A retaining wall that bows or bulges outward is more than an eyesore — it's a structural warning sign that should never be ignored. In most cases, the underlying cause is a build-up of pressure behind the wall that it was never designed to handle, and in many situations a professional assessment is the safest first step before you attempt any repairs yourself.
Why Retaining Walls Bow Outward
Bowing happens when the lateral (sideways) pressure pushing against the back of the wall exceeds what the wall can resist. Several factors commonly cause this in New Zealand conditions:
Hydrostatic Pressure from Poor Drainage
This is the most frequent culprit. When water saturates the soil behind a wall and has nowhere to escape, it creates enormous outward pressure. NZ's high-rainfall regions — think Auckland, the West Coast, and Wellington — make this especially common. A properly built wall should have drainage aggregate and weepholes to relieve this pressure. If these are blocked or were never installed, water builds up and the wall starts to move.
Inadequate Original Design or Construction
Many older retaining walls — particularly those built before modern standards — were undersized for the load they're holding. Under the NZ Building Code and standards like NZS 3604, walls over a certain height (generally 1.5 m, but check with your local council) require engineering input and a building consent. Walls built without these may simply not be strong enough.
Soil Movement and Surcharge Loading
Adding heavy loads near the top of a wall — such as a new driveway, vehicle parking, a deck, or even a large tree — increases the surcharge load on the retained soil. This extra pressure can cause a previously stable wall to start bowing. NZ's expansive clay soils (common in many parts of the North Island) are particularly prone to swelling when wet, adding further pressure.
Deterioration of Wall Materials
Timber retaining walls rot over time, especially in damp NZ conditions. Concrete block walls can suffer from mortar failure or reinforcing corrosion. As materials weaken, the wall loses its ability to resist the same pressures it once handled.
Root Intrusion
Tree and shrub roots growing into or behind a wall can physically push it outward over years. This is a slow but surprisingly powerful force.
How to Assess the Situation Safely
Before doing anything, stand back and look at the whole wall. Ask yourself:
- Is the bowing localised (one section) or along the whole wall?
- Is there cracking, crumbling, or visible leaning?
- Is there soil or water seeping through the wall face?
- Has the movement happened suddenly after heavy rain?
A wall that is actively moving, severely cracked, or retaining a slope above a house, driveway, or public area is a safety emergency. Keep people away from the area and call a professional immediately. WorkSafe NZ notes that wall collapses can cause serious injury or death.
| Severity | Signs | Recommended Action |
|---|---|---|
| Minor | Slight bow, no cracking, stable for years | Monitor closely, improve drainage |
| Moderate | Visible lean, some cracking, weepholes blocked | Get a professional assessment |
| Severe | Rapid movement, large cracks, soil seeping | Evacuate area, call a professional urgently |
What a Homeowner Can Safely Do
For minor issues on low walls (under 1 m) that are stable and not near any structure or public area, there are some things you can do yourself:
- Clear blocked weepholes — use a piece of wire or a garden hose to flush out debris blocking drainage holes at the base of the wall.
- Improve surface drainage — regrade soil at the top of the wall so rainwater runs away from the wall rather than pooling behind it.
- Remove surcharge loads — relocate heavy planters, stored materials, or vehicles parked close to the top of the wall.
- Monitor movement — mark a reference point with a pencil or tape and check it monthly to see if movement is ongoing.
For anything beyond these simple measures — especially repairing the structural integrity of the wall itself — you should get a qualified tradesperson involved. Attempting to push a bowing wall back into place without addressing the root cause will almost certainly fail, and could make the situation more dangerous.
Repair Options a Professional Might Use
A qualified retaining wall builder or structural engineer will assess the cause before recommending a fix. Common solutions include:
- Installing or clearing drainage — adding agricultural drain pipe and drainage aggregate behind the wall to relieve hydrostatic pressure.
- Deadman anchors or tie-backs — horizontal anchors driven into the retained soil to pull the wall back and hold it.
- Rebuilding the wall — sometimes the most cost-effective long-term solution, especially for timber walls that have rotted.
- Installing a new engineered wall — for walls over 1.5 m or in high-risk locations, a consented, engineered solution may be required.
Always check with your local council whether a building consent is required before carrying out repairs. MBIE Building Performance provides guidance on Schedule 1 exemptions, but rules vary by council and wall height.
When to Call a Professional
Call a licensed retaining wall builder or structural engineer if:
- The wall is over 1 m tall or retains a slope above a building, pathway, or public area
- The wall is actively moving or has moved suddenly
- You can see large cracks, gaps, or soil seeping through the wall face
- The wall is below a driveway or vehicle loading area
- You're unsure of the cause or the right repair method
BRANZ recommends that any retaining wall showing signs of structural distress be assessed by a competent professional before any repair work begins. You can find qualified retaining wall builders in your area right here on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
