Get the backfill wrong and a retaining wall doesn’t fail overnight — it fails slowly, then all at once. Water builds up behind the wall, hydrostatic pressure loads the structure it was never designed to carry, and the wall bows, cracks or tips. The fix is simple and cheap compared to a rebuild: free-draining aggregate, an ag-pipe wrapped in geofabric, and a path for water to get out. This guide covers what to use, why, and how to work out how much you need.
Why free-draining aggregate matters behind a wall
A retaining wall holds back soil. What it should never have to hold back is water. Soil behind a wall soaks up rain and runoff, and saturated soil is heavy — a lot heavier than dry soil. That water sitting against the back of the wall creates hydrostatic pressure that pushes outward and downward on the structure. On a masonry, block or timber-sleeper wall, that’s the load that causes bulging, leaning and cracking.
Free-draining aggregate — clean, angular crushed rock with no fines — solves this by giving water somewhere to go. Instead of soaking into and sitting in the backfill, water runs freely through the voids between the stones, down to an ag-pipe at the base, and away from the wall. The drainage zone keeps the soil behind the wall from ever becoming a saturated, pressurised mass. It’s the single most important thing you can do to make a retaining wall last.
The same logic applies to any ag-drain (agricultural drain / French drain) across a site: a trench, a perforated pipe, and a surround of clean stone that lets groundwater and surface water find the pipe and drain to a legal point of discharge.
Which aggregate: blue metal vs recycled
You want stone that stays open — angular, consistently sized, and washed or crushed clean of fines. Two families do the job at Elite:
20mm blue metal
20mm blue metal is the workhorse for retaining wall drainage and ag-drains. It’s crushed basalt, angular, and the larger size gives big voids that carry a lot of water and are far less likely to clog with fine sediment over time. For most retaining wall backfill zones and standard ag-pipe trenches, 20mm is the default choice.
10mm blue metal
10mm blue metal is the same clean crushed rock in a smaller size. Use it where the drainage gap is narrow, around thinner ag-pipe, or where you want a tighter, more uniform surround — for example a slimline strip drain against a wall footing. It still drains well; it just carries a bit less volume and packs a little tighter than 20mm. Not sure which size suits the job? Our 10mm vs 20mm blue metal guide breaks down the trade-offs.
Recycled aggregate
Recycled crushed concrete and brick is a cost-effective alternative for drainage backfill where you don’t need the clean look or the certainty of virgin quarried stone. It’s a sound choice for buried, non-structural drainage fill on landscaping and civil jobs, and it keeps material out of landfill. Just note it typically carries more fines than washed blue metal, so for the critical drainage layer right against the pipe, quarried blue metal is the safer bet — reserve recycled for bulk fill zones. Talk to us about current recycled aggregate stock on 1300 935 483.
Ag-pipe and geofabric: the basics
Aggregate alone drains — but pair it with an ag-pipe and geofabric and you get a system that keeps working for decades.
- Ag-pipe (slotted/perforated pipe): laid at the base of the drainage zone, along the back of the wall footing or in the bottom of the ag-drain trench. It collects the water that filters down through the aggregate and channels it to an outlet. Lay it with a consistent fall (a slight, continuous slope) toward the discharge point so water always moves — never let it sit level or run uphill.
- Geofabric (filter fabric): this is what stops the system silting up. Line the trench or the back face of the drainage zone with geofabric before you place the stone, so soil fines are filtered out and can’t migrate into the aggregate and clog the voids. Think of the stone as the highway and the geofabric as the fence that keeps the mud off it. Wrapping the ag-pipe itself in a fabric sock adds a second line of defence.
- Discharge: the pipe has to go somewhere legal — a stormwater connection, a rubble/soak pit, or daylight at a lower point. Water in with nowhere out is just a slower way to fail.
A typical build order behind a wall: geofabric against the retained soil, ag-pipe at the base on a bed of stone, then clean 20mm blue metal backfilled up the drainage zone, capped near the surface with topsoil or a low-permeability layer so surface water sheds rather than pours straight in.
Coverage table: how much aggregate do I need?
Drainage stone is sold by weight but measured on site by volume, so you need the density to convert. Sand and pebbles run about 1.5 t/m³; blue metal and other quarried drainage stone sit in a similar range, so 1.5 t/m³ is a safe planning figure for blue metal. In other words, 1 tonne of blue metal covers roughly 0.67 m³.
To size an order, work out the volume of your drainage zone or trench (length × width × depth in metres = cubic metres), then multiply by the density to get tonnes. The table below shows how far one tonne of blue metal (at 1.5 t/m³) spreads at common depths, plus the volume-to-tonnes conversion.
| Metric | Value (20mm/10mm blue metal @ 1.5 t/m³) |
|---|---|
| Density | 1.5 t/m³ |
| 1 tonne = | ~0.67 m³ of stone |
| 1 m³ = | ~1.5 tonnes |
| Coverage from 1 tonne at 50mm deep | ~13 m² |
| Coverage from 1 tonne at 75mm deep | ~9 m² |
| Coverage from 1 tonne at 100mm deep | ~6.7 m² |
Trench example: an ag-drain 20m long × 0.3m wide × 0.4m deep = 2.4 m³. At 1.5 t/m³ that’s about 3.6 tonnes of blue metal (allow a little extra for the pipe displacement and for topping up as fines settle in). To size your exact job in seconds, plug the dimensions into our material calculator.
Loose material is available from 500kg, and bulka bags come in 1 tonne and 1.5 tonne sizes — handy for tucking a drainage supply on site without a full tipper load. Larger volumes go out on our own fleet across Sydney, or pick up from the Jamisontown yard.
Road base vs drainage stone: when to use which
This trips people up, because both get used around retaining walls — but they do opposite jobs.
- Drainage stone (clean 10mm/20mm blue metal): designed to let water through. It’s a graded, single-size stone with open voids and no fines. Use it in the drainage zone, around ag-pipe, and anywhere the whole point is to move water. If it compacts solid and stops draining, it has failed.
- Road base (DGB20 / recycled road base): designed to lock up and bear load. It’s a blended, well-graded material — a mix of sizes plus fines — that compacts to a dense, stable, load-bearing layer. Use it under the wall footing, under paving, and for the trafficable base course. Its density is around 1.6 t/m³.
So on a typical retaining wall job you’ll often use both: recycled road base (DGB20) compacted as the levelling and bearing pad the first course sits on, and clean blue metal as the free-draining backfill behind the wall. Never substitute one for the other — road base behind the wall will trap water, and drainage stone under the footing won’t compact to a stable base. If you’re specifying the base layer, our DGB20 road base guide covers grading, compaction and depths.
Frequently asked questions
What’s the best aggregate for behind a retaining wall?
Clean, angular 20mm blue metal is the standard choice — its large voids drain freely and resist clogging. Use 10mm blue metal where the drainage gap is narrow or you want a tighter surround around thinner ag-pipe. The important part is that the stone is washed or crushed clean of fines, so the voids stay open and water keeps moving down to the ag-pipe at the base.
What’s the difference between 10mm and 20mm blue metal for drainage?
Both are clean, angular crushed basalt. 20mm has larger voids, so it carries more water and is less likely to silt up over time — it’s the default for retaining wall backfill and standard ag-drain trenches. 10mm packs a little tighter and suits narrow drainage gaps, thinner ag-pipe and slimline strip drains. Our 10mm vs 20mm blue metal guide covers it in detail.
Do I really need geofabric and an ag-pipe?
For anything structural, yes. The aggregate moves the water, the ag-pipe collects it and channels it to a legal discharge point, and the geofabric stops soil fines migrating into the stone and clogging the voids. Skip the fabric and the drainage zone silts up; skip the pipe and the water has nowhere to go. Together they’re what keeps a retaining wall or ag-drain working for decades rather than a few seasons.
How much drainage aggregate do I need?
Work out the volume of your drainage zone or trench — length × width × depth in metres gives cubic metres — then multiply by 1.5 to get tonnes (blue metal runs at about 1.5 t/m³). So one tonne is roughly 0.67 m³ of stone. For example, a 20m ag-drain trench at 0.3m wide × 0.4m deep is 2.4 m³, or about 3.6 tonnes. Allow a little extra for pipe displacement and settling, or run the numbers through our material calculator.
Can I use road base behind the wall instead?
No — road base is the opposite material. DGB20 and recycled road base are blended, well-graded mixes with fines that compact to a dense, load-bearing layer, which is exactly what you want under a footing or paving but exactly what you don’t want in a drainage zone, because it traps water. Keep clean blue metal behind the wall and save the road base for the bearing pad. See our DGB20 road base guide for the base-layer specs.
Do you deliver small loads?
Yes. Loose material is available from 500kg, and bulka bags come in 1 tonne and 1.5 tonne sizes, so you’re not forced into a full tipper load for a single drainage run. We deliver on our own fleet across Sydney, or you can pick up from the Jamisontown yard. Call 1300 935 483 to sort a load.