Retaining Walls Direct

After 50 years building retaining walls across Australia, the calculation is straightforward. Divide your wall height (in mm) by 75mm (standard sleeper height) to get the number of rows, then divide your wall length by your sleeper length.

• 1.0m high wall = 13 rows
• 1.2m high wall = 16 rows
• 1.5m high wall = 20 rows

Always add 5–10% for cuts and wastage, especially at corners. Browse our full range of concrete sleepers or use the calculator above for exact quantities.

Standard post spacing is 1.8m centres per AS 4678-2002 Earth-Retaining Structures. For a 10m wall, that is 7 posts — one at each end, plus one every 1.8m.

Reduce spacing to 1.5m or less when:

• Wall height exceeds 1.0m
• Heavy reactive clay soils (Brisbane, Melbourne)
• Driveway or vehicle surcharge loads apply
• Sandy or loose soils (coastal WA, SA) — get an engineer to confirm

Browse our steel H-beam and C-channel posts.

In our experience, undersized posts are the single most common cause of retaining wall failure in Australia. When in doubt, go up a size.

• Walls up to 1.0m, standard soil: 100×75 PFC C-channel galvanised posts
• Walls over 1.0m, reactive clay, or vehicle loads: 150 PFC or 100UC H-beam posts
• Walls over 1.5m or heavy surcharge: 150UC H-beam minimum

All posts should be hot-dip galvanised to AS 4680. See our full C-channel and H-beam post range or read our guide on post selection.

The rule under AS 4678-2002 is one-third of total post length in the ground. Posts must always be set in concrete — never compacted soil alone.

• 1.5m post: minimum 500mm embedment
• 1.8m post: minimum 600mm embedment
• 2.1m post: minimum 700mm embedment

In reactive clay (Melbourne, Brisbane) or loose sandy soils (coastal WA, SA), increase embedment by 20–30%. Use minimum N20 concrete for footings. In 50 years of experience, inadequate post depth is the leading cause of wall lean and failure after heavy rain.

Yes — in most Australian states, retaining walls over 1.0m require a building permit. Always check with your local council before starting.

• QLD: QBCC-licensed builder required over 1.0m — qbcc.qld.gov.au
• VIC: VBA building permit required over 1.0m — vba.vic.gov.au
• NSW: Development consent required in most councils — fairtrading.nsw.gov.au
• WA: Permit may be required from 500mm — buildingcommission.wa.gov.au
• SA/TAS: Generally 1.0m threshold — check your local council

Walls near boundaries or easements almost always require approval regardless of height. Building without a permit can result in fines and orders to demolish.

Yes — always. Poor drainage is the number one cause of retaining wall failure in Australia. Hydrostatic pressure will push any wall over without proper drainage — no matter how well it was built.

Minimum drainage system required:

• 20mm drainage aggregate (blue metal) directly behind the wall for the full retained height
• Slotted agricultural drain pipe at the base (65mm for walls up to 1.0m, 100mm for taller walls) directed to a legal discharge point
• Geotextile fabric (minimum 140gsm) between backfill soil and aggregate to prevent migration and blockage

Weep holes are a secondary measure only — they do not replace a proper drainage system. For walls over 600mm, clay soils, or high rainfall areas (QLD, VIC), drainage is non-negotiable. Browse our drainage products including geotextile fabric and ag pipe.