Treated pine decking boards can technically be used as horizontal retaining wall planks, but there's an important catch: standard decking boards are usually not treated to the correct hazard class for in-ground or ground-contact use. Using the wrong timber can lead to premature rot, structural failure, and potential property damage. Read on to understand what's required before you head to the timber yard.
Understanding Timber Hazard Classes
In New Zealand, treated timber is graded using a Hazard Class (H) system that indicates how much preservative treatment the timber has received and what conditions it can withstand. This system is governed by standards administered by Standards NZ.
Here's a quick guide to the most relevant hazard classes for retaining walls:
| Hazard Class | Typical Use | Ground Contact? |
|---|---|---|
| H3.1 / H3.2 | Above-ground exterior (decking, cladding) | No |
| H4 | Ground contact, freshwater contact | Yes |
| H5 | In-ground, high decay risk (posts, piles) | Yes |
Standard treated pine decking boards are typically H3.1 or H3.2. These are designed to resist moisture and UV exposure above ground — not prolonged soil contact. If you use them as retaining wall planks buried against or in contact with soil, they are very likely to rot within a few years, compromising the wall's structural integrity.
What Timber Should You Use?
For horizontal retaining wall planks that will be in contact with soil or retained earth, you should be looking at H4-treated timber as a minimum. H4 timber has a higher level of preservative treatment that resists the fungi and insects found in NZ soils.
If the planks are sitting in or very close to the ground (particularly in wetter regions like the West Coast, Northland, or areas with heavy clay soils), H5-treated timber may be more appropriate for longevity.
Check with your local timber merchant at Bunnings, Mitre 10, or PlaceMakers — they can confirm the hazard class stamped on the timber and recommend the right product for your application.
When Could Decking Boards Be Acceptable?
There is one scenario where H3.2 decking boards might be acceptable: if the planks are not in direct contact with soil — for example, in a sleeper-style wall where the boards span between posts and are elevated slightly above ground level with good drainage behind. Even then, you'd want to check with a professional, as moisture wicking from retained soil can still accelerate decay.
Always check the end grain of any cut timber is sealed with an appropriate preservative end-grain treatment, as cut ends are particularly vulnerable to moisture ingress.
Building Consent and NZ Building Code Considerations
Retaining walls in New Zealand may require a building consent, depending on their height and proximity to boundaries or structures. Under Schedule 1 of the Building Act, retaining walls up to 1.5 metres in height that do not support a surcharge (such as a driveway, vehicle load, or building) may be exempt from consent — but always confirm this with your local council before starting work, as conditions vary.
For walls over 1.5 metres, or those retaining a surcharge, a building consent is almost certainly required, and the design may need to be signed off by a Chartered Professional Engineer (CPEng). The MBIE Building Performance website has detailed guidance on consent requirements.
The NZ Building Code requires that retaining structures are fit for purpose, durable, and structurally sound. Using under-specified timber is a compliance risk that could also affect your home insurance.
Practical Tips for a Timber Retaining Wall
- Always check the hazard class stamp on the timber before purchasing — don't assume.
- Use H4 or H5 timber for any planks in contact with soil.
- Install geotextile drainage fabric behind the wall and provide adequate drainage (ag pipe at the base) to reduce hydrostatic pressure and extend timber life.
- Use galvanised or stainless steel fixings — standard zinc-plated screws will corrode quickly in treated timber and damp conditions.
- Treat all cut ends with an approved preservative before installation.
- Consider the long-term maintenance — even correctly specified treated timber will eventually need replacement.
When to Call a Professional
If your retaining wall is over 1 metre in height, retains a slope, is near a boundary, or supports any kind of surcharge load (driveway, building, etc.), it's strongly advisable to engage a qualified retaining wall builder or structural engineer. Getting the design and materials wrong can result in wall failure, which poses serious safety risks and can be very costly to remediate.
A Licensed Building Practitioner (LBP) can carry out or supervise restricted building work — check the Licensed Building Practitioners register to find someone qualified in your area. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
