Using the right geotextile fabric behind a retaining wall is one of the most important — and most overlooked — steps in getting a wall that lasts. The fabric acts as a filter, allowing water to drain freely while preventing soil from migrating into your drainage aggregate and blocking it up. Get it right and your wall will perform well for decades; get it wrong and you risk hydrostatic pressure build-up, wall movement, or even failure.
Why Geotextile Fabric Matters
Retaining walls hold back soil, and that soil is almost always wet at some point. Without proper drainage, water pressure (called hydrostatic pressure) builds up behind the wall and can push it over or cause it to bow outward. A geotextile fabric — placed between the retained soil and the drainage aggregate (usually 20–40 mm clean crushed rock) — stops fine soil particles from washing into the aggregate and clogging it over time.
The BRANZ guidelines and NZ Building Code requirements for retaining walls emphasise that drainage must be designed to handle the expected water load for the site. Geotextile fabric is a core part of that drainage system.
Types of Geotextile Fabric
There are two main types you'll encounter at NZ suppliers:
Nonwoven Geotextile
- Made from needle-punched or heat-bonded synthetic fibres (usually polypropylene or polyester)
- Best for drainage and filtration — the most common choice behind retaining walls
- Allows water through while filtering fine soil particles
- Available in various weights (grams per square metre, or GSM), typically 100–300 GSM for residential retaining walls
- Softer and more conformable — easier to work with on uneven surfaces
Woven Geotextile
- Made from interlocked synthetic yarns
- Higher tensile strength — better suited to separation and reinforcement applications (e.g., under driveways or in road construction)
- Less effective as a filter fabric because the uniform openings can allow fine particles through or block more easily
- Generally not the first choice directly behind a retaining wall unless specified by an engineer
For most residential retaining walls in NZ, a nonwoven geotextile of 130–200 GSM is the standard recommendation. Heavier weights (200–300 GSM) may be specified for walls retaining clay-heavy soils or in high-water-table situations.
Choosing the Right Weight (GSM)
The weight and pore size of the fabric need to match your soil type. As a general guide:
| Soil Type | Recommended GSM | Notes |
|---|---|---|
| Sandy or gravelly soil | 100–130 GSM | Good drainage, lower filtration demand |
| Silty or loamy soil | 130–200 GSM | Most common NZ residential situation |
| Heavy clay soil | 200–300 GSM | Slow-draining; higher filtration needed |
If you're unsure of your soil type, a local geotechnical engineer or experienced retaining wall builder can assess it. You can find qualified retaining wall builders on tradedirectory.co.nz.
Where to Buy Geotextile Fabric in NZ
Geotextile fabric is widely available from:
- Bunnings — stocks nonwoven geotextile rolls suitable for garden and retaining wall use
- Mitre 10 — carries landscape geotextile in various widths
- PlaceMakers — trade-focused supplier with a range of civil and drainage products
Always check the product specification sheet to confirm the GSM, material type, and intended application before purchasing.
How to Install Geotextile Fabric Behind a Retaining Wall
- Excavate and prepare the area behind the wall, ensuring the base is stable and the drainage aggregate layer is planned.
- Lay the fabric against the retained soil face, running it up the back of the wall and along the base. Overlap joins by at least 300 mm.
- Backfill with clean drainage aggregate (typically 20–40 mm crushed rock) against the fabric, keeping the fabric in place as you go.
- Fold the top of the fabric over the top of the aggregate before placing topsoil — this stops soil from filtering down into the drainage layer from above.
- Ensure a subsoil drain or ag pipe sits within the aggregate at the base, directing water to a legal discharge point.
Do not use the fabric as a substitute for drainage aggregate — it works with the aggregate, not instead of it.
Building Consent and NZ Building Code
In NZ, retaining walls over 1.5 metres in height generally require a building consent, though thresholds and conditions vary — walls retaining a slope, or near boundaries or structures, may require consent at lower heights. Even walls under 1.5 m may require consent depending on your council's rules and site conditions. Always check with your local council before starting work. The MBIE Building Performance website has useful guidance on when consent is required and what Schedule 1 exemptions may apply.
For consented walls, the drainage and geotextile specification may need to be designed or approved by a suitably qualified engineer or other professional. Note that LBPs are licensed for restricted building work related to the structure of buildings — retaining wall drainage design is typically an engineering matter rather than an LBP scope matter. Check the LBP register to find a qualified professional.
When to Call a Professional
Call a qualified retaining wall builder or geotechnical engineer if:
- Your wall will be over 1.0 m high (and especially over 1.5 m)
- The site has heavy clay soils, a high water table, or poor drainage
- The wall is near a boundary, building, or slope
- You're unsure about soil type, drainage design, or fabric specification
- A building consent is required
Getting the drainage design wrong can lead to costly wall failure and potential liability. A professional will specify the correct fabric weight, drainage aggregate, and pipe sizing for your specific site conditions. Find experienced retaining wall builders near you on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
