The wall holds the soil. The membrane holds the water.
Retaining wall waterproofing Sydney
A retaining wall against a habitable space is a basement wall that nobody called a basement. We waterproof it the same way: on the soil side, fully bonded, protected and drained, before the backfill goes in.
Why retaining walls leak into houses
Sydney builds on slopes. Split-level homes, garages cut into hillsides, lower-ground living rooms with a garden bed behind the wall. Every one of those walls has wet soil pressing against it for months at a time, and concrete and block are not waterproof. Water finds the cold joints, the block cores and the wall-to-footing junction, and it arrives inside as damp plaster, salt staining, mould and a smell that never leaves.
The usual fix, a waterproof paint on the inside face, is fighting hydrostatic pressure from the wrong side. It blisters and fails because water is pushing it off the wall. The membrane has to be on the soil side, where pressure pushes it on.
How we waterproof a retaining wall
- Prepare the wall. Block cores filled, render or bagging cured, sharp edges dressed, surface primed. Torch-on membrane needs a sound, dry substrate and we don't skip this.
- Torch-on membrane, soil side. Bitumen membrane fused to the wall, welded at every lap, carried continuously across the wall-to-footing junction and terminated above finished ground level.
- Protection board and drainage cell. The membrane is covered before backfill so rock and compaction can't puncture it, and a drainage layer takes water down to the ag drain instead of letting it stand against the wall.
- Ag drain and outlet. A subsoil drain at the footing, connected to stormwater, so the membrane is rarely under real pressure at all.
- Photos of everything. Once it's backfilled nobody will ever see this work again. You get the record.
New walls and existing walls
On new builds and landscaping projects we work in sequence with the builder or landscaper: wall, membrane, drainage, then backfill. It's a one-day window that saves a five-figure excavation later. On existing walls that are already leaking, the honest answer depends on access. If the soil side can be excavated, we tank it properly. If it can't, we'll tell you what interior drainage management can realistically achieve rather than sell you a paint system that fails by winter. Below-ground rooms and full basements are covered on our basement waterproofing page.
- Membrane on the soil side, where it works
- Continuous through the wall-to-footing junction
- Protection board and drainage cell before backfill
- Ag drain and stormwater connection coordinated
- Photo record of the work before it disappears
No fluff. No buzzwords.
Straight answers
Can you waterproof a retaining wall from the inside?
Interior coatings fight water pressure from the wrong side and usually fail. Where the soil side can be excavated, that's where the membrane goes. Where it genuinely can't, interior drainage management is sometimes a workable compromise and we'll be straight about its limits.
When should a retaining wall be waterproofed?
After the wall is built and cured, before any backfill. It's the only time the soil face is reachable. If your landscaper or builder is about to backfill, call us first.
Do garden retaining walls need waterproofing?
A freestanding garden wall with nothing behind it but more garden, no. A retaining wall with a garage, bedroom, rumpus room or any habitable space on the dry side, yes, and the earlier the better.
Does a retaining wall need drainage as well as a membrane?
Yes. Drainage removes the water; the membrane handles what's left. A membrane under permanent hydrostatic pressure with no drainage behind it is working ten times harder than it needs to.
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