Many residential jobs take a few hours. The patched holes are visible but small, especially with foam. Color matching is rarely perfect on older concrete.
Questions to ask before you hire
- Which method do you recommend for this slab, and why?
- What caused the settlement, and what will stop it from happening again?
- How many holes, and where?
- Is there a warranty against re-settling, and for how long?
- Will you seal joints and the gap next to the house?
Safety and trip hazards
Sunken sidewalks and steps are a common trip hazard, and uneven slabs near entrances are worth fixing promptly for that reason alone. In some cities, homeowners are responsible for public sidewalks in front of their property; check local rules if the sunken slab is a sidewalk.
Can the house slab be lifted with foam?
Foam and grout injection are sometimes used under house slabs, usually for voids or small settled areas, and under some engineered repair plans. For a house that has settled because the soil below cannot carry its weight, piers that transfer the load to deeper soil are the common solution. Ask a contractor or engineer to explain why one method fits your house better.
Garage floors and interior slabs
A sunken garage floor is common where the slab was poured over poorly compacted fill, or where water from the driveway runs under the edge. Many garage slabs can be lifted like driveways. Interior floor slabs in homes are different: they are often tied to the foundation, and settlement there can signal a bigger problem. Have an interior slab evaluated before choosing lifting as the fix.
Lifting, replacing or piering: how to decide
| Situation | Usually the best fit |
|---|---|
| Intact walkway, patio or driveway section sunk an inch or two | Mudjacking or foam lifting |
| Slab broken into many pieces or heavily cracked | Replacement |
| Pool deck settled next to the pool | Lifting, after checking the cause (often washout or leaks) |
| House walls cracking and doors sticking | Foundation evaluation, often piers |
| Garage floor sunk, house fine | Lifting, if the slab is intact |
Local price differences
Concrete lifting prices vary by region. Angi’s local data, for example, shows Phoenix homeowners paying about $4–$16 per square foot for mudjacking, with most spending around $1,069, while national averages are lower per square foot. Access, the size of the job, travel distance and whether the slab is load-bearing all affect the quote. Ask for the price per square foot and any minimum charge so you can compare offers. For a small walkway or step, the minimum charge often decides the price; for a large driveway, the price per square foot matters most.
Prevent it from happening again
- Extend downspouts away from slab edges.
- Seal joints and the gap between slab and house so water does not get underneath.
- Fix irrigation leaks and keep sprinklers off concrete edges.
- Keep soil and mulch from eroding along slab edges.
Sunken pool decks and patios are common in South Florida and Southwest Florida, where sandy fill washes out easily, and sunken driveways and walkways are common in Texas clay regions.