How to Level a Sunken Section of Your Concrete Driveway

A sunken section of concrete driveway is a common problem in New Zealand homes, caused by soil settlement, erosion, tree roots, or poor compaction during the original pour. The good news is that depending on the severity, you may be able to fix it yourself or have it professionally lifted without replacing the entire slab.

Why Does Concrete Sink?

Before choosing a repair method, it helps to understand what caused the sinking. Common culprits in NZ include:

• Soil erosion or washout — heavy rainfall (especially in wetter regions) can wash away the sub-base beneath a slab
• Poor compaction — if the ground wasn't properly compacted before the original pour, it will settle over time
• Tree roots — roots can both lift and, once they die and rot, cause voids that lead to sinking
• Clay soils — NZ has a lot of expansive clay soils that shrink and swell with moisture changes, causing movement
• Utility trenches — poorly backfilled trenches for pipes or cables can settle unevenly

Identifying the cause matters because if the underlying problem isn't fixed, the slab will sink again.

Assessing the Damage

Before starting any repair, assess the slab carefully:

• Measure the drop — use a long straight edge or spirit level. A drop of up to 50 mm is generally repairable; larger drops may require slab replacement.
• Check for cracks — hairline cracks are normal, but wide or structural cracks suggest the slab may be broken rather than just sunken.
• Look underneath — if you can see a void beneath the slab edge, that confirms sub-base loss.
• Check drainage — make sure water isn't pooling toward your house, which could breach NZ Building Code requirements around surface water management (Clause E1).

Repair Options

1. Polyurethane Foam Lifting (Polyjacking)

This is the most popular modern method. A contractor drills small holes (around 16 mm) through the slab, then injects expanding polyurethane foam underneath. The foam fills voids and lifts the slab back to level within minutes.

Pros: Fast, minimal disruption, lightweight, long-lasting, holes are small and easily patched. Cons: Requires specialist equipment — this is not a DIY job. As a rough guide, expect costs to vary significantly depending on the area to be lifted; get at least two quotes.

2. Mudjacking (Slabjacking)

A slurry of cement, soil, and water is pumped under the slab through larger drilled holes to lift it. This is an older technique still used by some NZ contractors.

Pros: Cost-effective for large areas, uses readily available materials. Cons: Heavier than foam (adds load to sub-base), larger drill holes, longer curing time, and the slurry can wash out again over time.

3. Grinding High Spots or Feathering Edges

If the adjacent slab has risen rather than the sunken one dropping, a concrete grinder can be used to reduce the trip hazard at the joint. This is a surface-only fix and doesn't address the underlying void.

4. Concrete Resurfacing or Topping

For minor settlement of 10–20 mm, a self-levelling concrete topping or polymer-modified screed can be applied over the sunken area. Products from Ardex or Sika are well-regarded for this application. Always follow the manufacturer's preparation and priming instructions — adhesion is critical outdoors.

Important: This method only works if the slab itself is stable. If there's a void underneath, a topping will crack and fail.

5. Full Slab Replacement

If the slab is badly cracked, broken into multiple pieces, or has dropped more than 50–75 mm, replacement is often the most cost-effective long-term solution. This involves breaking out the old concrete, re-compacting and re-filling the sub-base, and pouring a new slab.

DIY Concrete Patching: Step-by-Step

If the sunken area is small and stable (no void underneath), you can apply a topping yourself:

1. Clean the surface thoroughly — remove all dirt, oil, and loose material. A pressure washer works well.
2. Apply a concrete bonding primer — products like those from Ardex or Sika improve adhesion significantly. Don't skip this step outdoors.
3. Mix your topping compound according to the manufacturer's instructions. For feathered edges, use a flexible polymer-modified product.
4. Apply and screed to the desired level, blending into surrounding concrete.
5. Cure properly — keep the repair damp and out of direct sun for the recommended period (usually 24–48 hours minimum).
6. Seal if desired — a concrete sealer helps protect the repair from NZ's UV and rain.

Materials for a small patch repair are available from Bunnings, Mitre 10, or PlaceMakers.

Do You Need a Building Consent?

In most cases, repairing or resurfacing an existing driveway slab does not require a building consent under Schedule 1 of the Building Act. However, if you're replacing a large area of concrete or altering drainage, always check with your local council first — rules vary by territorial authority. See MBIE Building Performance for general guidance on consent exemptions.

When to Call a Professional

Call a qualified driveway layer or concrete contractor if:

• The slab has dropped more than 30–50 mm
• There is a visible void beneath the slab
• The concrete is cracked into multiple pieces
• The sunken area is near your house foundation or affects drainage toward the building
• You want polyurethane foam or mudjacking — these require specialist equipment

A professional can also identify whether a plumbing leak is causing the washout — if so, that's restricted work requiring a licensed drainlayer or plumber registered with the Plumbers, Gasfitters and Drainlayers Board.

You can find qualified local driveway layers and concrete contractors on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Building Code and consents
• BRANZ — Concrete and construction guidance
• Sika NZ — Concrete repair products
• Ardex NZ — Flooring and concrete repair systems
• WorkSafe NZ — Safe use of power tools and equipment
• Plumbers, Gasfitters and Drainlayers Board — Licensed tradespeople