CCB # - 214246
Uneven or settling concrete slabs occur when the support beneath the slab changes, creating voids, differential movement, or uplift that compromises the surface plane, safety, and drainage. In this guide, you’ll learn the common causes of concrete slab movement, how to identify visual and functional signs, practical repair options, and more.
Uneven concrete slabs are primarily caused by changes or failures in the supporting soil and subbase beneath the concrete, which create voids or differential bearing capacity, resulting in the concrete sinking, tilting, or cracking. When the subgrade loses its strength due to erosion, poor initial compaction, freeze-thaw cycles, or root intrusion, the slab no longer has uniform support, and point loads or moisture changes accelerate movement.
The result is a slab that no longer distributes loads evenly, resulting in trip hazards, water pooling, and accelerated surface deterioration. Understanding the root causes enables targeted fixes, such as stabilizing the soil, correcting drainage, and selecting a lifting method that restores uniform support without unnecessary demolition.
Soil erosion undermines concrete by removing the material that supports the slab, producing voids that allow the slab to sag or collapse into unsupported areas. Water sources (such as roof runoff that bypasses gutters, saturated planting beds, poor grading that directs flow toward slabs, or drainage failures) concentrate energy and carry fines away from the subbase, accelerating undercutting.
The typical sequence is: concentrated water flow → soil displacement → void formation beneath the slab → localized settlement or abrupt cracking, often near edges or joints. In the Willamette Valley, seasonal heavy rains combined with clay-lean soils can lead to gradual soil erosion if surface drainage isn’t properly controlled. Addressing erosion requires both repairing the concrete support and fixing water management so the same washout doesn’t recur.
Poor soil compaction leaves voids or loosely packed subgrade layers that compress under load, causing progressive settlement after the slab is in service. When poor installers skip adequate compaction or neglect proper subbase thickness, the load from vehicles, foot traffic, or storage areas compresses the subgrade unevenly, leading to differential settlement.
Signs include localized depressions, joints that open up, and a slab that feels springy in spots, which contrasts with properly compacted bases that remain stable over time. Preventing compaction-related settlement starts at installation—achieve specified compaction levels, use appropriate granular subbases, and verify through inspection or testing. When settlement has occurred, repair approaches must reestablish uniform support rather than simply filling cracks to be durable.
Uneven or settling slabs reveal themselves through visual cues and functional behavior that are easy to check with simple tools and observations. Cracks that follow settlement lines, sloping surfaces that cause runoff toward structures, pooling water in depressions, gaps between adjoining slabs, and trip hazards are the most common indicators that the supporting soil or subbase has changed.
Early detection avoids larger repairs. A small settlement can be remedied with lifting methods, whereas ignored problems often require partial replacement or extensive soil stabilization.
Crack patterns reveal cause. Hairline or shrinkage cracks are usually cosmetic, while wider, offset, or stair-step cracks often signal structural movement or settlement. A structural crack typically shows displacement across the crack plane, evidence of soil loss at an edge, or repeated widening after wetting and drying cycles.
Measure the crack width and note its progression over several weeks to detect active movement. Stable hairline cracks under 1/8 inch that don’t grow can often be sealed, while wider settlement-related cracks may require leveling or full panel replacement. If cracks coincide with pooling or adjacent sloping, the underlying subgrade likely needs attention to prevent recurrence. Understanding the type of crack determines whether to monitor, seal, lift, or replace it.
Pooling water identifies low spots and often points to either surface grading issues or subbase settlement beneath the slab that has created depressions. When water persists, it increases freeze-thaw stress (where applicable), promotes erosion of fine particles from the subbase, and can accelerate deterioration of joints and edges.
A simple straightedge test and observing flow after a rain event quickly indicate whether the problem is a local low spot or a broader grading issue. Short-term fixes—such as adding surface slope or patching—may help temporarily, but durable repair usually requires lifting the slab and correcting drainage to prevent the same pooling from recurring.
Concrete lifting is most commonly performed using polyurethane foam injection (polyjacking) or mudjacking (cementitious slurry), both of which correct levelness by filling voids and restoring bearing capacity beneath the slab. Polyurethane foam injection utilizes high-density, expanding foam to fill voids and compact surrounding soil, requiring small drill holes and offering fast cure times. Mudjacking pumps a cementitious grout beneath the slab through larger holes, providing a heavier fill that can be economical for large areas but has longer cure times and can add permanent weight to the subgrade.
Choosing the right method depends on void size, soil type, load requirements, access, and desired downtime. Polyjacking tends to be preferred where minimal disruption, quick return to service, and moisture resistance are priorities, while mudjacking remains appropriate for budget-constrained, large, low-load areas.
Polyurethane lifting fills voids with an expanding polymer that both lifts the slab and grouts the subbase, producing rapid stabilization and reuse. Technicians drill small ports through the slab, inject metered foam that expands to fill voids and compact surrounding fines, and then cut and grind port plugs flush for a clean finish.
Benefits include lightweight material that avoids adding permanent load, closed-cell moisture resistance that reduces future washout, and minimal surface repair with a fast return-to-service for driveways and sidewalks. Typical on-site timelines are short, often completed in a single day for small residential jobs, making polyjacking an attractive option for homeowners who need prompt and durable results.
Mudjacking pumps a cementitious slurry beneath the slab to raise and re-support concrete by filling voids with a heavier grout that sets over time. Concrete contractors often drill larger access holes, use hydraulic pumps to push grout under targeted panels, and monitor lifts until the desired plane is reached, with the surface ports later patched with cement.
Mudjacking’s advantages include lower material costs in some markets and proven performance on large, low-traffic slabs. However, trade-offs include longer cure times, larger patch areas, and a heavier, permanent fill that can stress weak subgrades. Mudjacking can be a sensible and economical choice when weight addition is acceptable and large, contiguous areas need re-leveling.
Preventing uneven slabs begins with controlling water, protecting subbase integrity, and performing routine maintenance that catches small issues before they enlarge into costly repairs. Effective drainage—through gutters, downspout extensions, grading away from slabs, and French drains where needed—reduces erosion and soil displacement beneath concrete.
Regular inspection, joint maintenance, vegetation control near slab edges, and sealing surfaces where appropriate help extend slab life by minimizing water ingress, root intrusion, and freeze-thaw damage.
Here are some preventive actions homeowners can take now:
Proper drainage intercepts and redirects water before it concentrates against or under a slab, preventing the soil displacement that creates voids and undermines support. Techniques include ensuring roof runoff is carried away with downspout extensions, grading the landscape to slope away from structures, and installing subsurface drains or French drains where surface solutions are insufficient.
Do-it-yourself checks—observing flow during rain and using a long, straight edge to verify slope—help detect problem areas early. Where concentrated flow or persistent pooling occurs, professional drainage correction can protect the investment of any concrete repair. Addressing drainage first is often the most cost-effective way to prevent recurring settlement.
Regular maintenance extends the life of concrete slabs by addressing small problems before they become structural, including resealing control joints, removing vegetation at slab edges, and correcting minor low spots to restore surface drainage. Seasonal inspections, especially after winter or heavy rains, allow timely patching and regrading while the repairs remain small and inexpensive. Small preventive maintenance, such as joint resealing and surface patching, mitigates water ingress that leads to erosion and freeze-thaw damage, often delaying or avoiding the need for slab lifting altogether. A consistent maintenance schedule—inspecting each season and performing targeted repairs when issues are identified—offers the best return on investment and keeps surfaces safe and functional.
Addressing uneven concrete slabs is essential for maintaining safety, aesthetics, and property value. By understanding the causes and implementing effective repair methods, such as polyurethane foam injection or mudjacking, you can achieve lasting results. Regular maintenance and proactive measures further enhance the longevity of concrete surfaces. For a tailored solution to your concrete needs, contact us today to receive a free estimate.