Drainage behind retaining walls in the Perth Hills
Updated October 2026
Water, not weight, is what kills retaining walls. A wall is designed for the pressure of soil. Saturate that soil and you add the pressure of the water in it, and the load can rise well beyond what the wall was built for. That is why almost every failed wall traces back to the same missing thing, and why this page exists separately from the material pages.
In the Perth Hills the problem is sharper than on the coastal plain, for a reason a council has put in writing.
Why water has nowhere to go up here
Perth is a sand city. Sand drains. The Hills does not work that way. The ground on the Darling Plateau is lateritic duricrust and pea gravel over weathered granite, and the City of Kalamunda records in its Local Planning Strategy that reticulated sewerage is constrained in the Hills because hard granite sits close to the surface and is costly to dig through.
Read that as a drainage statement and it is the whole story. Rock close to the surface means water cannot keep going down. It gets to the rock and then it travels sideways, along the interface, until it meets something. On a sloping block in Gooseberry Hill, Darlington or Bedfordale, the thing it meets is the back of your wall.
The Shire of Mundaring names the same problem from the other direction, flagging perched water tables and heavy soils as Hills constraints in its own strategy. A perched water table is exactly what it sounds like: water sitting on a layer it cannot get through, at a level nobody expected.
What proper drainage actually includes
If you only read one list on this site, read this one, because it is the list to hold a quote against.
- Ag-pipe at the base of the wall, laid behind the face with a fall, sitting in the gravel rather than on the soil.
- Free draining gravel backfill against the back of the wall, not the site clay and not the spoil that came out of the hole.
- Geotextile fabric wrapping the gravel so fine material cannot wash in and silt the drain up. A drain full of clay is not a drain.
- A legal discharge point. The water has to go somewhere lawful, and that is the part that gets skipped. Rainwater is what a stormwater drain is for, but where a stormwater connection is not available the discharge has to be designed, not improvised.
- Weep holes or a drainage layer on masonry walls, so pressure behind the face has a release.
- Reticulation kept off the wall. The most common homeowner contribution to a failed wall is a sprinkler aimed at the fill behind it, running all summer.
The septic question, which only matters here
Plenty of the Hills is not on reticulated sewerage. Outside the Kalamunda and Mundaring town centres, most properties run their own septic tank and leach drains or a secondary treatment system, sometimes called an ATU.
That changes a drainage design. A leach drain is a disposal field, and it does not want your wall's stormwater running into it. The tank, the drains and the irrigation area also decide where a machine can get in, where spoil can be stacked and where the wall's drainage can discharge. Your local government holds the system records for your address, and the Shire of Mundaring notes that records for homes built from 2008 are digital and readily available while older homes need an archive search, which takes about a week. Worth starting early if nobody knows where the drains run.
A further local note. Much of the Darling Range here sits in gazetted public drinking water source catchments, and the City of Kalamunda records that Bickley, Piesse Brook, Kalamunda, Pickering Brook and Carmel fall inside the Middle Helena Catchment Area. In a catchment, earthworks and drainage are looked at more closely. We will not tell you what the rules are, because that depends on your site, but it gets checked before work starts.
Retrofitting drainage to an existing wall
Sometimes possible, sometimes not, and an honest builder will tell you which. Where a wall is structurally sound and the problem is water, excavating behind it to install ag-pipe and gravel can buy it decades. Where posts have rotted or snapped, where the face is bulging across several bays, or where soil is escaping through the wall, drainage is no longer the fix. See retaining wall repairs for where that line sits, and wall leaning or collapsed if it is already moving.
Guide pricing
Drainage is usually a line item inside a wall quote rather than a job of its own, which is precisely why it gets dropped to win a price. Retrofit drainage on an existing wall is priced per job, because it depends entirely on access, wall height and what is behind it. The retaining wall cost guide covers how to read drainage inclusions across competing quotes.
How the referral works
Perth Hills Retaining Walls is a referral service. Tell us what the wall is doing through the form and we pass it to an experienced, insured local builder who works your suburb. They call you back with a free quote, no obligation. Before any digging, your builder should lodge a free Before You Dig Australia (BYDA) enquiry so underground services are located.
Frequently asked questions
Why do Perth Hills walls fail when the same wall on the plain survives?
Because the water behaves differently. Sand lets water keep going down. Granite close to the surface stops it, so it runs sideways along the rock and arrives at the back of the wall instead. Add a slope and you have a wall taking soil load and water pressure at the same time.
Can I just drill holes in my wall to let the water out?
Not as a fix. Weep holes are part of a designed drainage system on a masonry wall, installed with a drainage layer behind the face. Drilling a failing wall after the fact relieves very little, can let soil wash out through the holes, and tells you nothing about why the water is there. Have it assessed.
Where is the wall's water supposed to go?
To a lawful discharge point. Rainwater is what a stormwater drain is for, and where a connection is available that is usually the answer. Where it is not, the discharge has to be designed for the site, and it must not be directed at a neighbour's land or into a leach drain. Your builder works this out before the wall is built, not after.
My reticulation runs along the top of the wall. Does that matter?
Yes, more than most people expect. Watering the fill behind a wall every day through summer is a slow structural load test. Move the line, cap the heads nearest the wall, or redesign that bed. It is the cheapest thing on this page.