A walkway slab in North Charleston had dropped almost two inches at one corner, low enough that water pooled against the house every time it rained instead of running off toward the yard. The slab itself was in decent shape — no cracking across the panel, no rocking, just a corner that had settled into a void left behind after an old drain line was replaced and the trench backfill was never compacted properly. That is close to the textbook case for lifting rather than replacing: good concrete sitting on bad ground. It is also a case that gets mishandled a lot, because lifting only works if the diagnosis behind it is right.
What lifting actually does
Slabjacking and polyurethane lifting both work the same basic way: material is pumped through small holes drilled in the slab, fills the void or compresses the soft zone underneath, and raises the panel back toward its original grade. Neither process fixes the concrete itself — it was never broken to begin with, in a case where lifting is the right call. What both processes fix is the support underneath a slab that is otherwise structurally sound.
Cement grout versus polyurethane
Traditional slabjacking pumps a cement-based grout slurry, which is heavy, inexpensive per unit volume, and adds significant weight to whatever it fills — useful when the goal is also to build up bearing capacity in a large void, but slower to cure and often requiring larger access holes, typically 1-1/2 to 2 inches. Polyurethane foam is lighter, expands to fill voids more precisely, cures in a matter of minutes rather than days, and uses much smaller injection holes, often around 5/8 inch. It costs more per unit of lift but usually less overall once you account for smaller holes, faster turnaround, and the fact that the foam does not add meaningful dead load on soil that is already struggling to carry the slab. On slabs over crawlspaces or areas with limited equipment access, the lighter material is often the only practical option.
When a slab qualifies for lifting
Lifting works when the panel itself is intact — no through-cracks splitting it into separate pieces, no corner spalling severe enough to compromise the edge, and reasonably uniform settlement rather than a slab that has folded or tilted unevenly across its width. The ground below has to be a definable void or a soft, compressible zone rather than ground that is still actively subsiding. A slab that dropped once, from a one-time cause like poor trench backfill or a washed-out utility bed, and has been stable since, is a good candidate. A slab still dropping a little more every season is not, because lifting it now just means lifting it again later.
When lifting is the wrong tool
A slab already broken into multiple pieces along uneven crack lines cannot be lifted as a unit — each piece would rise at a different rate, and the injection process can make the cracking worse rather than restoring a flat surface. Ground that is still actively moving, including areas over old coal mine workings where subsidence is ongoing rather than historical, should not be treated with lifting until the movement itself has stopped or been independently assessed; the National Concrete Masonry Association publishes technical guidance on evaluating substrate conditions before any repair method involving fill or support beneath masonry and flatwork gets chosen. A slab sitting on ground with ongoing settlement risk should get a geotechnical opinion before anyone drills a hole in it.
Holes, cost, and the water problem that does not go away
Injection holes, whether for cement grout or polyurethane, are typically patched with a color-matched mortar plug after the lift is complete, and on broom-finished or exposed aggregate work the patches are rarely invisible but are usually not obtrusive either. Cost runs roughly $5 to $7 per square foot for polyurethane lifting and somewhat less per square foot for cement grout on larger areas, though grout jobs often need more holes and more volume to achieve the same lift. A typical residential correction might run $600 to $2,500 depending on the size of the affected area and how much material the void takes. None of that addresses why the void formed in the first place. If the cause was water — a downspout dumping next to the foundation, a swale that never drained, a broken drain line — that has to get fixed separately, or the same corner settles again in a few years regardless of how well the lift itself was done.
Frequently asked questions
Is polyurethane lifting always better than cement grout? Not always — grout costs less per unit volume on large voids and adds bearing mass, while polyurethane is lighter, faster, and better suited to smaller access holes and weight-sensitive ground.
Can a cracked slab be lifted? A slab with hairline cracking but still structurally one piece can sometimes be lifted; a slab broken into multiple separate pieces generally cannot be lifted evenly and needs a different repair path.
Does lifting fix the reason a slab settled? No — lifting restores grade on ground that has already moved, but the underlying cause, often related to drainage or old mine workings, needs separate attention; see mine subsidence and what it does to flatwork for how to tell the two apart.