Lichen on sandstone: the slow damage under the patina
Lichen isn't sitting on your sandstone — it's anchored into it. Rhizines work between the grains and the colony holds moisture against porous stone through every wet-dry cycle. On a heritage wall the patina argument is fair. On steps it's a slip hazard. The fix is low pressure, correct chemistry and patience — never blasting or sealing, both of which the NSW Heritage salt-attack guide warns against.
Lichen is not dirt sitting on the stone
Lichen isn't grime. It's a fungus and an alga living as one organism, and it doesn't rest on your sandstone — it holds on. The fungal threads called rhizines work down between the sand grains, into the pores and the soft natural cement that binds them. On glazed tile, lichen can only cling to the surface. On sandstone it gets a handhold in the material itself, which is why a stiff broom takes the top off a colony and leaves the anchor behind, ready to regrow from the same spot.
The damage isn't the grip so much as what the grip keeps. A mature colony behaves like a mat laid over porous stone: it takes up water, then releases it slowly into the face beneath instead of letting it evaporate. Stone under lichen stays damp long after the wall beside it has dried. Sydney sandstone is built to breathe — wet, then dry, over and over. Extend the wet half of that cycle for years and the face softens and sheds, grain by grain. Nothing dramatic happens on any given weekend. That's the problem.
On a heritage wall the patina argument is real
There is a genuine conservation case for leaving lichen alone, and it isn't sentimental. Weathered sandstone develops a hardened outer face over decades — the honey-coloured skin that makes an old wall look like it grew where it stands. Scrub a boundary wall back to raw, even colour and it reads as new stone in an old garden, which is a worse outcome than the growth was. On a heritage wall well clear of foot traffic, doing nothing is a defensible decision, and we will say so when it applies.
But be clear about what the patina actually is. The patina is the weathered stone. The lichen is a tenant sitting on it, holding water against it, and the two are separable — a careful clean lifts the growth and leaves the aged face intact. The version worth resisting is the one where a wall gets stripped back to fresh, pale, open-pored stone, because that surface then drinks water and recolonises faster than what it replaced. Keep the patina. Evict the tenant.
On steps the argument ends
Lichen and the algal film that grows alongside it turn slick the moment they are wet, and sandstone treads in a shaded side return stay wet a long time after rain. It is usually the third or fourth step down — the one in permanent shade, the one you take in the dark carrying shopping. A patina you admire from the terrace is a different object from a patina you stand on in wet shoes. Steps, thresholds and pool surrounds are function first, character second.
So split the property in your head rather than treating it as one decision. Treads, landings and the path to the bins get cleaned on a schedule. The far wall, the rock face and the retaining wall behind the garden bed can keep their growth if you like the look of it. We do exactly this across the North Shore and the Eastern Suburbs — steps done properly, the boundary wall left as found — because those two surfaces are being asked to do completely different jobs.
Blasting and sealing are the two intuitive wrong answers
Two fixes feel obvious, and both are warned against in Salt Attack and Rising Damp, the guide published by the Heritage Council of NSW and partner agencies. The first is blasting. Lichen holds tight, so force seems like the answer — but a high-pressure jet strips the weakened outer face along with the growth, and on salt-affected stone that face is precisely what you can least afford to lose. The stone does not recover. The colony, rooted below the surface, frequently does.
The second is sealing. Coating a wall after a clean feels like insurance, and the guide is direct about why it is not: an impermeable skin traps salts and moisture behind it, the decay carries on out of sight, and when the coating eventually lets go it takes sound stone with it. Sealing lichen-affected stone is worse again, because you are locking in the moisture the colony spent years putting there. Sandstone survives by letting water leave as vapour. Don't shut that door.
Low pressure, correct chemistry, patience
What works is unglamorous. Pressure closer to heavy rain than a cutting jet, a treatment matched to what is actually growing, and dwell time instead of scrubbing. Lichen dies slowly and lets go slowly. A good sandstone clean often looks unfinished on the day and keeps improving for weeks as the dead colony weathers off the face. Anyone promising an instant transformation on a heavily colonised wall is either about to blast it or about to disappoint you, and usually both in that order.
The honest limit: heavily colonised stone does not come back quarry-fresh. Where a colony sat for twenty years you may keep faint discs and darker haloes, because the lichen etched and stained the grain and that record stays in the stone. It should not be blasted out to chase an even colour. Sandstone is quoted per job — growth, access and stone condition vary that much — and the Schmick instant quote returns a firm, bookable price in about 60 seconds. Bundling the steps with other work on one visit saves 5-15%.