For timber retaining walls in New Zealand, H5 treated timber is the minimum required for any timber that is in direct ground contact. This applies to posts, sleepers, and any structural member embedded in or resting on soil. Using the wrong treatment class is one of the most common — and costly — mistakes DIYers make with retaining walls, so it pays to get this right from the start.
Understanding the H Class Treatment System
New Zealand uses a Hazard Class (H class) system to classify how much preservative treatment timber needs based on its exposure to moisture, insects, and decay. The higher the H number, the more protection the timber has. The classes most relevant to retaining walls are:
| H Class | Typical Use | Ground Contact? |
|---|---|---|
| H3.1 | Above-ground, exposed to weather | No |
| H3.2 | Above-ground, exposed to weather, wetting likely | No |
| H4 | Ground contact, low decay risk | Yes (limited) |
| H5 | Ground contact, high decay risk | Yes (recommended) |
| H6 | Marine/saltwater exposure | Specialist use |
For retaining walls, H5 is the standard you should be specifying. H4 may be acceptable in some low-risk situations (such as very dry, free-draining soils), but H5 is what most builders and councils expect, and it's what the BRANZ guidelines support for ground contact structural applications.
Why H5 and Not H4?
Retaining wall timbers face a tough combination of conditions: constant soil contact, moisture from retained earth, and often limited airflow. These conditions accelerate fungal decay and can attract wood-boring insects, including borer and termites in warmer parts of the country.
H4 timber uses copper azole or similar preservatives at a lower retention level. It offers reasonable protection in above-ground or intermittent ground contact situations, but for a structural retaining wall — where the timber is permanently embedded in soil — it may not last as long as you'd hope.
H5 timber has a higher preservative retention level, making it suitable for prolonged ground contact where decay and insect hazard is high. For posts driven or concreted into the ground, H5 is the right choice. You can expect a well-built H5 retaining wall to last 20–30 years or more, depending on site conditions and timber species.
What to Look for When Buying
When purchasing timber for your retaining wall, check for the following:
- The H class stamp or tag — reputable suppliers will clearly mark the treatment class on the timber or its packaging.
- The treatment type — common treatments for H5 include copper azole (CA) and copper chrome arsenate (CCA). Note that CCA-treated timber has restrictions on its use in some situations (e.g., near food gardens or playgrounds) — check with your supplier.
- Kiln-dried vs. wet-treated — H5 timber is often sold wet from the treatment process. Allow it to dry before painting or sealing if needed.
- Species — treated radiata pine is the most common and cost-effective option in NZ. Hardwoods like kwila (merbau) have natural durability but still benefit from treatment for ground contact.
You'll find H5 treated timber at most major building suppliers including Bunnings, Mitre 10, and PlaceMakers.
NZ Building Code and Consent Requirements
Retaining walls are subject to the NZ Building Code and may require a building consent, depending on their height and proximity to boundaries or structures. As a general guide:
- Retaining walls up to 1.5 metres in height may be exempt from consent under Schedule 1 of the Building Act — but this depends on surcharge loading, proximity to boundaries, and other factors.
- Walls over 1.5 metres, or those retaining slopes near buildings or boundaries, will almost certainly need consent.
Always check with your local council before starting work. Consent requirements vary by region and site conditions, and getting this wrong can cause serious problems when you sell your property.
The structural design of retaining walls often references NZS 3604 (Timber Framed Buildings) and relevant geotechnical standards. Your council or a structural engineer can advise on what applies to your specific situation.
Cutting and Drilling Treated Timber
If you need to cut or drill H5 timber on site, the cut ends and drilled holes must be treated with an approved end-grain preservative before installation. This is a critical step — untreated cut surfaces are vulnerable to decay and will significantly shorten the life of your wall. End-grain preservative products are available at most hardware stores.
When working with treated timber, always:
- Wear gloves, eye protection, and a dust mask
- Avoid burning offcuts — treated timber smoke is hazardous
- Dispose of offcuts responsibly (check with your local council for disposal options)
- Review WorkSafe NZ guidance on working safely with treated timber
When to Call a Professional
While a small garden retaining wall can be a satisfying DIY project, there are situations where you should engage a qualified retaining wall builder or structural engineer:
- The wall will be over 1.0–1.5 metres high
- The site has poor drainage, unstable soils, or a steep slope
- The wall is near a boundary, building, or driveway
- You're unsure about drainage design, footing depth, or structural requirements
- A building consent is required
A licensed builder or LBP (Licensed Building Practitioner) can ensure the work meets code and is signed off correctly. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
