If your concrete slab rises, cracks, or becomes uneven during winter or after heavy rain, you’re likely dealing with heave. This upward movement is usually caused by frost, expanding clay soil, or poor drainage under the slab. Left unchecked, it can damage floors, tilt structures, or become a trip hazard. In this guide, we explain how to stop slabs from heaving — using site fixes, slab supports, and long-term prevention methods.
Before you fix it, confirm it’s actually heaving — not sinking or settling. Heaving slabs rise or tilt upward, often seasonally. Look for raised corners, gaps under door thresholds, or uneven paths in winter that level out by summer. Settlement usually causes depressions or cracking inward. Knowing which is which determines the right fix.
Most slab heaving begins with excess moisture in the subgrade. Redirect downpipes, fix grading to slope water away from the slab, and add surface drains or French drains where needed. If water sits below the slab, frost or swelling soil will push it upward. Fixing water issues is often the most cost-effective long-term solution to stop seasonal movement.
Tree roots can lift paths, patios, or shed slabs over time. Identify if heave is near visible roots or consistent with one side of a slab. Roots may not only displace the slab, but also affect soil moisture cycles — worsening freeze-thaw effects. Remove offending vegetation and consider installing a root barrier if replanting nearby.
In frost-prone areas, the ground beneath exterior slabs can freeze and expand — lifting the slab from underneath. Install rigid insulation (extruded polystyrene or EPS) vertically along slab edges to reduce frost penetration. In some cases, horizontal insulation laid beneath the slab is used during construction, but edge insulation can still help retroactively.
For slabs already displaced but structurally sound, foam injection (polyjacking) can be used to relevel and stabilise. Holes are drilled and high-density foam injected to fill voids and lift the slab back to position. It also blocks water and insulates the sub-base to reduce future frost damage. This is especially effective for paths, garages, or slabs near thresholds.
If a slab has cracked or shifted beyond repair, replacement may be needed. When re-pouring, excavate deeper and install a well-compacted granular sub-base (MOT Type 1 or hardcore) with proper drainage. Use reinforcement mesh and expansion joints to control future movement. Consider pouring over insulation boards if frost is a known issue.
For new slabs or full replacements, installing a drainage mat beneath the slab can prevent moisture from collecting in fine soil particles and creating frost heave. These synthetic layers act as capillary breaks and drainage planes. This is especially effective in clay-rich soils or poorly draining areas.
If you’re building a new slab, avoid laying directly on topsoil, peat, or organic material. These expand and contract significantly with moisture. Always remove soft material and replace with compacted granular fill before pouring. A floating slab on poor subsoil is almost guaranteed to heave or crack in winter.
Heave tends to follow seasonal cycles. Even after repairs, keep an eye on changes. Monitor joints and measure slab elevation if needed. Maintain nearby drainage systems, clean channels, and reseal joints to stop water returning beneath the slab. Prevention is easier than further repair.
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