A homeowner in Frenchs Forest once showed us a roof where a previous cleaner had pressure washed the lichen off. Half the discs were gone. Where each one had been, there was a pale crater in the tile where the surface had come away with it. The roof was cleaner and permanently damaged at the same time.
Lichen is the growth people most often ask us to “just blast off”, and it’s the one where that approach does the most harm. Here’s why, and what works instead.
What lichen actually is
Lichen isn’t a plant and it isn’t moss. It’s a partnership between a fungus and an alga living as one organism. The fungus provides structure and anchors into whatever it’s growing on; the alga makes food from sunlight. That’s why lichen can live on bare rock, roof tiles, concrete and sandstone where nothing else grows: it doesn’t need soil.
The anchoring is the problem. Lichen sends fine threads called rhizines into the pores of the surface. On a concrete or terracotta tile, on a sandstone paver, on old render, those threads penetrate a millimetre or more. It also secretes weak acids that slowly etch the surface it’s attached to, which is how lichen breaks down rock over centuries and how it dulls a roof over a decade.
Why Sydney roofs are covered in it
Lichen wants moisture, light and a rough porous surface. Sydney supplies all three. The Northern Beaches and North Shore get sea mist and overnight dew most of the year; the Eastern Suburbs get salt spray that holds moisture on the surface. Concrete roof tiles from the 1960s to the 1990s are porous and rough, and they’re everywhere. Sandstone is porous by nature. Shaded south-facing pitches under trees are ideal.
On the ground it’s the same story: sandstone steps and pavers in Vaucluse and Hunters Hill, concrete driveways in Belrose, retaining walls anywhere with overhanging gums.
Why pressure washing fails
Two reasons. First, the lichen is stronger than the surface. To remove it with force you have to exceed the strength of the bond, and on a porous tile or stone that bond is stronger than the surface itself, so the surface goes first. Second, whatever you don’t remove is still alive. Blasting knocks the visible body off and leaves the anchoring threads behind, and it grows back from them.
Scraping is worse. Wire brushes and scrapers on sandstone leave scratch marks that stay forever.
What works: kill it, then let it go
The method that works is slower and less satisfying to watch. The lichen is treated with a biocide, usually a sodium hypochlorite mix on roofs and concrete, or a gentler quaternary ammonium or percarbonate treatment on sandstone and heritage surfaces. The treatment is left to dwell so it penetrates the organism, then the surface is rinsed at low pressure.
Here’s the part customers need to know before the job: it won’t all come off on the day. Dead lichen loses its grip gradually. Over the following weeks and months the discs dry, shrink, and weather or wash away with rain. A roof treated in spring often looks its best by the end of summer. We tell every customer this before we start, and we put it in the photo report, because a roof that looks 70 percent clean on day one and 95 percent clean at month three is exactly what a correct job looks like.
Sandstone needs a different hand
Lichen on sandstone is common across the Eastern Suburbs and the older harbourside suburbs, and sandstone is soft. We don’t use strong hypochlorite on heritage stone; it can leave a bleached patch that reads as damage. We use a milder biocide with a longer dwell, sometimes two visits, and we accept that some deeply-established colonies on very old stone are best left to fade rather than forced.
Keeping it away
Lichen returns where conditions suit it. Trimming back trees that shade a roof, keeping gutters clear so tiles dry, and on sandstone a breathable sealer that reduces water uptake all slow it down. A maintenance treatment every two to three years is far cheaper than another full clean, and far cheaper than replacing tiles that were pressure-washed into craters.
Written by the crew at The Prestige Standard. The Frenchs Forest roof described above was a real assessment; we photograph every job before and after so customers can see the difference between removal and damage. This is general guidance based on our experience across Sydney’s Eastern Suburbs, North Shore and Northern Beaches.
