Cracks in a concrete block retaining wall are common in New Zealand, especially given our varied soils, seismic activity, and wet winters. Some cracks are purely cosmetic and easy to fix yourself, while others signal structural failure that needs urgent professional attention. Before reaching for the mortar, it pays to understand what you're dealing with.
Assess the Crack Before You Do Anything
Not all cracks are equal. Take time to examine the wall carefully before starting any repair work.
- Hairline cracks (less than 1–2 mm wide) in mortar joints are usually caused by normal shrinkage or minor movement and are generally cosmetic.
- Wider cracks (3 mm or more), cracks running diagonally through blocks, or cracks that are wider at one end than the other suggest structural movement.
- Horizontal cracks running along mortar joints are a serious warning sign — they can indicate the wall is bowing under soil pressure and may be close to failure.
- Leaning or bulging sections mean the wall has moved and must be assessed by a structural engineer before any repair.
If you're unsure, photograph the cracks and monitor them over a few weeks by marking the ends with a pencil and date. If they grow, call a professional immediately.
Understand Why Retaining Walls Crack
Identifying the cause helps you fix the right problem. Common causes in NZ include:
- Poor or blocked drainage — water building up behind the wall increases soil pressure dramatically. Check whether your wall has weep holes (small drainage gaps at the base) and whether they are clear.
- Inadequate footing depth — walls built without a proper concrete footing can shift with frost, wet soil, or seismic movement.
- Overloading — heavy vehicles, soil stockpiles, or new structures placed near the top of the wall add unexpected load.
- Age and weathering — older mortar becomes brittle and crumbles over time.
- Seismic movement — New Zealand's earthquake-prone environment can shift even well-built walls.
NZ Building Code and Consent Considerations
Retaining walls in New Zealand are regulated under the NZ Building Code. As a general guide, retaining walls over 1.5 metres in height (measured from the bottom of the retained soil to the top of the wall) typically require a building consent from your local council. Walls under this height may be exempt under Schedule 1 of the Building Act, but this depends on factors such as proximity to boundaries, slopes, and surcharge loads.
Always confirm consent requirements with your local council before carrying out significant repairs or rebuilding. You can find general guidance at MBIE Building Performance. For walls over 1.5 m or in complex ground conditions, a Licensed Building Practitioner (LBP) or structural engineer must be involved. Check the LBP register at Licensed Building Practitioners.
Tools and Materials You'll Need
| Item | Purpose |
|---|---|
| Cold chisel and hammer | Raking out loose mortar |
| Wire brush | Cleaning joints and crack faces |
| Masonry repair mortar or hydraulic cement | Filling cracks and repointing |
| Pointing trowel | Applying and finishing mortar |
| Safety glasses and gloves | Personal protection |
| Garden hose | Dampening surfaces before repair |
For crack fillers and repair mortars, products from Sika or Ardex are widely available at Bunnings, Mitre 10, and PlaceMakers. Always follow the manufacturer's instructions for mixing ratios and curing times.
Step-by-Step: Repairing Minor Cracks and Repointing
This process suits hairline to moderate cracks in mortar joints where the wall is structurally sound and not moving.
1. Clear and Clean the Area
Using a cold chisel and hammer, carefully rake out any loose, crumbling, or cracked mortar to a depth of at least 15–20 mm. Remove all dust and debris with a wire brush and compressed air or a hose.
2. Dampen the Surface
Lightly wet the cleaned joint or crack with water. This prevents the dry masonry from drawing moisture out of the new mortar too quickly, which causes weak, crumbly repairs.
3. Mix and Apply Repair Mortar
Mix your masonry repair mortar or use a pre-mixed product according to the manufacturer's instructions. Press the mortar firmly into the crack or joint using a pointing trowel, working it in layers if the gap is deep. Avoid overfilling — finish flush with or very slightly recessed from the block face.
4. Cure and Protect
Keep the repair damp for at least 24–48 hours (mist with water or cover with damp hessian) to allow proper curing. Avoid working in direct hot sun or frost.
5. Improve Drainage
Clear any blocked weep holes. If the wall has none, consider drilling 50 mm diameter holes through the base of the wall at regular intervals (every 1–1.5 m) to allow water to escape. Poor drainage is the number one cause of retaining wall failure in NZ.
When to Call a Professional
Some situations are beyond DIY repair and require a qualified builder, structural engineer, or geotechnical engineer:
- The wall is leaning, bulging, or has moved from its original position.
- You see horizontal cracks along mortar joints.
- The wall is over 1.5 m high and showing significant cracking.
- The wall retains soil near a boundary, driveway, or structure.
- You are in a seismic or landslide-prone area (check with your council).
- Cracks are growing or recurring after previous repairs.
A structural engineer can assess whether the wall needs rebuilding, additional reinforcement, or drainage improvements. Don't delay — a failing retaining wall can cause serious property damage or injury. Find qualified local builders and engineers on tradedirectory.co.nz.
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