Building a simple timber retaining wall on flat ground is a manageable DIY project for most handy homeowners — provided the wall stays within certain height limits and you follow good building practice. In New Zealand, retaining walls up to 1.5 metres high may be exempt from building consent under Schedule 1 of the Building Act, but there are important conditions, so always confirm with your local council before you start.
Do You Need a Building Consent?
Under the Building Act 2004, Schedule 1 exemptions allow some retaining walls to be built without a building consent. As a general guide, a retaining wall that retains less than 1.5 metres of ground and does not support a surcharge (such as a driveway, vehicle load, or building) may be exempt — but the exact threshold and conditions vary, so confirm with your local council. However, this depends on your specific site conditions, soil type, and local council rules.
Always check with your local council before starting. You can also find guidance on consent requirements at MBIE Building Performance. Getting this wrong can cause costly problems when you sell your home or make an insurance claim.
Choosing the Right Timber
For ground contact, you need H4 or H5 treated timber — these are the NZ Hazard Class ratings for timber in contact with the ground or in wet conditions. Common choices include:
- H4 treated pine sleepers (typically 200 × 100 mm or 200 × 75 mm) — widely available at Bunnings, Mitre 10, and PlaceMakers
- Hardwood railway sleepers — durable but heavy; check they are free of contaminants if recycled
- Round timber posts (H5 treated) for the vertical deadmen and anchor posts
Avoid untreated timber or H3-only timber in ground contact — it will rot quickly in NZ's damp conditions.
Tools and Materials You'll Need
- H4 treated timber sleepers
- H5 treated timber posts (for deadmen/anchors)
- Galvanised coach screws or structural bolts (minimum 150 mm)
- Geotextile drainage fabric
- 20 mm drainage chip (washed gravel)
- Perforated ag-pipe (agricultural drainage pipe)
- String line and spirit level
- Post-hole borer or spade
- Circular saw
- Drill/driver
- Wheelbarrow and tamper
Step-by-Step Construction
1. Plan and Mark Out
Mark the wall line with a string line. Check the area for any underground services (power, water, gas) before digging — call Dial Before You Dig (visit their website for current contact details) or visit their website to request a service location.
2. Dig the Foundation Trench
Excavate a trench approximately 200–300 mm deep along the wall line. The base course of sleepers should be partially buried to anchor the wall and resist forward movement.
3. Install the First Course
Lay your first row of sleepers in the trench, checking they are level along the length and plumb on the face. This base course is the most critical — take your time here.
4. Install Deadmen (Anchor Ties)
For walls over about 600 mm high, deadmen are essential. These are horizontal sleeper sections running back into the retained soil at right angles to the wall, typically every 1.2–1.8 metres along the wall and at every second course. They prevent the wall from tipping forward under soil pressure. Secure them with galvanised bolts.
5. Build Up the Courses
Stack subsequent courses of sleepers, staggering the joints like brickwork so vertical joints never line up. Fix each course to the one below using galvanised coach screws (at least two per sleeper length). Backfill as you go — don't leave the wall unsupported.
6. Install Drainage — This Is Critical
Poor drainage is the number one cause of retaining wall failure in New Zealand. Behind the wall, install:
- A layer of geotextile fabric against the soil face to prevent fine particles migrating
- At least 300 mm of drainage chip (washed gravel) behind the wall
- A perforated ag-pipe at the base of the wall, sloped to daylight at one or both ends
Without proper drainage, hydrostatic pressure builds up behind the wall and will eventually push it over — especially in NZ's wet winters. BRANZ provides useful guidance on drainage best practice for retaining structures.
7. Backfill and Compact
Backfill in layers of no more than 150–200 mm, compacting each layer before adding the next. Avoid heavy machinery close to the wall during this stage.
Key Measurements at a Glance
| Wall Height | Consent Likely Needed? | Deadmen Required? | Drainage Required? |
|---|---|---|---|
| Up to 600 mm | Usually not (confirm with council) | Not usually | Yes |
| 600 mm – 1.5 m | Check with council | Yes | Yes |
| Over 1.5 m | Almost certainly yes | Yes — engineer required | Yes |
When to Call a Professional
Some retaining wall projects are beyond DIY scope. Call a qualified landscaper or structural engineer if:
- The wall will be over 1.0–1.5 metres high
- The retained soil supports a building, driveway, or vehicle loads
- Your site has unstable, expansive, or sloping ground
- You are unsure about soil type or drainage conditions
- A building consent is required (an LBP may need to be involved — see Licensed Building Practitioners)
A qualified landscaper can design and build a wall that will last for decades and meet NZ Building Code requirements. You can find experienced local landscapers on tradedirectory.co.nz.
Safety Note
Always wear appropriate PPE when handling treated timber — gloves, safety glasses, and a dust mask when cutting. Wash hands thoroughly after handling H4/H5 treated timber. Dispose of offcuts responsibly; do not burn treated timber.
Licensed Building Practitioner (LBP)
