Sydney is a city of slopes, so retaining walls are everywhere — sandstone, besser block, timber sleeper, concrete sleeper, rendered masonry.
They are also the surface where cleaning is most often the wrong first move, because the marks on the face are usually a report on what is happening behind it.
Read the staining first
White bloom. Efflorescence — salts carried out of the wall by water moving through it. Normal on new masonry for the first year or two. Persistent on an older wall means water is tracking through, and that points at the drainage.
A damp patch that never dries, or a vertical streak. Water is concentrating at that point — often a blocked or absent agricultural drain behind the wall.
Rust streaks. Corroding reinforcement inside the wall or a corroding fixing. The cause is internal and cleaning the stain is temporary — here.
Crumbling or flaking face. Salt crystallisation inside the material, and washing accelerates it. On masonry that is the same mechanism as fretting brickwork; in reinforced concrete it is concrete cancer.
Bulging, leaning, or stepped cracking. Structural. Stop, and get an engineer.
Weep holes are not optional
The small holes along the base of a masonry retaining wall are its pressure relief. Water that builds up behind an unrelieved wall exerts enormous hydrostatic pressure, and that is how retaining walls fail.
Two rules. Never direct a pressure washer at a weep hole — it forces water and debris the wrong way into the drainage layer. And never let them be blocked by a garden bed, mulch or paving built up against the wall.
Clearing blocked weep holes is genuinely useful maintenance and takes minutes. Same principle as weep holes and subfloor vents.
By material
Sandstone. Very common in older Sydney gardens, and it blackens biologically in shade. Low pressure, no acid, dilute hypochlorite for growth. High pressure erodes the face permanently — here, and why it goes black in here.
Besser block. Porous and alkaline, no acid, moderate pressure at most — cleaning a besser block wall.
Concrete sleepers. Reasonably robust. Watch for rust streaks from the reinforcement and from the steel posts they sit in.
Timber sleepers. Low pressure, timber cleaner. Check for rot at ground level — that is where they go. Treated pine sleepers also leach, and the runoff is corrosive to Zincalume.
Rendered walls. Soft wash, and tap for drumminess first — how to clean render.
Stone cladding over a block wall. The most delicate case, because water behind the veneer debonds it — cleaning stone cladding.
The sealing question
If you seal a retaining wall, use a breathable impregnating sealer only.
This matters more here than anywhere. A retaining wall has permanently damp soil on one side and the only escape route for that moisture is the face. Seal the face with a film and the water evaporates inside the wall instead, crystallising salts within the material and breaking it apart.
The argument in full is in sandstone sealer, and it applies to every retaining wall material.
The garden side
Most retaining wall problems are solved from above rather than at the face.
Check that the ag drain outlet is clear and actually discharging somewhere. Check that surface water is not being directed at the back of the wall by a downpipe, a path or a reshaped garden bed — blocked downpipes is a common culprit.
Keep soil and mulch below the weep hole line.
For a clean and an honest read on what the staining is telling you, see stone cleaning or concrete cleaning.
