A lot of what looks like rendered masonry on Sydney houses built since the 1990s is rendered polystyrene: EPS panel fixed to the frame, with a thin reinforced render coat and a texture finish over it.
From the street it is indistinguishable from solid render. Under a pressure washer it is not, and the difference shows up as permanent dents.
How to tell before work starts
Two quick checks settle it:
- Knock on it. Solid render over brick or block sounds dead and solid. Foam sounds hollow and slightly drummy.
- Look at the reveals. Window and door reveals on foam systems are usually thicker than the wall opening suggests, and edges are noticeably crisper and squarer than rendered brickwork.
Age is a reasonable guide too. Full masonry render is more common on older stock; foam systems became widespread on project homes, additions and upper-storey work from the 1990s onward.
Any contractor quoting a render wash should be doing this check as part of the quote. If nobody has knocked on the wall, nobody has established what they are about to clean.
What goes wrong
The render coat over foam is thin, measured in millimetres, and the substrate behind it has almost no compressive strength. High pressure produces:
- Dents and depressions that stay after the wall dries, most visible in raking afternoon light.
- Cracking of the thin render coat, then water behind it.
- Water driven in at joints and around penetrations, into the wall cavity.
- Texture coat stripped in patches, leaving a permanently uneven finish.
The water ingress is the expensive part. Foam systems depend on their sealed outer face; once water is behind it, it does not readily dry out, and the repair is a re-render rather than a touch-up.
How it should be cleaned
Foam cladding cleans up well by soft wash, because the problem on these walls is nearly always biological growth rather than embedded dirt. Treatment applied at low pressure, dwell time, gentle rinse. No mechanical action against the surface at all.
That is the same approach used across render cleaning and house washing; the difference is that on foam it is not a preference, it is the only safe option.
Joints, sealant and flashings
On these systems the sealant at control joints, window perimeters and penetrations is the waterproofing. Sealant has a service life shorter than the cladding, and by the time a house is fifteen years old some of those joints have usually failed.
Directing water at a perished joint at pressure puts it straight into the wall. Worth inspecting during the quote, and worth repairing before a wash rather than after.
Where it shows up most
Second-storey additions on older houses are the classic case, where the ground floor is brick and the upper level is foam. The two require different methods on the same elevation, and treating the whole facade identically damages the top half.
Coastal properties add salt to the equation. Salt does not damage the foam, but it accumulates in the texture coat and feeds growth, which is why exposed Northern Beaches and Eastern Suburbs houses need this done more often than inland equivalents.
If it is already dented
Dents in the render coat do not come back out. Repair means patching and re-texturing the affected area, and matching an aged texture finish is difficult enough that the result is often visible. Prevention is genuinely the whole story here.
If a previous clean has marked a wall, photograph it in raking light early or late in the day, when the damage is most apparent.
Written by the team at The Prestige Standard, exterior cleaning contractors working across Sydney. Fixed written quotes within 24 hours with every surface itemised, $20 million public liability. See our service areas or call 0494 810 609.
