Retaining wall drainage limits the water pressure that builds up behind a wall. A typical Australian detail puts a 100 mm slotted or perforated agi pipe at the base, wrapped in filter fabric and surrounded by about 300 mm of free-draining gravel or crushed rock. The pipe falls to a council-approved stormwater outlet, and a compacted soil or clay layer caps the drainage at the top.
Water trapped behind a retaining wall pushes on it as hydrostatic pressure, a load the wall usually isn't designed to resist. RMIT University's building notes say all types of retaining walls require a system to minimise the build-up of that pressure. Drainage is one of the simplest and most important parts of a retaining wall design.
What should be behind a retaining wall?
Behind a retaining wall there should be a free-draining layer of gravel or crushed rock, filter fabric against the retained soil, a slotted agi pipe at the base and a compacted seal on top. Some walls also have weep holes through the face. Each part does a different job:
| Part | What it does | In published Australian details |
|---|---|---|
| Free-draining gravel or crushed rock | Lets water drop quickly to the base of the wall | About 300 mm thick: 20–25 mm crushed rock or free-draining granular material (Austral Masonry); 10 mm crushed rock (City of Canterbury Bankstown) |
| Filter fabric (geotextile) | Stops soil washing into the gravel and clogging it | Between the retained soil and the drainage layer |
| Agi pipe (subsoil drain) | Collects the water at the base and carries it away | 100 mm slotted PVC or perforated polyethylene pipe |
| Outlet | Releases the water at a lawful point of discharge | A council-approved stormwater system, the kerb and channel, or a gully |
| Top seal | Keeps silt and surface water out of the drainage layer | A compacted soil plug (Austral Masonry) or at least 150 mm of compacted clay (City of Canterbury Bankstown) |
| Weep holes | Let water out through the wall face as a backup | Vertical joints left free of mortar on their lower half, at most 1000 mm apart (Brisbane City Council, block walls) |
What drainage pipe goes behind a retaining wall?
The drainage pipe behind a retaining wall is usually a 100 mm slotted or perforated subsoil pipe, often called agi pipe or ag pipe, laid at the base of the wall. Three published Australian details compare like this:
| Item | Austral Masonry concrete sleeper guide (2023) | City of Canterbury Bankstown S-222, concrete wall | Brisbane City Council BSD-2222, block wall |
|---|---|---|---|
| Pipe | 100 mm slotted PVC ag pipe | 100 mm slotted PVC in a geofabric sock | 100 mm corrugated perforated polyethylene pipe in geotextile, or a 300 x 50 mm strip drain |
| Fall | Uniform 1 in 100, in runs of up to 30 m | Uniform, at least 1% | At least 1 in 200 to the outlet |
| Drainage layer | 300 mm of 20–25 mm crushed rock or free-draining granular material, with filter fabric | At least 300 mm of 10 mm crushed rock in geofabric around the drain | Full height and length: a drainage composite, or 300 mm of free-draining filter sand or gravel, separated from the soil by geotextile |
| Outlets | At most 20 m apart on dry sites and 5 m on wet sites, to a council-approved stormwater system | Low end through the hob or into stormwater; upper end brought to the surface and capped | To the kerb and channel, a stormwater pipe or a gully, at 25 m intervals |
These are typical details for the conditions each document states. The wall's engineer sets the pipe, fall and outlets for the actual site.
How do you install agi pipe behind a retaining wall?
Agi pipe is installed at the base of the wall, on a steady fall to its outlet, inside the free-draining layer and before the wall is backfilled. Austral Masonry's guide for its concrete sleeper walls sets out this order:
- Lay filter fabric at the base of the wall, so it separates the drainage gravel from the soil.
- Place the slotted agi pipe at the base, laid to a uniform fall of 1 in 100.
- Backfill with free-draining gravel to 200 mm from the top of the wall.
- Compact a soil plug over the drainage layer to stop silt getting in.
- Drain the low end of each run to a stormwater system the council approves as a lawful point of discharge.
City of Canterbury Bankstown's detail also brings the upper end of each pipe run to the surface and caps it. Austral Masonry says subsoil drains should be flushed at regular intervals. Where the engineer's drawing differs from a product guide, follow the drawing.
Do retaining walls need weep holes?
Some retaining walls need weep holes, and published council details add them as a backup outlet to the agi pipe, not a replacement for it. Brisbane City Council's standard block wall drawing says to install weep holes in addition to the longitudinal drain, for maintenance and overflow. Where that drain can't reach an outlet, the drawing lines the drain up with the weep hole inlets so the water discharges through the weep holes. RMIT University's building notes also list a series of weep holes in the base of the wall as a way to reduce water pressure.
Where should retaining wall drainage discharge?
Retaining wall drainage should discharge to a lawful point of discharge, such as a council-approved stormwater system, the kerb and channel or a gully. Austral Masonry's detail drains the low end of each agi pipe run to a stormwater system the council approves as a lawful point of discharge. Brisbane City Council's block wall drawing outlets the drain to the kerb and channel, a stormwater pipe or a gully. Ipswich City Council says owners must not divert or redirect overland flow from its natural path onto neighbouring properties. Check the lawful point of discharge for your site with your council.
What are common retaining wall drainage problems?
The most common retaining wall drainage problems we see are:
- drainage installed without filter fabric, which silts up over time;
- an agi pipe with nowhere for the water to actually discharge;
- backfill of clay or site soil instead of free-draining gravel.
The signs are water pooling behind the wall, damp patches, or water seeping through the face instead of draining through the weep holes. Our guide to the signs your retaining wall is failing covers the other warning signs, and a retaining wall inspection checks how the drainage is performing.
Does every retaining wall need drainage?
Yes, in practice. RMIT University's building notes say all types of retaining walls require a system to minimise the build-up of water pressure. That covers timber sleeper walls too, since the guidance applies to all types. Even a low, lightly loaded wall can fail from drainage neglect. Drainage is specified as standard in every design we prepare.
What rules apply to retaining wall drainage in Australia?
In Australia, retaining wall drainage is part of the wall's structural design, and the main Australian Standard for that design is AS 4678 Earth-retaining structures. Clause H1D3(2) of the National Construction Code (NCC) 2025 Volume Two says an earth retaining structure associated with the construction of a building or structure meets Performance Requirement H1P1 if it is designed and built to AS 4678. The NCC's explanatory information says AS 4678 covers earth retaining structures between 800 mm and 15 m high. The same note says H1D3(2) is only one way to meet H1P1. Standards Australia lists AS 4678-2002 as pending revision.
Published details carry their own assumptions. Austral Masonry's sleeper wall design table assumes no hydrostatic (water) pressure behind the wall. Brisbane City Council's standard drawing says an appropriately qualified designer or a Registered Professional Engineer of Queensland (RPEQ) should confirm it suits a specific project. Approval rules for the wall itself differ by state and council: see council approval and compliance.
When should you call an engineer about retaining wall drainage?
Call a retaining wall engineer when the site doesn't match the conditions a standard detail assumes. Austral Masonry's guide says a registered engineer should design the wall where the materials, soil conditions, loadings, drainage or geometry vary from its tables. Common triggers are:
- groundwater or a wet site, where Austral Masonry puts outlets at most 5 m apart instead of 20 m;
- no lawful point of discharge near the low end of the wall;
- an existing wall that seeps water through its face, leans or bulges;
- drainage repairs that need digging beside an existing wall.
Our retaining wall design and engineering certification specifies drainage and backfill as standard, and retaining wall repair and replacement covers drainage remediation on existing walls. To get a quote, send the wall height and length, site photos and where the water can discharge.
Retaining wall drainage FAQs
How big should weep holes be in a retaining wall?
There is no single size: the wall's design sets it. Brisbane City Council's standard block wall drawing forms each weep hole by leaving the lower half of a vertical joint free of mortar, at most 1000 mm apart and about 200 mm above finished ground. Moreton Bay Regional Council's stone-pitched wall drawing, made for council works, uses 100 mm uPVC pipes at the same maximum spacing.
Can you use clay or site soil as backfill behind a retaining wall?
Not in the published details. Brisbane City Council's standard block wall drawing calls for free-draining, non-plastic, predominantly granular backfill. City of Canterbury Bankstown's concrete wall drawing uses well-graded gravel or sand behind its crushed rock drainage layer. Clay or site soil in place of free-draining gravel is one of the most common drainage faults we see.
How do you fix retaining wall drainage?
Start with the drain itself. Austral Masonry says subsoil drains should be flushed at regular intervals to keep a retaining wall's drainage working. A missing pipe, a pipe with no outlet or clay backfill usually means the drainage has to be dug out and rebuilt. Austral Masonry also says no excavation should go below a 45° line from the base of the wall, so get an engineer's assessment before digging.
Do concrete block retaining walls need drainage?
Yes. Brisbane City Council's standard drawing for core-filled concrete block walls includes a full-height filter drainage layer, a longitudinal drain at the base and weep holes for maintenance and overflow.