How to Fix a Sunken Concrete Path Section Yourself

A sunken concrete path section is more than an eyesore — it's a genuine trip hazard and, left untreated, it will only get worse. The good news is that minor settlement can often be tackled by a capable DIYer over a weekend, without needing a building consent in most cases. Here's what you need to know before you grab a shovel.

Why Do Concrete Path Sections Sink?

Understanding the cause is the first step, because the fix depends on it. Common reasons include:

• Soil erosion or washout beneath the slab, often from poor drainage or leaking pipes
• Tree root activity lifting one edge and leaving a void under another
• Poor compaction of the sub-base when the path was originally laid
• Clay soils shrinking during dry summers — a common issue across many NZ regions
• Heavy vehicle loads on a path not designed for them

If you suspect a leaking pipe is the culprit, get that sorted first — otherwise any repair will fail again quickly. Plumbing work that involves drains or supply pipes may be restricted work requiring a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board.

Assessing the Damage

Before deciding on a repair method, check the following:

• How much has it sunk? A drop of up to about 50 mm is generally manageable for DIY. More than that, or if the slab has cracked badly, you may need professional help.
• Is the slab still structurally sound? Tap it with a hammer — a hollow sound suggests a large void underneath.
• Are the edges crumbling? Spalled or crumbling concrete won't hold a repair well.
• Is there an obvious void? Probe around the edges with a screwdriver or thin rod.

Your Main Repair Options

Option 1 – Lift and Re-bed the Slab

If the slab is intact and the void beneath it is accessible, you can lift the slab, re-compact the base, and re-lay it. This is the most reliable DIY method for path slabs.

What you'll need:

• Pry bar or slab lifter
• Compactor or hand tamper
• Crusher dust or AP20 compactable gravel
• Dry mortar mix (or a bagged bedding mix from Bunnings or Mitre 10)
• Safety gloves and eye protection

Steps:

1. Use a pry bar to carefully lever up the sunken slab. Have a helper steady it — concrete slabs are very heavy.
2. Remove loose soil and debris from the void.
3. Fill the void with crusher dust or AP20 gravel in layers no deeper than 75 mm, compacting each layer firmly.
4. Spread a thin, even bed of dry mortar mix (or a purpose-made bedding sand) over the compacted base.
5. Lower the slab back into position, checking it is level with adjacent sections using a spirit level.
6. Tap it down gently with a rubber mallet.
7. Fill the joints with fine sand or a jointing compound and compact it in.

Option 2 – Grout Injection (Mudjacking / Slab Lifting)

For larger or heavier slabs, grout injection (sometimes called mudjacking or slab lifting) involves pumping a cement-based slurry under the slab to fill voids and raise it back to level. This is typically a job for a specialist contractor, though it's worth knowing about as an alternative to full replacement.

Option 3 – Grind the High Edge

If only one edge has lifted slightly (say, 10–15 mm) and the slab itself is sound, grinding down the raised edge with an angle grinder fitted with a diamond cup wheel can remove the trip hazard quickly. This doesn't fix the underlying settlement but is a practical short-term solution.

Filling Cracks and Gaps

Once the slab is re-levelled, fill any cracks or control joints with a flexible polyurethane sealant or a purpose-made concrete crack filler. Products from Sika are widely available and well-suited to NZ conditions. Avoid rigid fillers in expansion joints — they need to flex.

Sub-base and Drainage Tips

A well-drained sub-base is the key to a long-lasting concrete path. When re-bedding your slab:

• Ensure the path still has a slight crossfall (typically 1:50 to 1:40) to shed water away from the house
• Check that surface water isn't pooling against the foundation — this relates to NZ Building Code clause E1 (Surface Water)
• Consider adding a strip drain or channel drain if water is consistently tracking under the path

For guidance on drainage and weathertightness, BRANZ is an excellent NZ resource.

Do You Need a Building Consent?

In most cases, repairing or replacing an existing residential concrete path does not require a building consent under Schedule 1 of the Building Act 2004. However, rules can vary — always confirm with your local council before starting work, especially if the path is close to a boundary or retaining structure. Check MBIE Building Performance for general guidance.

Safety Notes

• Concrete slabs are extremely heavy — a standard 600 × 600 × 75 mm slab can weigh over 60 kg. Use proper lifting technique and get a helper.
• Wear gloves, safety glasses, and a dust mask when cutting, grinding, or mixing concrete.
• If using an angle grinder, review safe use guidelines at WorkSafe NZ.
• Wet concrete is caustic — avoid prolonged skin contact.

When to Call a Professional

Call a qualified concreter if:

• The slab is heavily cracked, crumbling, or larger than you can safely lift
• There is a significant void suggesting ongoing erosion or a drainage problem
• Multiple sections have sunk, pointing to a systemic sub-base failure
• You suspect a broken underground pipe is the cause (call a licensed plumber or drainlayer)
• The path is load-bearing or adjacent to a retaining wall or foundation

A professional concreter can assess whether repair or full replacement is the better long-term investment. You can find qualified local concreters on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Building Research Association of NZ
• MBIE Building Performance – Building Consent information
• WorkSafe NZ – Tool and equipment safety
• Sika NZ – Concrete repair and sealant products
• Bunnings NZ – Concrete and paving materials
• Plumbers Gasfitters & Drainlayers Board – Licensed tradespeople