If a concrete slab has sunken or settled, resurfacing it without addressing the problem first is a costly mistake. Sunken sections cause uneven overlays, cracking, water pooling, and structural failure. Before any microcement, overlay, or resurfacer can be applied, the base slab must be stable, properly leveled, and free of voids. In this guide, we'll walk you through exactly how to repair sunken concrete sections the right way — whether indoors or outdoors.
Start by carefully inspecting the slab. Use a long straightedge, laser level, or water level to measure settlement depth across the affected area. Note the maximum deflection (how low it’s sunk) and whether surrounding areas have cracked, tilted, or separated. Minor settlement under 20mm can sometimes be corrected with surface leveling. Major settlement or widespread voiding often needs slab lifting or sub-base filling.
Sunken slabs usually result from poor compaction, soil erosion, water leaks, organic subsoils, or freeze-thaw damage. Investigate nearby plumbing lines, drainage systems, or evidence of washout under the slab. Fixing the cause is just as important as fixing the slab — otherwise, the problem will recur. Seal leaks, stabilize soils, or improve drainage before any lifting or resurfacing work is done.
Minor sunken spots (under 10mm) can often be leveled with patch repair or feather edge resurfacing. Moderate settlement may require bonded screeds or self-leveling overlays. Major sunken slabs may need slabjacking (mudjacking) or polyurethane foam lifting, where material is injected beneath the slab to raise it back into position. Always match the repair technique to the severity of settlement to avoid further cracking or collapse.
Clean the sunken area thoroughly before patching or lifting. Grind or scarify any smooth laitance, remove contaminants, and vacuum all dust and debris. If large cracks are associated with settlement, chase and fill them first. Proper surface prep ensures strong bonding of fillers, screeds, or overlays. Skipping cleaning is one of the top reasons leveling and repairs fail later.
For light settlement, use polymer-modified patch mortars or self-leveling compounds. Prime the surface if required. Trowel or pour material evenly to bring low spots flush with the surrounding slab. For more serious settlement, hire professionals to perform slabjacking (pumping a grout slurry underneath) or polyurethane injection (expanding foam lifting). These techniques recompact the soil and raise slabs accurately with minimal disruption.
Allow patching or leveling compounds to fully cure per manufacturer instructions. Then lightly grind the repaired areas flush with the rest of the slab. Recheck levels with a straightedge or laser. Minor spot touch-ups may be needed to achieve perfectly flat surfaces. Only once the slab is sound, level, and stable should you proceed with priming and resurfacing.
After repairing, address the root causes of the original sinking. Improve surface drainage, reroute downspouts, fix plumbing leaks, stabilize soils, and avoid loading edges with heavy weights. Preventing water infiltration and subsidence extends the life of both the slab and your new resurfacing layer — reducing risk of future cracks, hollow spots, or overlay failures.
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