Do spray-on roof treatments work?

Updated 2026-10-06

Yes, spray-on roof treatments work, but slowly, and only on things that are alive. They kill moss, lichen and algae, then rely on rain to wash the dead growth away over weeks to months. They won’t shift dirt, rust or tired paint. Here is how they work, how to use one safely and when professional roof cleaning is the better call.

How does a spray-and-leave roof treatment work?

It is a biocide you spray on and don’t rinse off. Most products sold in New Zealand are based on a quaternary ammonium compound, commonly benzalkonium chloride, which kills the cells and spores of moss, lichen, algae and mould. Left on the surface it keeps working, and over the following weeks rain breaks up and rinses away what it killed. There is no scrubbing and no pressure involved.

The safety data sheets we checked for two products sold here both list a benzalkonium chloride compound as the active ingredient, at under 10% in a concentrate you dilute and up to 25% in a ready-made product. Their hazard ratings differ: the concentrate is rated a mild skin irritant, while the other is classed as corrosive to skin and eyes and very toxic to aquatic life. Both sheets say to keep the product out of drains.

Because the product is meant to stay put, hosing it off straight away defeats the point, and makers tell you to leave it to soak in. That is also why the weather window matters: rain in the first few hours dilutes it and carries it off the roof before it has done its job.

How long until a spray-on treatment makes a difference?

Usually days to a few weeks for a colour change, and months for a clean roof. One roofing maker’s guidance says some growth dies straight away and disappears after a few rainfalls, while black algae and lichen can take many rainfalls and up to about six months. A treatment maker’s technical sheet gives the same six-month upper limit. Roof faces sheltered from the rain clear slowest, because rain does the removing.

• First days to weeks: moss yellows or browns and lichen starts to pale. A colour change is the sign it is working.
• One to two months: if growth was heavy and some survives, a second application is normal practice.
• Up to about six months: the last of the dead lichen and algae staining weathers away.

If you need a clean roof by a fixed date, such as a sale or a repaint, start early or plan on a soft wash to rinse off the loosened growth.

What spray-on roof treatments don’t do

They don’t clean. A treatment kills living growth, so it does nothing for dirt, soot, pollen, bird droppings, rust, chalky or peeling paint, or marks left by old growth. It won’t fix a failed coating or a leak, and it won’t make an old roof look new. If the problem on your roof isn’t alive, you need a wash, a repair or a repaint instead.

Two cases trip people up. A stone-chip roofing maker notes that dust from building sites, fertiliser drift and pollen are often mistaken for growth on newer roofs; a treatment won’t touch them, but a wash will. The same maker says white deposits spreading over a stone-chip metal tile roof usually mean the roof is due for recoating, not cleaning.

Black streaks are a mixed bag. Where they are algae, they fade over the months after treatment. Where they are dirt or rust staining, they stay put until the roof is washed or repainted.

Using a spray-on roof treatment safely

Stay off the roof, read the label and plan for the weather, the drains and any water tank before you start. Spray from the ground or from a ladder at the gutter line, on a dry day with no rain forecast. Divert downpipes away from the stormwater, disconnect any downpipe feeding a drinking-water tank, wet nearby plants first and wear gloves and eye protection.

For tanks, BRANZ’s Level guidance says to disconnect the pipe to the tank first and rinse the roof thoroughly with clean water before reconnecting. One treatment maker suggests keeping the tank disconnected for three to four weeks, or until at least 13 mm of rain has fallen.

For drains, Auckland Council asks you to handle moss and mould removers like any harmful chemical: don’t apply them when rain is forecast and don’t let them reach the stormwater, which flows untreated to streams and the sea. Infringement fines of up to $2,000 apply to individuals.

For height, WorkSafe says there is no minimum height below which a fall can be ignored. If you can’t reach the roof from the ground or from a well-footed ladder at the gutter, leave it. For older roofs, spray-and-leave is one of the right methods, as long as nobody scrapes or blasts the growth off afterwards: WorkSafe’s asbestos alert rules out high-pressure spraying on asbestos roofing, and decramastic or pre-2000s roofs may contain it.

Spray-and-leave treatment vs a professional soft wash

A DIY spray-and-leave treatment suits a small, low roof you can cover from the ground and are happy to watch clear over months. A professional soft wash suits bigger, higher or heavily grown roofs, or anyone who wants it clean sooner: the crew treats the whole roof evenly, can rinse off loosened growth at low pressure and controls the run-off.

• Coverage: DIY reaches what you can spray from the ground; a crew with planned roof access covers every face, including the high ridge and the back of the house.
• Speed: spray-and-leave relies on the weather; a soft wash can rinse off loosened growth on the day or on a return visit.
• Safety: DIY should stay off the roof entirely; contractors plan access and fall protection for each roof.
• Run-off: either way, downpipes and drains need to be controlled. Ask any contractor how they will do it.
• Cost: a bottle from the hardware store costs far less than a visit, but a crew can do a whole roof in one go.

Plenty of owners do both: a soft wash to get the roof back to clean, then a spray-and-leave treatment from the ground every year or two on the faces that grow back first.

Should you treat the roof or repaint it?

Treat when the coating is sound and the problem is growth; repaint when the coating itself has failed. Signs of a failed coating include faded, chalky paint that rubs off on your hand, bare or rusting patches on iron, and worn, porous concrete tiles. A treatment will kill growth on a tired roof, but only a properly prepared repaint protects it again.

Every repaint starts with a clean and a treatment anyway, because paint won’t stick to moss or grime. Our roof painting page covers the full process. On concrete tiles a repaint is mostly about looks: one tile maker notes it refreshes the colour without improving waterproofing and needs redoing every five years or so. Clay tiles have their colour fired in and only ever need cleaning.