Yes, you can install a simple retaining wall using railway sleepers, and it's one of the more popular DIY landscaping projects in New Zealand. However, there are important rules around height, building consent, timber treatment, and drainage that you need to understand before you start digging.
Understanding NZ Building Consent Rules First
This is the most critical step. In New Zealand, retaining walls are governed by the NZ Building Code and your local council's rules. As a general guide under Schedule 1 of the Building Act 2004, retaining walls that retain less than 1.5 metres of ground may be exempt from building consent — but this is not a blanket rule.
Exemptions depend on:
- The height of retained soil (not the wall height above ground)
- Surcharge loading (e.g., is a driveway, structure, or vehicle near the wall?)
- Proximity to boundaries and neighbouring properties
- Your local council's specific requirements
Always check with your local council before starting, even for a seemingly small wall. Some councils have stricter rules than the national minimum. You can also find guidance on consent exemptions at MBIE Building Performance.
Choosing the Right Sleepers
Not all sleepers are equal, and this matters a lot in the NZ context.
New Treated Timber Sleepers
New landscaping sleepers are typically H4 or H5 treated pine, which are rated for in-ground contact. H4 is suitable for ground contact in most situations; H5 is used where there's a higher risk of decay (e.g., permanently wet ground). Look for the treatment stamp on the timber. Suppliers like Bunnings, Mitre 10, and PlaceMakers stock treated landscaping sleepers.
Recycled Railway Sleepers
Authentic old railway sleepers are typically hardwood (often jarrah or similar) and were treated with creosote, a substance now restricted due to health and environmental concerns. Under WorkSafe NZ guidelines, creosote-treated timber should not be used in situations where people or animals have regular skin contact, near food gardens, or near waterways. If you're unsure about the treatment used, opt for new treated timber instead.
Planning Your Retaining Wall
Before you buy materials, consider:
- Wall height: Keep DIY walls to a single tier of 600–900 mm where possible. Taller walls need engineering input.
- Tiered walls: If you need to retain more than 1 metre, consider stepping the wall into two or more tiers with a gap of at least 600 mm between them.
- Drainage: Poor drainage is the number one cause of retaining wall failure. You must install a drainage layer (coarse gravel or scoria) behind the wall and a agricultural drain pipe (ag pipe) at the base to carry water away.
- Deadmen anchors: For walls over 600 mm, burying horizontal sleeper anchors (called deadmen) perpendicular into the bank significantly improves stability.
Step-by-Step: Basic Sleeper Retaining Wall
- Mark out and excavate the base trench — typically 200–300 mm deep for the first course.
- Compact the base with a hand tamper or plate compactor.
- Lay the first course of sleepers, checking they are level. This base course is the most important.
- Backfill behind with coarse gravel as you build up each course.
- Install ag pipe at the base of the gravel layer, sloping to an outlet point.
- Stagger the joins between sleepers on each course (like brickwork) for strength.
- Fix courses together using long galvanised or stainless steel coach screws (300–500 mm) or rebar driven vertically through the sleepers into the ground.
- Install deadmen at regular intervals if the wall is over 600 mm tall.
- Finish backfilling with suitable soil and compact gently.
| Wall Height | Consent Likely Needed? | Deadmen Needed? | Engineer Recommended? |
|---|---|---|---|
| Under 600 mm | Usually no | No | No |
| 600 mm – 1.0 m | Check with council | Yes | Consider it |
| 1.0 m – 1.5 m | Check with council | Yes | Recommended |
| Over 1.5 m | Almost certainly yes | Yes | Yes |
Drainage: Don't Skip This Step
New Zealand's rainfall — especially in regions like Auckland, Wellington, and the West Coast — means hydrostatic pressure behind a retaining wall can be enormous. A wall without proper drainage will eventually fail, often dramatically. Use a minimum 200 mm layer of free-draining aggregate (AP20 chip or similar) behind the wall, and ensure your ag pipe has a clear outlet that doesn't direct water onto a neighbour's property.
For guidance on drainage requirements, BRANZ is an excellent NZ-specific technical resource.
When to Call a Professional
While a low single-tier sleeper wall is a manageable DIY project, you should engage a qualified landscaper or structural engineer if:
- The wall will be taller than 1.0–1.5 metres
- There is a structure, driveway, or vehicle load near the top of the wall
- The wall is near a boundary or could affect a neighbour's property
- The ground is unstable, on a steep slope, or has poor drainage
- Building consent is required (an LBP-licensed builder or engineer may need to be involved — see Licensed Building Practitioners)
A professional landscaper will also know your local council's specific requirements. You can find qualified local landscapers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
