Polyjacking: Polyurethane Foam Slab Lifting

Polyjacking is a concrete lifting method that injects expanding polyurethane foam beneath a sunken slab to fill the void underneath and raise the concrete back to its original elevation. The foam is a two-part polyurethane mixture that expands after injection, filling gaps between the bottom of the slab and the subgrade. Unlike mudjacking, which pumps heavy cement slurry under the slab, polyjacking uses a hydrophobic foam material that weighs only 2 to 4 pounds per cubic foot. The result is slab void filling without adding significant load to the already-compromised soil beneath.

The method works because the foam expansion ratio — typically 15:1 to 20:1 depending on the product — generates enough force to lift a concrete slab while the closed-cell foam structure provides permanent support once it cures. Foam cure time is measured in minutes, not days. A polyjacked slab reaches 90 percent of its final strength within 15 minutes of injection, meaning homeowners can walk on the slab almost immediately and park vehicles on it the same day. This speed is one of the reasons polyjacking has largely replaced mudjacking for residential slab lifting in both Kansas City and Des Moines markets.

How Does Polyjacking Actually Work?

Polyjacking works by injecting a two-component polyurethane mixture through small holes drilled in the concrete slab, where it expands to fill the void beneath the slab and lifts the concrete upward. The two liquid components — an isocyanate and a polyol resin — are pumped through heated hoses and mixed at the injection gun tip. When the components combine, a chemical reaction produces expanding foam that fills every cavity it contacts. The expanding foam generates upward pressure against the bottom of the slab, and the technician controls the lift by monitoring the slab surface with a level or laser while injecting.

Injection port placement determines whether the foam reaches all void areas beneath the slab. Ports are drilled in a grid pattern, typically 3 to 6 feet apart, with additional ports placed at the lowest settlement points. Each port is only 5/8 inch in diameter — roughly the size of a dime — which is dramatically smaller than the 1- to 2-inch holes required for mudjacking. The small hole size preserves more of the slab's structural integrity and leaves far less visible patching when the work is complete.

The polyurethane foam density used for slab lifting ranges from about 2 to 4 pounds per cubic foot in its expanded state, compared to over 100 pounds per cubic foot for mudjacking slurry. This weight difference matters because the soil that failed beneath the slab is now being asked to support both the slab and whatever material was injected to fill the void. Adding 100-plus pounds per cubic foot of slurry to soil that already could not hold the slab's weight sometimes accelerates future settlement. The lightweight foam fills the same void without meaningfully increasing the load on the weakened subgrade.

Injection pressure monitoring during the lift prevents over-correction and slab cracking. The technician controls foam volume and injection rate at each port, watching the slab surface for movement. Over-lifting is correctable only by drilling and grinding — not by pushing the slab back down — so precise injection control matters. Experienced crews inject in short pulses, checking elevation between each pulse, rather than pumping continuously.

What Problems Does Polyjacking Fix?

Polyjacking fixes slab settlement caused by void formation beneath the concrete — situations where the soil has compressed, eroded, or washed away, leaving the slab unsupported. The most common residential applications are garage floors, driveways, sidewalks, patios, and pool decks that have sunk below their original grade. In each case, the concrete itself is typically intact; the problem is entirely in the soil beneath it. Polyjacking addresses the void without requiring demolition or replacement of functional concrete.

Tripping hazards at sidewalk joints, driveway-to-garage transitions, and patio-to-house connections are among the most frequent polyjacking applications. When one slab section settles and the adjacent section holds its elevation, the resulting lip creates a tripping hazard and a potential liability for the homeowner. Polyjacking lifts the settled section back to match the adjacent slab, eliminating the height difference. Most of these projects take two to four hours and involve fewer than a dozen injection ports.

Interior basement floor slabs that have settled away from the perimeter foundation walls also respond well to polyjacking. The gap between the floor slab and the wall indicates the slab has dropped as the subgrade beneath it compressed. Polyjacking fills that sub-slab void and lifts the floor back toward level. This is distinct from foundation wall settlement, which requires piering — polyjacking addresses only the slab, not the structural foundation.

What Problems Is Polyjacking Not the Right Fix For?

Polyjacking does not repair structural foundation settlement — situations where the footing and foundation wall are sinking because bearing soil beneath the footing has failed. If your foundation walls are dropping, the appropriate repair is piering (either push piers or helical piers) to transfer the building's load to stable bearing strata below the failed soil. Polyjacking lifts slabs; piering stabilizes structural foundations. They address different failure modes at different depths, and confusing one for the other leads to wasted money and unresolved problems.

Polyjacking cannot restore a slab that is severely cracked, broken into multiple pieces, or deteriorated from freeze-thaw damage. The foam requires a reasonably intact slab to push against. If the concrete is fractured into small sections, the foam will lift some pieces and not others, or push through the cracks rather than building pressure beneath the slab. Slabs with a single crack or one control joint step can often be polyjacked successfully, but slabs broken into a mosaic of fragments typically need replacement.

Active water intrusion problems are not solved by polyjacking alone. If water is actively flowing beneath a slab and eroding the subgrade, polyjacking fills the current void but does not stop the water. The foam itself is hydrophobic and will not deteriorate in wet conditions, but water will continue eroding soil around and beneath the foam, potentially creating new voids adjacent to the repaired area. Drainage correction should happen before or alongside polyjacking when water is the root cause of settlement.

When Is Polyjacking the Right Choice?

Polyjacking is appropriate when a structurally sound concrete slab has settled due to void formation beneath it, and the goal is to restore the slab to its original elevation. The ideal candidate is a slab that is intact or has only minor cracking, has settled between 1/4 inch and several inches, and sits over soil that has simply compressed or eroded rather than actively washing away. Most residential driveways, sidewalks, garage floors, and patios meet these criteria.

Soil conditions that involve fill soil compaction over native subgrade are particularly well-suited to polyjacking. When fill was placed during construction and has gradually compressed under the slab's weight, the resulting void is stable — the soil is not actively moving, it has simply consolidated. Polyjacking fills this consolidated void permanently. The closed-cell foam structure does not compress under residential loads, so the lift is maintained as long as the surrounding soil remains stable.

Polyjacking is also the preferred method when minimizing disruption matters — rental properties, commercial walkways, and driveways that cannot be out of service for days. Because the foam cure time to 90 percent strength is roughly 15 minutes, the surface is functional almost immediately. Mudjacking requires 24 to 48 hours of cure time before the slab can bear traffic. For a commercial property or a homeowner who parks in the garage daily, that difference in downtime is significant.

How Do Kansas City and Des Moines Conditions Affect Polyjacking?

In Kansas City, polyjacking is most commonly performed on settled garage slabs and driveways built on Wymore-Ladoga clay, a high-plasticity soil series that dominates the southern and eastern metro. Wymore-Ladoga clay undergoes significant shrink-swell cycling with seasonal moisture changes, which creates voids beneath slabs as the soil alternately pushes upward and pulls downward. Garage slabs on this soil type are particularly vulnerable because garage floors receive less rainfall to moderate soil moisture, leading to more extreme drying and shrinkage beneath them. Polyjacking fills the voids created by these cycles, and the hydrophobic foam material resists the moisture that drives the cycle.

Des Moines metro suburbs — Ankeny, Waukee, and Grimes in particular — developed rapidly since the 1990s and commonly need polyjacking where fill soil over glacial till was inadequately compacted during construction. The pace of development in these suburbs meant that fill placement and compaction did not always meet engineering specifications. Garage slabs and driveways poured over this insufficiently compacted fill began settling within five to fifteen years as the fill consolidated under load. Polyjacking is the standard correction because the fill has reached a stable density by the time the homeowner notices the settlement — the void simply needs filling, not the soil reworking.

Both metro areas have soil and construction conditions that make polyjacking one of the most frequently performed foundation repairs. In the Kansas City metro, approximately 40 percent of homes built on Wymore-Ladoga soils will experience some degree of slab settlement within 20 years of construction. In the Des Moines metro, homes built during the 1990s and 2000s development boom are now entering the 15- to 25-year age range where fill-over-till settlement becomes apparent. For specific cost information in either market, see the cost and economics page.

What Are the Steps in a Polyjacking Project?

1. Step 1: Assess slab condition and identify voids. The technician examines the slab for cracks, structural soundness, and the pattern of settlement. A level survey across the slab surface maps the low spots and estimates the void volume beneath. This assessment determines whether polyjacking is appropriate or whether the slab damage requires replacement instead.
2. Step 2: Plan injection port placement. Based on the settlement map, the technician marks port locations on the slab surface. Ports are concentrated near the deepest settlement and spaced to ensure foam reaches all void areas. Typical spacing is 3 to 6 feet, with tighter spacing in areas of deeper settlement or more complex void geometry.
3. Step 3: Drill injection ports through the slab. A 5/8-inch masonry bit drills through the concrete thickness at each marked location. The small diameter minimizes slab damage and leaves ports that are easily patched after the work is complete. Drilling takes only seconds per port in standard 4-inch residential slabs.
4. Step 4: Inject polyurethane foam through each port. The two-part polyurethane mixture is injected through specialized heated hoses and a mixing gun. The foam expands as the chemical reaction progresses, filling the void and generating lifting pressure. The technician injects in controlled pulses, checking slab movement between each pulse to prevent over-lifting.
5. Step 5: Monitor slab elevation throughout injection. Using a level, laser, or string line, the technician tracks the slab surface during each injection pulse. The goal is to lift the slab to match adjacent surfaces or reach a pre-determined target elevation. Injection pressure monitoring ensures the foam is filling voids rather than escaping through cracks or joints.
6. Step 6: Plug injection ports and complete cleanup. Each port is sealed with a concrete patch compound or a polyurethane plug. Any foam that escaped through joints or cracks is trimmed flush with the surface. The foam reaches 90 percent cure within 15 minutes, so the slab is walkable immediately and can handle vehicle traffic within a few hours.

How Do You Know if Polyjacking Was Done Well?

A quality polyjacking job restores the slab to within 1/4 inch of its target elevation across the entire surface, with no new cracks caused by over-lifting. After the work is complete, placing a 4-foot level on the slab should show a bubble within or very close to the center lines. Some slight variation is acceptable — the goal is functional levelness, not laboratory precision — but visible slopes or rocking when you walk across the slab indicate incomplete lifting.

Injection port patches should be flush with the slab surface, tightly sealed, and neat in appearance. Each 5/8-inch port hole should be filled completely and finished smooth. Sloppy patching — lumpy fills, unfilled ports, or excessive patch material spreading beyond the hole — suggests careless work. The ports are small enough that they become nearly invisible once patched properly and the concrete weathers.

No excess foam should be visible on the slab surface, in joints, or squeezed out from slab edges after cleanup. During injection, some foam may escape through control joints or the slab perimeter. A thorough crew trims all excess foam flush with the surface and removes any foam debris. Foam left protruding from joints is both cosmetically unappealing and creates a trip hazard at joint lines.

The contractor should provide documentation of the pre-lift survey, the injection port layout, the foam product used, and the post-lift elevation measurements. This documentation establishes a baseline for future comparison and verifies that the work achieved its target. If settlement recurs years later, these records help the next contractor understand what was done and where additional support may be needed. Contractors who do not document their work make warranty claims and future repairs more difficult.

Frequently Asked Questions About Polyjacking