Sinking Concrete: Why It Settles and How to Lift It
What Sinking Concrete Looks Like
You might notice a section of your driveway has dropped, creating a lip where two slabs meet. Sidewalk panels might be uneven, with one section noticeably lower than its neighbour. Your garage floor might slope toward the house instead of toward the door, or a patio slab might have settled, creating a gap between the concrete and the house wall.
Water pooling is often the first sign. When concrete sinks, it creates low spots that collect rainwater. On driveways, you’ll see puddles that didn’t used to form. On sidewalks, water might pool near the foundation instead of draining away. On porches and stoops, the slab might tilt toward the house, directing water against the foundation wall.
Look at the joints between slabs. If one slab has dropped relative to its neighbour, you’ll see a step or lip at the joint. In some cases, the slab has settled enough that the joint has opened up, creating a wide gap where weeds grow through.
Cracks in the slab are common as well. As the soil beneath shifts unevenly, the rigid concrete breaks at its weakest points. These cracks often run across the centre of the slab or radiate from the corners.
Why This Happens
Concrete slabs sit directly on the soil or on a layer of compacted fill material. When that base erodes, compresses, or washes away, the slab has nothing to support it and begins to sink.
Soil washout is the most common cause in BC. Water flowing beneath slabs, whether from poor drainage, broken downspouts, or underground springs, erodes the soil and creates voids. The slab sits on top of empty space until it cracks and settles into the gap.
Poor compaction during construction causes settlement in the years after a home is built. If the fill material beneath the slab wasn’t compacted properly, it gradually compresses under the weight of the concrete. This is especially common in new developments where large areas are graded and filled quickly.
Soil shrinkage happens during dry periods. Clay soils contract significantly when they lose moisture, pulling away from the bottom of the slab and creating gaps. When rain returns, the soil expands but doesn’t always push the slab back to its original position.
Tree roots can create channels beneath slabs where water flows more easily, accelerating erosion. When roots die and decompose, they leave void spaces that the slab settles into.
Freeze-thaw cycles in BC’s climate contribute to the problem. Water that seeps beneath slabs freezes and expands, then thaws and drains away, progressively loosening the soil base.
How Serious Is It? A Quick Guide
When in doubt, a free assessment takes the guesswork out of it.
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What You Can Do, and When to Call Us
Monitor at Home
Measure the step. Place a straightedge or level across the joint between slabs and measure the difference. Record the date and check periodically to see if the gap is growing.
Watch where water goes. After a heavy rain, check how water flows across your concrete surfaces. Note any new pooling or areas where water is being directed toward the foundation.
Fill small voids. If you can see gaps at the edge of a slab where soil has washed out, fill them with compactable material like crushed gravel. This slows further erosion, though it doesn’t address voids beneath the slab.
Fix your drainage first. Clean gutters, extend downspouts, and repair any grading issues that are directing water under or around your slabs. Solving the water problem prevents the settlement from getting worse.
Call a Professional
Any slab that’s settling toward the house. Concrete that slopes toward the foundation directs water against the basement wall, which creates bigger problems than the slab itself.
Trip hazards on walkways or driveways. A lip of 15mm or more at a joint is enough to catch a foot. If visitors, mail carriers, or family members use the surface regularly, it’s a liability issue.
Water pooling where it shouldn’t. If sunken concrete is collecting water near the house or creating standing puddles, the drainage problem will continue to erode the soil beneath and worsen the settlement.
Garage floors sloping toward the house interior. This directs any water (from a car, snowmelt, or spill) toward the foundation wall rather than toward the garage door.
Concrete that’s sinking progressively. If the settlement is getting worse over time, the cause (usually water erosion) is still active and the slab will continue to drop.
Common Questions About Sinking Concrete
In most cases, sunken slabs can be lifted back to level using polyurethane foam injection. Small holes are drilled in the slab, and expanding foam is injected beneath it. The foam fills the void, compacts loose soil, and lifts the slab back to its original position. It’s faster, less disruptive, and more cost-effective than tearing out and pouring new concrete.
A series of small holes (about the size of a penny) are drilled through the sunken slab. A specialized polyurethane foam is injected through the holes, expanding beneath the slab to fill voids and lift the concrete. The foam cures in minutes, the holes are patched, and the surface is ready to use the same day.
The polyurethane foam we use is a permanent, waterproof material that doesn’t erode, wash away, or decompose. However, if the underlying cause of the sinking (like poor drainage directing water beneath the slab) isn’t addressed, new voids can form beside the repaired area. That’s why we assess the drainage situation as part of every project.
Not necessarily. Exterior slabs like driveways, sidewalks, and patios sit on their own soil base and can settle independently without affecting your home’s foundation. However, a sinking garage floor or slab that’s directing water toward the house can contribute to foundation problems. And if the same soil conditions are affecting both the slab and the foundation, both should be assessed together.
The cost depends on the size of the area, how far the slab has settled, and the number of injection points needed. Concrete lifting is typically a fraction of the cost of full replacement, with less disruption and faster results. We provide a free assessment and detailed quote so you know exactly what to expect.
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