Moss turns up every Sydney winter on the shaded south side of things — roof tiles, paths, retaining walls, the bottom few courses of a brick fence. It holds water against whatever it is growing on, and on a path it is genuinely slippery.
The products sold to deal with it range from effective to pointless depending on what you are actually looking at.
First, make sure it is moss
Three different organisms get treated as the same problem and they are not.
Moss is a green, soft, cushiony plant with visible structure. It has rhizoids that grip the surface but no true roots penetrating it, and it holds a remarkable amount of water.
Algae is a flat green or black film with no structure. It wipes off wet. We have written on this in that green film on your walls is not dirt.
Lichen is a flat, crusty, grey-green or pale disc that looks painted on. It anchors into the surface, and almost nothing you spray on it will make it leave. That is covered in why lichen will not pressure-wash off.
This matters because the disappointment people report with moss killers is usually a lichen problem being treated with a moss product.
What actually kills moss
Iron-based products. Ferrous sulfate or iron sulfate, sold as moss killer and as lawn moss treatment. Very effective — moss blackens within a day or two.
The catch, and it is a significant one: iron sulfate stains. Permanently, orange-brown, on concrete, pavers, sandstone and render. If you are treating moss on a path or a roof, this is the wrong product, and the resulting rust stains are harder to remove than the moss was. See rust stains on concrete and stone for what you would then be dealing with.
Keep iron-based products for lawns.
Quaternary ammonium compounds. The active in most no-rinse outdoor treatments. Effective on moss, slower than iron, no staining. Works over weeks rather than days. Our assessment of these is in does Wet & Forget actually work.
Sodium hypochlorite. Fast, effective on moss, and what professional soft washing uses at a controlled concentration. It is also the harshest option for plants and metal — see the notes on hypochlorite-based cleaners.
Potassium salts of fatty acids. The gentlest option, sold as an organic moss and algae treatment. Works on light growth, struggles with a thick mat.
Vinegar. Works, sort of, on light moss. It is also an acid, which means it etches sandstone, limestone, travertine and concrete. Not worth it on hard surfaces.
What to use where
Tile roof. Hypochlorite-based soft wash, professionally applied, or a quaternary ammonium treatment if you are doing it yourself and can wait. Never iron sulfate. Never high pressure — see why we never pressure wash a roof. And disconnect any rainwater tank first.
Colorbond roof. Gentle detergent and a soft wash. Keep hypochlorite contact time short and rinse thoroughly, because it is corrosive to the coating edges and fixings.
Concrete paths and driveways. Hypochlorite, or mechanical removal with moderate pressure. Concrete tolerates this well.
Pavers. Treat chemically first and let the moss die before touching it with pressure, otherwise you blast out the jointing sand along with the growth and create a bigger job. Replacing jointing sand across a paved area is slow work.
Sandstone and soft stone. Gentlest effective option, low pressure, no acid, no iron. Sandstone is unforgiving — see how high pressure permanently damages Sydney sandstone.
Timber decks. Purpose-made deck cleaner, not a general moss killer. See timber decks: when to clean, when to sand.
Protecting the garden
Every effective moss killer will damage plants, and moss grows in exactly the shaded damp spots where gardens also are.
Soak the surrounding beds and lawn with fresh water before you start — wet foliage and saturated soil dilute anything that lands. Cover what you can. Rinse everything down thoroughly afterwards.
Iron-based products will also stain paving, pots, furniture and the mortar in a brick path, so if you are using one on a lawn, keep it off every hard surface nearby.
Where the runoff goes
Rinse water carrying biocide should go to garden or lawn, pre-soaked, not into a stormwater drain. In NSW, discharging it to stormwater can be an offence, and stormwater in most of Sydney goes to a creek or the harbour without treatment.
On a sloped driveway that drains straight to the street, this takes active management rather than good intentions. The rules are set out in where the dirty water goes.
Stopping it coming back
Moss needs shade and moisture. Remove either and it struggles.
Cut back overhanging branches to get light and airflow onto the surface. Fix the drainage or the leaking tap that keeps the area damp. Clear the gutter that is overflowing down the wall.
On a roof, zinc or copper strips along the ridge release trace metal ions in rain that inhibit regrowth down the slope. Modestly effective, and not a substitute for cleaning what is already there.
And treat annually once clean. That is where retail products earn their money.
For established growth on a roof or a large paved area, see roof moss treatment or paver cleaning.
