Drainage Kits, Ag Pipe & Weep Holes

Complete retaining wall drainage kits — ag pipe, geotextile fabric, and weep holes — essential for every concrete sleeper retaining wall. Drainage failure is the leading cause of retaining wall failure in Australia. A correctly installed drainage system is not optional — it is the single most important factor in how long your wall lasts.

Before ordering, read our retaining wall drainage guide — it covers correct ag pipe placement, geotextile installation, weep hole spacing, and the most common drainage mistakes that cause walls to fail prematurely. For the complete technical reference, see the complete drainage and structural integrity guide.

What's in a drainage kit?

A correctly installed retaining wall drainage system has four components — omitting any one significantly reduces system effectiveness:

• Ag pipe (agricultural drainage pipe) — slotted flexible pipe laid at the base of the wall to collect and channel groundwater away. Use 100mm slotted ag pipe for standard residential walls; 150mm for longer walls, high-rainfall areas, or sloped sites with large catchment areas. Install with slots facing downward or to the sides, wrapped in a geotextile sock, with a continuous fall of at least 1:100 toward the outlet. Discharge to a legal point — never onto a neighbouring property. See the complete ag pipe sizing and installation guide.
• Geotextile filter fabric — a permeable membrane laid against the excavated soil face behind the sleepers. Prevents fine soil particles from migrating into the drainage aggregate and blocking the system over time. Without it, the drainage layer can become effectively silted up within 5–10 years. The fabric should extend from the base of the excavation to the top of the drainage zone and be folded over the top of the aggregate before backfilling.
• Drainage aggregate — 20mm clean crushed rock (not crusher dust) filled behind the wall face to a minimum 300–400mm depth. On reactive clay sites in Melbourne's western suburbs, Brisbane's Woogaroo clay corridor, or Adelaide's northern plains, increase to 400–500mm. See the backfill guide for the full specification by soil type.
• Weep holes — installed through or between the lowest sleeper course to provide a secondary drainage path at the wall face. Space at 1.8m centres. For taller walls, add a second row at mid-height. Inspect and clear annually. Weep holes allow water that has built up in the drainage zone to escape directly through the wall face rather than continuing to build pressure.

Why drainage is non-negotiable

Water building up behind a retaining wall creates hydrostatic pressure — a force that pushes outward on the wall face with increasing intensity the longer drainage is absent or inadequate. This is the leading cause of retaining wall failure in Australia across every soil type, every state, and every wall material.

• Prevents hydrostatic pressure — the number one cause of retaining wall failure in Australia. The 2021–2022 La Niña delivered record rainfall across Sydney and south-east Queensland, causing widespread retaining wall failures on underdrained sites. Correctly drained walls on the same sites performed significantly better.
• Required by most councils — drainage is mandated for walls over 600mm in most Australian states under AS 4678-2002 Earth Retaining Structures. Check your state permit requirements before building.
• Critical on clay sites — reactive clay soils (Wianamatta Shale in western Sydney, Woogaroo clay in Brisbane, Guildford Clay in Perth, Keswick Clay in Adelaide) expand dramatically when saturated. Without drainage, hydrostatic pressure builds rapidly after heavy rainfall. These sites require a minimum 400mm drainage aggregate zone and geotextile fabric is non-negotiable.
• Critical on sloped sites — sloped blocks have larger catchment areas above the wall. The ag pipe must be sized to handle peak flow from the entire upslope catchment, not just the immediate wall area. See the sloped site retaining wall guide for drainage sizing on sloped blocks.
• Critical for tiered walls — each tier of a tiered wall needs its own independent drainage system. Water from the upper tier must drain away without saturating the lower tier's backfill. See the tiered retaining wall guide for drainage requirements between tiers.
• Protects your entire wall system — reduces moisture exposure on concrete sleepers, steel posts, and concrete footings across the design life of the wall.

Drainage specification by soil type and state

How to install retaining wall drainage — the correct sequence

1. Set posts and cure footings. Allow minimum 48 hours before loading.
2. Lay geotextile fabric. Place against the excavated soil face behind where sleepers will sit — from the base to the top of the drainage zone.
3. Position the ag pipe. Place at footing level with slots facing down or to the sides, with a continuous fall of at least 1:100 to the outlet. Connect to discharge point before backfilling.
4. Install sleepers and weep holes. Install weep holes in the lowest course as you go, at 1.8m centres.
5. Backfill with drainage aggregate. Fill 300–500mm of 20mm clean crushed rock around and over the ag pipe. Do not compact — compaction reduces permeability.
6. Fold geotextile over the aggregate. Before placing general backfill above the drainage zone, fold the fabric over the top of the aggregate to prevent soil migration downward.
7. Backfill in lifts. Place general backfill in 150–200mm layers and compact each layer before adding the next. Never dump full backfill volume against the wall at once.

Full step-by-step: how to build a concrete sleeper retaining wall.

Complete your retaining wall system

• Drainage kit for retaining walls — ag pipe, geotextile fabric and weep holes in one order. Everything you need, nothing missed.
• Geotextile filter fabric — also available separately for larger orders or replacement.
• Concrete retaining wall sleepers — steel-reinforced sleepers in charcoal, woodgrain, sandstone and smooth finishes.
• Galvanised steel retaining wall posts — C-channel and H-beam posts, hot-dip galvanised as standard.
• H-beam steel posts — for walls over 1.2m, reactive clay sites, or engineer-specified applications.
• Retaining wall kits and bundles — sleepers, posts and drainage together in one order.

Drainage guides and resources

• Retaining wall drainage guide — correct ag pipe placement, geotextile installation, weep hole spacing, and common mistakes.
• Ag pipe sizing and installation guide — what size ag pipe, how to install, fall requirements, and outlet options.
• Retaining wall backfill guide — what to use behind the wall and how to compact correctly.
• Complete drainage and structural integrity guide — the full technical reference covering every drainage failure mode and how to prevent it.
• Sloped site retaining wall guide — drainage sizing for sloped blocks and larger catchment areas.
• Tiered retaining wall guide — independent drainage for each tier.
• How to build a concrete sleeper retaining wall — full step-by-step including drainage installation sequence.

Drainage specification varies by wall height, soil type, rainfall zone, and council requirements. For walls over 1m, on reactive clay sites, in high-rainfall areas, or for tiered and sloped sites, confirm your drainage design with a qualified structural engineer before ordering. All drainage products comply with AS 4678-2002 Earth Retaining Structures requirements.