How-to guide
Retaining Wall Guidelines
How to plan, spec and build a concrete sleeper retaining wall — from post spacing and drainage to permits and step-by-step installation.
Installation steps
Plan your wall
Measure the length and height of your proposed wall. Mark out the wall line with stakes and string. Check for underground services (call Dial Before You Dig on 1100 before any excavation). Determine whether you need a building permit — in Victoria, retaining walls over 1.0m high generally require a permit; some councils have lower thresholds. Contact Mitchell Shire Council if you are unsure.
Walls over 1.0m high in Victoria typically require a building permit and may need an engineer's report. Check with your local council before you start.
Choose your materials
Select your concrete sleeper finish (Plain Grey, Charcoal, Stackstone, Blockface or Woodgrain) and thickness (80mm standard or 100mm heavy duty). Choose your steel post type — 100UC H posts for line posts, 100UC C posts for ends and corners. For walls over 1.2m, consider 150UC posts. Use our estimator tool to calculate quantities.
Use 100mm sleepers and 150UC posts for any wall over 1.2m high, or where the soil is heavy clay or likely to become waterlogged.
Set out post positions
Standard post spacing is 1.2m, 1.5m, 1.8m, 2.0m, 2.4m or 2.7m — matching your sleeper length. Mark each post position along the wall line. Post burial depth should be at least one-third of the total post length above ground; for a 1.0m wall, bury posts at least 500mm. For walls over 1.2m, increase burial depth accordingly or consult an engineer.
Excavate and set posts
Dig post holes to the required depth using a post-hole digger or auger. Holes should be 200–250mm in diameter for 100UC posts. Set posts plumb and at the correct spacing. Pour concrete (rapid-set concrete is ideal) around each post and allow to cure fully — at least 24 hours before loading with sleepers. Check posts are plumb and aligned before the concrete sets.
Use a spirit level and string line to keep posts perfectly plumb and aligned. Misaligned posts are very difficult to correct once the concrete has set.
Install sleepers
Once posts are set and concrete has cured, slide sleepers into the post flanges from the top, working from the bottom course upward. Use plastic levelling wedges to keep each course level and to take up any gaps between sleepers. For heavy sleepers (2.4m and longer), use a sleeper grabber lifting clamp — do not attempt to lift heavy concrete sleepers by hand.
Always work from the bottom course up. Never try to slide sleepers in from the side — they must be lowered in from the top.
Install drainage
Drainage is critical — it is the number one cause of retaining wall failure when it is done poorly. Install geofabric drainage cloth against the back face of the wall before backfilling. Lay ag pipe (slotted drainage pipe) along the base of the wall, sloping to an outlet point. Cover the ag pipe with 200mm of clean gravel before backfilling with soil.
Never skip the drainage layer. A wall without proper drainage will eventually fail as water pressure builds up behind it.
Backfill and compact
Backfill behind the wall in 200mm layers, compacting each layer before adding the next. Do not use heavy machinery close to the wall — hand compact or use a plate compactor at least 500mm back from the wall face. Avoid backfilling with clay-heavy soil if possible; use a free-draining material such as crushed rock or sandy loam.
Post spacing reference
Standard post spacing for concrete sleeper retaining walls. Post spacing equals sleeper length. Burial depth is a minimum guide — increase for taller walls or heavy soil conditions.
| Sleeper length | Post spacing | Post type | Min. burial depth |
|---|---|---|---|
| 1.2m | 1.2m | 100UC | 500mm |
| 1.5m | 1.5m | 100UC | 500mm |
| 1.8m | 1.8m | 100UC | 600mm |
| 2.0m | 2.0m | 100UC | 700mm |
| 2.4m | 2.4m | 100UC / 150UC* | 800mm |
| 2.7m | 2.7m | 150UC* | 900mm |
| 3.0m | 3.0m | 150UC* | 1000mm |
* 150UC posts recommended for walls over 1.2m high or where soil conditions are heavy. Consult an engineer for walls over 1.5m.
Drainage — the most important step
Poor drainage is the single most common cause of retaining wall failure. Water pressure building up behind a wall can exert enormous force — far more than the wall was designed to handle. A properly installed drainage system eliminates this risk entirely.
Geofabric cloth
Line the back face of the wall with geotextile drainage fabric before backfilling. This prevents fine soil particles from migrating into the drainage layer and blocking it over time.
Ag pipe
Lay slotted ag pipe (agricultural drainage pipe) along the base of the wall, sloping at least 1:100 to a stormwater outlet or daylight point. Cover with 200mm of clean gravel.
Gravel backfill
Use free-draining material (crushed rock or coarse gravel) for the first 300mm of backfill directly behind the wall. This creates a drainage column that feeds into the ag pipe.
Never backfill directly against the wall without installing drainage first. Once the wall is backfilled, adding drainage is extremely difficult and expensive.
Permits in Victoria
Under the Victorian Building Regulations, a building permit is generally required for retaining walls over 1.0m in height. However, there are important exceptions and local variations — always check with Mitchell Shire Council before starting work.
Under 1.0m
Generally no permit required in Victoria, but check with your local council. Some councils have lower thresholds, and walls near boundaries or drainage lines may still require approval.
Over 1.0m
A building permit is generally required. You will need to engage a registered building practitioner and may need an engineer's report depending on wall height and soil conditions.
Near a boundary
Walls within 1.0m of a property boundary may require a permit regardless of height. Check with your council and consider your neighbour's drainage and land stability.
Surcharge loads
If the wall will support a surcharge load (driveway, shed, pool, etc.), an engineer's report is required regardless of wall height.
This is a general guide only. Always confirm permit requirements with Mitchell Shire Council or a registered building practitioner before commencing work.
Have more questions?
Browse our full FAQ or use the estimator to calculate materials for your project.