How to Fix a Concrete Slab That Has Lifted at the Edges

Lifted or curled edges on a concrete slab are a frustrating but common problem for NZ homeowners. The good news is that minor edge lifting can often be stabilised or repaired with the right approach — but more serious movement may need a licensed concreter to assess and fix it properly.

Why Do Concrete Slab Edges Lift?

Understanding the cause is the first step to choosing the right fix. The most common reasons include:

• Soil shrinkage or erosion – In dry summers, clay-heavy NZ soils can shrink and pull away from the slab edge, leaving it unsupported.
• Tree root intrusion – Roots growing beneath or alongside the slab can push edges upward over time.
• Poor compaction during construction – If the sub-base wasn't properly compacted before the pour, settlement and movement are likely.
• Water ingress and washout – Heavy rain or poor drainage can wash away the material beneath slab edges, causing them to drop or tilt. This is especially relevant in wetter NZ regions.
• Concrete curling – Differential drying between the top and bottom of a slab can cause edges to curl upward, particularly in thinner slabs.
• Frost heave – Less common in most of NZ, but in colder inland or southern areas, freeze-thaw cycles can lift slabs.

Assessing the Damage First

Before reaching for a bag of concrete mix, take time to assess the situation:

1. Measure the lift – Use a straight edge or spirit level. A lift of less than 10–15 mm may be manageable as a DIY repair; anything greater warrants professional assessment.
2. Check for cracking – Surface cracks radiating from the lifted edge suggest structural movement, not just cosmetic curling.
3. Look underneath – If you can see a void beneath the slab edge, the sub-base has washed out or settled significantly.
4. Check drainage – Is water pooling near the slab? Fixing drainage is essential before any repair, otherwise the problem will return.

For guidance on what the NZ Building Code requires for concrete floor slabs (particularly around moisture and drainage), refer to Clause E1 (Surface Water) and Clause E2 (External Moisture).

DIY Repair Options for Minor Edge Lifting

Option 1: Grinding Down a Raised Edge

If the slab has curled slightly upward and the edge is a trip hazard, grinding it down is often the simplest fix. You can hire an angle grinder with a diamond cup wheel from most hire shops. Work slowly, keeping the grind gradual and feathered so there's no abrupt step. This doesn't fix the underlying cause but removes the hazard.

Safety note: Always wear eye protection, a P2 dust mask, and hearing protection when grinding concrete. Concrete dust contains silica, which is a serious respiratory hazard. See WorkSafe NZ for guidance on working safely with silica dust.

Option 2: Filling a Void and Re-bedding the Edge

If the edge has lifted because the sub-base has washed away, you can attempt to fill the void:

1. Clear out any loose material, debris, or roots from beneath the slab edge.
2. Use a flowable grout or cementitious filler — products from Sika or Ardex are well-suited for this kind of repair and are available through trade suppliers and some hardware stores.
3. Pour or pump the grout into the void, working from one end to the other.
4. Allow to cure fully before loading the slab.

Note: This approach works best when the slab itself is structurally sound and the lift is minor. It does not push the slab back down — it simply supports it in its current position.

Option 3: Polyurethane Foam Lifting (Slab Jacking)

Slab jacking (also called foam lifting or poly lifting) involves injecting expanding polyurethane foam beneath the slab to fill voids and gently raise it back to level. This is a specialist technique — while the concept is straightforward, the equipment and skill required mean it's generally best left to a professional. Some NZ concreters and specialist ground-engineering companies offer this service.

Option 4: Crack and Joint Sealing

Once the slab is stabilised, seal any cracks or open joints at the edge using a flexible polyurethane sealant (such as those available from Sika). This helps prevent water getting back under the slab and repeating the cycle.

Fixing the Drainage Problem

No concrete repair will last if water continues to undermine the slab. Check that:

• Downpipes discharge well away from the slab edge
• Ground slopes away from the slab (a fall of at least 1:50 is generally recommended)
• Garden beds adjacent to the slab aren't retaining water against the edge

If you need to install or modify stormwater drainage, be aware that some drainage work is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed drainlayer. Check with the Plumbers Gasfitters & Drainlayers Board if you're unsure.

Does This Work Need a Building Consent?

In most cases, repairing an existing slab does not require a building consent under Schedule 1 of the Building Act — but this depends on the scope of work. If you're replacing a significant portion of the slab or altering the structure, check with your local council first. MBIE Building Performance has useful guidance on consent exemptions.

When to Call a Professional

Call a licensed concreter or structural engineer if:

• The lift is greater than 15–20 mm
• There are significant cracks, especially diagonal or structural-looking ones
• The slab is part of a building foundation or supports a structure
• You suspect tree roots are the cause (removal may be needed first)
• The void beneath is large or the sub-base has significantly failed
• DIY repairs haven't held

A qualified concreter can assess whether the slab needs to be broken out and re-poured, or whether a less invasive fix is appropriate. You can find experienced local concreters on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – NZ Building Code
• WorkSafe NZ – Silica Dust Safety
• Sika NZ – Concrete Repair Products
• Ardex NZ – Flooring and Repair Systems
• BRANZ – Building Research
• Plumbers Gasfitters & Drainlayers Board