A retaining wall without proper drainage is a wall waiting to fail. Water that builds up behind a wall creates hydrostatic pressure — a force that can push even a well-built wall over time. Getting the drainage right from the start is just as important as the wall's structural design, and in many cases it's a legal requirement under the NZ Building Code.
Why Drainage Matters So Much
New Zealand's climate — particularly in regions like Auckland, Wellington, and the West Coast — means retaining walls regularly face heavy rainfall. When water saturates the soil behind a wall and has nowhere to go, pressure builds rapidly. This is one of the leading causes of retaining wall movement, cracking, and collapse. Good drainage protects your investment and keeps your property safe.
BRANZ research consistently highlights poor drainage as a key factor in retaining wall failures across NZ.
Does Your Wall Need a Building Consent?
Before you start any work, check whether your retaining wall requires a building consent. Under the NZ Building Code (MBIE), retaining walls that retain more than 1.5 metres of ground, or any wall retaining ground near a boundary or surcharge load (like a driveway or building), generally require consent. Drainage design is typically part of the consented plans.
Even exempt walls should still be built to good practice standards — a collapsed wall can cause serious harm and liability.
The Three Key Components of Retaining Wall Drainage
Effective drainage behind a retaining wall relies on three elements working together:
1. Free-Draining Backfill
Backfill is the material placed directly behind the wall. Never use clay or heavy silty soil as backfill — these hold water and dramatically increase pressure. Instead, use:
- Crushed aggregate (typically 20–40 mm clean chip or drainage metal)
- Scoria (lightweight and very free-draining, popular in NZ)
- Free-draining gravel
The drainage layer should extend at least 300–600 mm back from the wall face, from the base up to near the surface. Your engineer or consent documents may specify exact dimensions.
2. Ag Pipe (Perforated Drainage Pipe)
Agricultural pipe (ag pipe) is a perforated flexible pipe laid at the base of the wall to collect and carry water away. Key installation points:
- Lay ag pipe at the base of the drainage aggregate, sitting on a bed of clean chip
- Wrap the pipe in geotextile filter fabric (sock) to prevent fine particles blocking it over time
- Ensure the pipe has a fall (slope) of at least 1:100 toward an outlet — ideally to a stormwater drain, soakpit, or daylight at a lower point on the property
- Never discharge directly onto a neighbour's property
You can find ag pipe and geotextile fabric at Mitre 10, Bunnings, or PlaceMakers.
3. Weepholes
Weepholes are openings through the wall face that allow water to escape if the ag pipe system becomes overwhelmed. They act as a secondary relief valve.
- For concrete block or timber walls, space weepholes roughly every 1–2 metres along the base course
- Keep weepholes clear — never block them with soil or landscaping
- In masonry walls, a missing mortar joint at the base course is a simple way to create weepholes
Geotextile Filter Fabric — Don't Skip It
Geotextile fabric (also called filter cloth or drainage membrane) is placed between the native soil and the free-draining aggregate. It allows water to pass through while stopping fine soil particles from migrating into and clogging your drainage layer. Without it, even the best drainage system can silt up within a few years. Wrap it around the aggregate layer like a blanket, overlapping joins by at least 300 mm.
Surface Water Management
Drainage behind the wall is only part of the picture. You also need to manage surface water so it doesn't all funnel behind the wall:
- Grade the ground behind the wall slightly away from it where possible
- Install surface channels or swales to intercept runoff before it reaches the wall
- Consider downpipe outlets — never direct roof water directly behind a retaining wall
A Quick Comparison: Drainage Options
| Method | Best For | Key Advantage |
|---|---|---|
| Crushed aggregate backfill | All wall types | Simple, effective, widely available |
| Ag pipe with sock | Walls over ~1 m high | Actively carries water to outlet |
| Weepholes | Masonry/concrete block walls | Low-cost secondary relief |
| Geotextile fabric | All drainage systems | Prevents silting of aggregate |
| Surface grading/swales | All sites | Reduces water entering system |
When to Call a Professional
Retaining wall drainage can seem straightforward, but there are situations where you should always engage a qualified professional:
- Walls over 1.5 m high — these almost certainly require a building consent and engineering input
- Walls near boundaries, buildings, or slopes — failure risk is higher and liability is significant
- Poor or unknown soil conditions — a geotechnical assessment may be needed
- Connecting to stormwater systems — in many councils this is restricted work requiring a licensed drainlayer (see PGDB)
- Any wall that has already moved or cracked — get a structural assessment before attempting repairs
For walls requiring a building consent, the design and construction may need to be carried out or supervised by a Licensed Building Practitioner (LBP). You can find qualified retaining wall builders and drainlayers in your area on tradedirectory.co.nz.