Soil washing out between retaining wall sleepers is one of the most common problems NZ homeowners face, especially in areas with heavy rainfall or clay-heavy soils. The good news is that with the right combination of drainage, geotextile fabric, and proper backfill, you can stop erosion in its tracks — whether you're building a new wall or fixing an existing one.
Why Soil Washes Out in the First Place
Water is the main culprit. When rain saturates the soil behind a retaining wall, hydrostatic pressure builds up and forces water — along with fine soil particles — through any gaps between the sleepers. Poor drainage and missing or damaged geotextile fabric are almost always to blame. NZ's variable climate, from the wet West Coast to Auckland's heavy summer downpours, makes this a widespread issue.
Use Geotextile (Filter) Fabric — Your Most Important Tool
Geotextile fabric (sometimes called filter cloth or landscape fabric) is the single most effective barrier against soil loss. It allows water to pass through freely while holding soil particles back.
How to install it:
- Line the entire back face of the sleepers with geotextile fabric before backfilling.
- Overlap joins by at least 300 mm so there are no gaps.
- Fold the fabric over the top of the drainage aggregate before adding topsoil.
- Use a non-woven geotextile rated for retaining wall use — these are available at Bunnings and Mitre 10.
If your wall is already built and soil is washing out, you may be able to push geotextile fabric down behind the sleepers using a flat spade, though this is easier said than done on an established wall.
Get Your Drainage Right
Geotextile fabric alone won't solve the problem if water has nowhere to go. Proper drainage behind the wall relieves hydrostatic pressure and reduces the volume of water pushing through the gaps.
Key drainage steps:
- Drainage aggregate: Backfill the first 300–500 mm directly behind the sleepers with clean, free-draining aggregate (typically 20 mm crushed stone or similar). This creates a drainage blanket.
- Ag pipe (agricultural drain): Lay a perforated ag pipe (wrapped in sock filter if possible) at the base of the wall, sloping to a discharge point. This carries water away before it builds up.
- Weep holes: If you're building a new wall, leave small gaps or drill weep holes near the base of the sleepers to allow water to escape. Space these roughly every 1–1.5 m.
Good drainage design is also a requirement under the NZ Building Code, particularly Clause E1 (Surface Water) and E2 (External Moisture), which aim to prevent water damage to structures and neighbouring properties.
Correct Backfill Material Matters
What you put behind the wall is just as important as the drainage layer. Avoid using:
- Pure clay — it holds water and swells, increasing pressure on the wall.
- Topsoil alone — it compacts poorly and erodes easily.
Instead, use a well-graded granular fill or a mix of topsoil and aggregate for the upper layers, placed in compacted lifts of around 150–200 mm. This reduces settlement and limits the amount of fine material that can migrate toward the wall face.
Planting and Surface Stabilisation
Once the wall is built and backfilled, stabilising the soil surface above and around the wall helps enormously:
- Ground cover plants with fibrous root systems bind the soil and reduce surface runoff.
- Mulch (bark or wood chip, 75–100 mm deep) slows rainwater impact and reduces erosion.
- Grass or turf on the slope above the wall intercepts rainfall before it saturates the soil.
For steeper banks, consider erosion control matting (coir or jute mesh) pegged over newly planted areas while plants establish.
A Quick Comparison: Drainage Solutions
| Method | Best For | Approximate Cost Guide |
|---|---|---|
| Geotextile fabric | All walls — essential | Low (DIY-friendly) |
| Drainage aggregate backfill | New builds and rebuilds | Low–moderate |
| Perforated ag pipe | Walls with high water table | Moderate |
| Weep holes in sleepers | New timber or concrete sleeper walls | Low |
| Ground cover planting | Surface stabilisation above wall | Low–moderate |
Cost ranges are a guide only and will vary by site, materials, and labour.
Building Consent — Do You Need One?
In New Zealand, retaining walls over a certain height generally require a building consent (the threshold can depend on factors such as surcharge loading, proximity to boundaries, and local council rules — check with your council for the specific trigger height that applies to your site). Walls below the relevant threshold may be exempt under Schedule 1 of the Building Act, but this depends on your specific site conditions, soil type, and proximity to boundaries or structures. Always check with your local council before starting work — don't assume your wall is exempt. You can find general guidance at MBIE Building Performance.
For technical guidance on retaining wall design and construction, BRANZ is an excellent NZ resource.
When to Call a Professional
If your retaining wall is:
- Over 1.5 m high
- Showing signs of leaning, cracking, or bulging
- On a steep or unstable slope
- Near a boundary, driveway, or structure
…then you should engage a qualified retaining wall builder or a geotechnical engineer before doing anything else. A wall failure can be dangerous and costly. Licensed Building Practitioners (LBPs) can carry out restricted building work — check the LBP register to find a qualified professional. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
