Concrete Slab Leveling

A concrete slab sinks when the soil under it moves or washes out. On the clay soils north and east of Austin, that happens in two main ways. Dry weather shrinks the clay and leaves gaps under the slab. Wet weather and leaky downspouts soften or erode the base so the slab drops into the void. Either way, one edge or corner ends up lower than its neighbor, and you get a trip hazard, a pooling spot or a step that was never designed. Concrete slab leveling, sometimes called slab lifting or concrete raising, fixes the drop without removing the concrete. Small holes are drilled through the slab, a material is injected to fill the void and push the slab up, and the holes are patched. The slab keeps its original finish, and the job usually takes hours rather than days.

There are two common materials. Mudjacking, also called slabjacking, pumps a heavy slurry of water, soil or sand and cement under the slab. Polyurethane foam leveling, sometimes called polyjacking or polylifting, injects a two-part expanding foam that grows and hardens beneath the concrete. Both can raise a slab. The difference shows up in weight, hole size, cure time and long-term behavior. We focus on foam. Contractor sources describe polyurethane foam as very light, around 2 to 4 pounds per cubic foot, so it adds little load to soft clay. Mudjacking slurry is much heavier, which can matter on weak ground. Foam holes are also smaller, about five eighths of an inch, compared with roughly an inch and five eighths for mudjacking. Those numbers come from contractor literature, so treat them as typical rather than universal.

Cure time is another practical difference. Foam generally hardens within minutes, and many sources say a slab can carry foot traffic shortly after the work. Mudjacking slurry needs longer to set before the slab sees weight. Foam is also water resistant, so it does not wash out the way a cement slurry can when downspouts or irrigation keep feeding water under a slab. That matters here because clay lots are prone to periodic soaking. Mudjacking has real strengths. It is often less expensive per slab, and it can be a good fit for large, heavy slabs where a lot of fill is needed. The trade-off is weight, bigger holes and a longer wait. Which material is the better choice depends on the slab, the soil under it and the budget. A good contractor will explain the pros and cons rather than push one answer.

Leveling does not solve every problem. If the cause of the settlement is still active, the slab can drop again. A downspout dumping next to a patio, a broken irrigation line, or runoff flowing under a walkway will keep eroding the base. That is why leveling is best paired with drainage work. Extending the downspout, regrading the bed, or sealing the joint where water enters is often what makes the lift last. Slabs also differ in condition. Concrete that is badly cracked, crumbling or thin can break when lifted. In those cases, replacement may be the honest recommendation. We check the slab before quoting, tap it for hollow sounds, and explain the risks so you can choose with eyes open.

One firm boundary: we level exterior slabs that are separate from your house, such as patios, sidewalks, steps, pool decks, and driveways. We do not lift the house slab or do structural foundation repair, which includes piers and slab jacking under the home. Mudjacking and foam can only lift slabs that are not structurally attached to the foundation of the house, and lifting a house calls for engineering. If a patio is tied to the house with rebar, or if you see cracks in interior floors and walls, stop and get a structural engineer or a foundation repair company to look first. They can tell you whether the movement is cosmetic or serious, and our work is then planned around their findings.

How can you tell when a slab is a good candidate? Look for a step at a joint, a slab that tilts toward the house, or a hollow sound when you tap it with a hammer or the heel of a shoe. If the concrete is mostly intact and the settling is under a few inches, leveling is usually practical. If the slab is shattered, or the soil under it is washing away fast, the underlying cause comes first. Before any drilling, Texas law calls for an 811 notice at least two business days before digging. Leveling holes are shallow, but buried lines can run under patios and walks. Check HOA and city rules too, especially for slabs near an easement.