Joining two sections of sleeper retaining wall at an angle — whether at a 90° corner or a more acute or obtuse angle — is very achievable as a DIY project, provided the wall is under 1.5 metres high and you plan the corner carefully. The key is using a robust corner post or interlocking sleeper arrangement to transfer loads between both wall runs and prevent the joint from opening up over time.
Understand the Load at a Corner
A corner in a retaining wall is a high-stress point. Soil pressure pushes outward on both wall runs, and those forces meet at the joint. If the corner isn't properly tied together, it can splay open or rotate. This is why the jointing method matters just as much as the sleepers themselves.
Important: Under the NZ Building Code, retaining walls over 1.5 metres high (measured from the bottom of the footing to the top of the wall) generally require a building consent. Always check with your local council before starting — some councils have lower thresholds or additional requirements depending on slope, soil type, or proximity to boundaries. You can find guidance at MBIE Building Performance.
Choose Your Jointing Method
There are three main approaches for angled sleeper wall joints:
1. Corner Post Method (Most Common)
A corner post — typically a 150×150 mm or 200×200 mm treated timber H5-rated post, or a galvanised steel post — is set in concrete at the corner point. Each wall run butts up to or wraps around this post, and the sleepers are bolted or coach-screwed directly into it. This is the most straightforward method and works well for any angle.
- Set the post first, plumb and in concrete, before laying sleepers.
- Allow concrete to cure for at least 48 hours before loading the post.
- Use M12 or M16 galvanised coach screws or bolts to fix sleepers to the post.
- For non-90° angles, cut the post faces at the matching angle so sleepers sit flush.
2. Interlocking (Stacked Bond) Method
For a 90° corner, sleepers from alternating courses can overlap at the corner — similar to how bricks are laid in a running bond. One course has the sleeper from Wall A extending past the corner, and the next course has Wall B's sleeper extending past. This creates a mechanical interlock.
- Each overlapping sleeper should extend at least 300–400 mm past the corner.
- Drill and rod through with 16 mm or 20 mm threaded galvanised rod, tying courses together vertically.
- This method only works cleanly at or near 90°.
3. Mitre or Angle-Cut Method
For non-right-angle corners, sleepers from each run can be mitre-cut to meet neatly at the angle. This looks tidy but provides less mechanical strength on its own — always combine it with a corner post or through-rods for structural integrity.
Step-by-Step: Corner Post Method
- Mark out your wall lines and find the exact corner point.
- Dig the post hole — typically 600–900 mm deep depending on wall height and soil conditions. A deeper hole gives more resistance to overturning.
- Set the post in concrete, checking it is plumb in both directions. Use temporary bracing to hold it while the concrete sets.
- Lay your first course of sleepers on a compacted gravel base or concrete footing, butting them firmly against the post. Pre-drill and fix with coach screws.
- Continue laying courses, staggering vertical joints where possible, and fixing each sleeper to the post and to the course below using coach screws or rods.
- Install deadman anchors (horizontal sleepers running back into the bank) every 1–2 metres along each wall run for walls over about 600 mm high. These resist the outward soil pressure.
- Backfill in layers, compacting each layer to avoid overloading the wall during construction.
Timber Treatment and Fixings
All timber in ground contact must be H5 treated (e.g. CCA or equivalent) to resist rot and insect attack in NZ conditions. Check the treatment stamp on your sleepers. Fixings must be hot-dip galvanised or stainless steel — never use bright zinc-plated fixings in ground contact as they corrode quickly in our damp soils.
You can find treated sleepers and appropriate fixings at Bunnings, Mitre 10, or PlaceMakers.
Drainage Is Critical
Poor drainage is the number one cause of retaining wall failure in New Zealand. Water build-up behind the wall dramatically increases soil pressure. Always:
- Lay a 100 mm perforated ag-pipe (agricultural drain) at the base of the wall, wrapped in geotextile fabric.
- Backfill immediately behind the wall with free-draining gravel (20 mm chip or similar) for at least 300 mm.
- Provide weep holes through the wall every 1.5–2 metres if using a solid construction.
For more on drainage and wall performance, BRANZ has useful technical guidance.
When to Call a Professional
You should engage a qualified retaining wall builder if:
- The wall (either run) is or will be over 1.5 metres high.
- The ground is unstable, on a steep slope, or has poor soil (clay, fill, or loose material).
- The wall is close to a boundary, building, or any structure.
- You are unsure about the angle geometry, post sizing, or deadman spacing.
- A building consent is required — in that case, engineering input is often needed.
A licensed building practitioner (LBP) may be required to carry out or supervise restricted building work. Check the Licensed Building Practitioners register. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
