Cracked or sunken concrete in the Seattle area can often be lifted with cement slurry or polyurethane foam, sealed, patched or resurfaced. Replacement is better for badly broken slabs. Because water washing out soil is a common cause here, the repair should include fixing downspouts or drainage. Moving slopes or foundation cracks call for an engineer first.
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Not every cracked or sunken slab has to be torn out. Depending on the cause, concrete can often be lifted, sealed, patched or resurfaced with less disruption than a full replacement. The key is understanding why it failed. In the Seattle area, the answer usually involves water.
Why concrete cracks and sinks around Seattle
- Water under the slab. Dense glacial till drains slowly, so rain and runoff can sit under slabs or wash fine soil out from beneath edges, leaving voids.
- Settling fill. Backfill next to foundations and fill placed when hillside lots were graded can compress over the years, often at the garage door or along steps.
- Downspouts and runoff. A downspout that empties at a slab edge, or runoff from the lot above, is one of the most common causes of a sunken corner.
- Tree roots. Large street and yard trees lift walks and driveways.
- Slopes. On hillside lots, slabs and walls can creep downhill if drainage or support is poor.
- Freeze-thaw and deicers. Freezes are occasional here, but salt and water in non-air-entrained concrete can cause flaking, known as scaling.
Repair options
- Slab lifting (mudjacking): a cement-based slurry is pumped under a sunken slab through drilled holes to raise it and fill voids.
- Polyurethane foam lifting: expanding foam is injected through smaller holes. It's lightweight and cures quickly.
- Crack and joint sealing: flexible sealants keep water out, which matters a lot in a climate this wet.
- Patching: spalled spots can be patched with repair mortar, though patches rarely match perfectly.
- Resurfacing: a thin cement-based overlay can renew a scaled or worn surface that is still structurally sound.
- Section replacement: badly broken panels are cut out at joints and repoured.
When lifting makes sense
Lifting works best when the slab is mostly intact, has sunk as a unit, and the soil underneath has stopped moving. If water is still washing soil out, fix that first by extending downspouts, regrading or adding drainage, or the slab will sink again. Badly broken slabs are usually better replaced.
Trip hazards and public sidewalks
An offset joint on a walk or driveway is a common cause of falls. Contractors can lift the low side, grind the high edge or replace the panel. In the City of Seattle and in Tacoma, the adjacent property owner is responsible for the public sidewalk, and repairs in the right-of-way require a permit. In other cities, check with public works about who is responsible before work begins.
When it might be structural or geotechnical
Cracks in a patio or garage floor are usually not structural. But cracks in foundation walls, doors and windows that suddenly stick, retaining walls that lean, or ground cracks on a slope can signal foundation movement or slope instability. Seattle has mapped steep slope and landslide-prone areas, and parts of Renton and other Eastside cities sit over old coal mine workings that local codes address. In those situations, start with a structural or geotechnical engineer, not a concrete crew.
Questions to ask
- What caused the problem, and how will the repair address it?
- Will you lift, patch or replace, and why?
- How will drill holes or patches look afterward?
- Is there a warranty if a lifted slab settles again?
Have a sunken slab or a trip hazard in Seattle or a nearby city? Mark your request as urgent and request a concrete repair contractor.
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Frequently asked questions
Is mudjacking or foam lifting better?
Both can lift sunken slabs. Cement slurry lifting is long established and often less expensive; polyurethane foam uses smaller holes, is lighter and cures quickly. The best choice depends on the slab, the voids and the contractor's experience.
Why does my slab keep sinking at the same corner?
Usually water. A downspout, runoff from higher ground or water running along dense till under the slab can keep washing soil away. Fix the water source, then lift or replace the slab.
When should I call an engineer instead of a concrete contractor?
If you see cracks in foundation walls, sticking doors and windows, a leaning retaining wall or ground cracks on a slope, start with a structural or geotechnical engineer. Those signs can point to movement a slab repair won't fix.
Sources and further reading
- Washington State Department of Labor & Industries: Register as a contractor
- Washington Utilities and Transportation Commission: Call before you dig
- Seattle Department of Transportation: Property owners' responsibilities
- Streets Illustrated (Seattle right-of-way manual): Construction permits
- Seattle Department of Construction and Inspections: Environmentally Critical Areas code
- Seattle Public Utilities
- City of Bellevue: Right-of-way permits
- City of Redmond: Right-of-way use permit
- City of Tacoma: Sidewalks
- City of Renton Municipal Code: Critical areas regulations
- Washington State University Extension
- USDA NRCS Web Soil Survey
- American Concrete Institute (ACI)
- Portland Cement Association (PCA)