Moss, lichen or mould: what’s on your roof?
Moss grows as soft green cushions, usually in valleys, along tile laps and on the shady side. Lichen forms flat, crusty patches in grey, pale green, yellow or orange that are stuck fast to the surface. Black or dark green streaks running down the roof are mostly algae or mould. All three are living growth, and the same type of treatment deals with all of them, just at different speeds.
Lichen is not a plant at all. Te Ara’s entry on lichens describes each one as a fungus living in partnership with a green alga or a cyanobacterium: the partner makes sugar from sunlight, and the fungus builds the body and supplies water and minerals. Lichens colonise man-made surfaces, metal included, and they simply shut down in dry spells and start again when the rain returns.
Mosses are small plants held on by fine threads called rhizoids rather than true roots. Te Ara notes that many favour damp, shady places, and a handful of city species grow better in gutters, cracks and drains than they do in the bush.
Knowing which one you have matters most for expectations. Moss browns off and lets go fairly quickly once treated. Lichen hangs on long after it has died, so a lichen-covered roof takes the longest to look clean.
Why do moss and lichen grow on North Shore roofs?
Because the climate suits them. NIWA describes Auckland as subtropical, with warm, humid summers, mild winters with few frosts and plentiful rain through the year, so a roof rarely stays dry for long. Add the Shore’s bush-clad gullies, big trees over older sections and south-facing roof planes that miss the winter sun, and spores have everything they need.
The usual trouble spots are the south and east faces, the strip under an overhanging tree, the area around chimneys and aerials, and anywhere leaves gather and hold water. Low-pitch roofs that drain slowly and older tiles with rough, porous surfaces stay damp longer, so they tend to colonise first.
Growth starts before you can see it. One roofing manufacturer’s maintenance guide says spores can sit on a roof for up to two years before anything shows, which is why a roof can look fine one winter and furry the next. A treatment kills those spores as well as the visible growth.
Parts of the roof the rain doesn’t rinse, such as under solar panels, around flues and beneath aerials, collect dirt and growth faster too. Steel roofing makers call these unwashed areas and ask owners to give them extra cleaning.
Does moss damage a roof?
It can, mainly by keeping the roof wet. Moss and lichen hold water against the surface for longer after every shower, and on painted or coated roofs that extra wet time works on the coating. The metal roofing industry’s lichen bulletin warns lichen can work into a painted surface and cause permanent damage, and a stone-chip tile maker says growth can create moisture traps that lead to corrosion.
Loose moss also ends up in the gutters. A clump sitting over a downpipe outlet is enough to make a gutter overflow in a downpour, and tile makers list blocked gutters, downpipes and valleys as a common cause of leaks. That is why a gutter clean often follows a roof treatment once the dead growth has washed down.
What we can’t tell you is how many years moss takes off a particular roof. It depends on the material, the coating and how long the growth has been there, and we haven’t found an independent New Zealand figure we would stand behind. Treat any confident number you read online with caution.
If the roof already leaks, has cracked tiles or has rusted through, a treatment won’t fix it. Repairs, leaks and re-roofing need a licensed roofer. We don’t do that work, but we can point you to one.
How does a roof moss treatment work?
The spray kills the growth; it doesn’t wash it off. The crew applies a moss and mould killer at low pressure and leaves it on the roof without rinsing. Moss usually changes colour within days to a few weeks. Rain and wind then loosen the dead material, and roofing makers say stubborn lichen and black algae can take many rainfalls and up to about six months to clear completely.
Most roof treatments sold in New Zealand use a quaternary ammonium compound, often benzalkonium chloride, as the active ingredient. It kills the cells and spores of moss, lichen, algae and mould, and stays on the surface to keep working. At least one New Zealand product’s safety data sheet classes it as very toxic to aquatic life, which is why the weather and run-off rules further down this page matter.
Heavy growth sometimes needs a second application one to two months after the first, and manufacturers’ guidance allows for that. If you want the roof to look clean sooner, a treatment can be paired with a gentle soft wash that rinses loosened growth away at low pressure. We’ll talk through both options when we quote.
Older roofs: the asbestos check before any cleaning
Before anyone cleans an older roof, find out what it is made of. WorkSafe says asbestos-containing materials may be present on buildings from before the 2000s, and decramastic roofing laid before asbestos was phased out may contain it. High-pressure water spraying, including water blasting, must never be used on asbestos-containing roofing or cladding under the Health and Safety at Work (Asbestos) Regulations 2016.
WorkSafe’s safety alert was prompted by a 1970s house with decramastic tiles that had been water blasted. Debris spread around the property and inside the home, testing found chrysotile asbestos in the tiles, and specialist remediation was needed.
So on these roofs we only quote low-pressure methods: a spray-on treatment or a soft wash, never a blaster. If the house was built before 2000, or the roof is decramastic (pressed metal tiles with a stone-chip finish), tell us when you enquire. If you’re not sure whether the material contains asbestos, check first, for example with an asbestos survey or a test. If suspect material turns up partway through, the job stops for specialist advice.
Weather and run-off rules for roof moss treatment
Treatments go on a dry roof when no rain is forecast, and none of it is allowed into the stormwater. Auckland Council’s guidance on pressure washing says moss and mould removers should be handled like any other harmful chemical, kept out of the stormwater system and not applied when rain is forecast. Stormwater runs straight to streams and the sea.
In practice the crew checks the forecast, diverts or blocks downpipes that drain to the stormwater, and keeps spray off decks, paths and gardens that drain to a sump. When you compare quotes, ask each contractor how they will stop run-off reaching the drains. Under the Resource Management Act the council can issue infringement fines of up to $2,000 for individuals and $4,000 for companies, and landowners are responsible for work done on their land.
If the roof feeds a drinking-water tank, disconnect the tank inlet before treatment. BRANZ’s Level guidance on rainwater tanks says to disconnect the pipe to the tank first and rinse all surfaces with clean water before reconnecting. One roofing maker suggests leaving the tank disconnected for at least three heavy rainfalls. If the product label asks for longer, follow the label.
What roof moss treatment costs in Auckland
A moss and lichen treatment on a small to average single-storey roof usually costs $250 to $450, based on figures Auckland operators publish. Adding it to a roof wash on the same visit is cheaper, around $100 to $250 extra. Big, steep or two-storey roofs cost more because of access and the amount of product needed, and we confirm the figure once we’ve seen the roof.
| Job | Typical range |
|---|---|
| Moss and lichen treatment, small to average single-storey roof | $250–$450 |
| Treatment added to a roof wash on the same visit | $100–$250 |
A guide only, rounded from figures Auckland operators publish (checked October 2026). Ex-GST figures have had GST added. Larger, steeper or two-storey roofs cost more. We confirm your price after seeing the roof.
How a roof moss treatment job runs
- We look at the roof from the ground, from a ladder at the gutter line or from your photos, and ask how old the house is.
- Roof type and asbestos check: pre-2000s roofs and decramastic tiles are flagged for low-pressure methods only.
- Downpipes to the stormwater or a rainwater tank are diverted or disconnected, and nearby drains, decks and plants are protected.
- On a dry day with no rain forecast, the crew applies the treatment at low pressure and leaves it on.
- The roof is left to weather. Moss browns off within days to weeks; lichen can take months to let go.
- If needed, a second application for heavy growth, a soft wash to rinse off loosened growth, or a gutter clean once the dead material has washed down.
Questions people ask about roof moss treatment
Does roof treatment really work?
Yes, as long as you judge it over months rather than days. A quaternary ammonium treatment kills moss, lichen and algae along with the spores on the surface, and the colour change usually shows within days to a few weeks. The roof only looks clean once rain has washed the dead growth off, which roofing makers put at up to about six months for lichen. It won't fix rust, failed paint or broken tiles.
How long does it take for moss to die after treatment?
Moss usually starts to brown off within days and is dead within a few weeks, depending on how thick it is and the weather. Getting it off the roof takes longer, because that relies on rain and wind, so thick cushions can sit there dead for a while. Lichen is slower again: roofing makers allow many rainfalls and up to about six months for it to clear.
What time of year is best for roof moss treatment?
Any time you get a dry, settled spell with no rain forecast for the day of application. Many owners book in autumn so the growth is dealt with before the cooler, damper months, or in spring once they notice it. Light rain over the following weeks then helps rinse the dead growth away. Avoid booking straight into a wet week, because rain soon after spraying washes the treatment off.
Will roof treatment stain my gutters or paint?
Staining isn't the usual problem with quaternary ammonium treatments: their makers list painted surfaces among the places they can be used. Bleach-based cleaners are different, and one roofing maker warns hypochlorite can affect the coating and cause discolouration on its stone-chip tiles. The more common side effect is dead moss washing into the gutters, so plan a gutter check a few weeks after treatment.
Is roof moss treatment safe for rainwater tanks?
Only if the tank is disconnected first. Take the downpipe off the tank inlet before the roof is treated and keep it off until the roof has been rinsed with clean water. BRANZ's Level guidance says rinse thoroughly before reconnecting, and one roofing maker suggests waiting for at least three heavy rainfalls. Follow the product label if it asks for longer, and tell us about the tank when you enquire.