Moss, mould or lichen?
Moss is the soft green cushion, usually on shaded slopes and under overhanging trees. Lichen is the flat, crusty growth that grips far harder. NZ Steel says lichen on a COLORSTEEL roof is almost impossible to remove completely by physical means without damaging the paint, so the chemistry has to do the work.
Dark patches can also just be dirt. NZ Steel and Roofing Industries both describe a quick bleach spot test: if the dark area disappears, it's lichen or fungus. On a roof, that's a test for the roofer.
Which treatment suits your roof?
| What the maker points to | What to avoid | |
|---|---|---|
| COLORSTEEL | A 2% sodium hypochlorite wash on a soft broom, washed off with plenty of water after 3 to 5 minutes | Stronger mixes, which NZ Steel says may harm the coating |
| Concrete and clay tiles | Resene lists tile roofs among the surfaces its Moss & Mould Killer suits | Chemicals the tile maker hasn't approved: Monier's warranty excludes damage from unsuitable ones |
| Gerard pressed metal tiles | Gerard MossBoss or a quaternary ammonium sanitising wash | Hypochlorite (bleach) cleaners, which Gerard says should never be used |
| Decramastic and asbestos cement | Biocide from a low-pressure sprayer, keeping the surface wet, per WorkSafe | High-pressure spraying, blasting or abrasive methods |
What a moss treatment job looks like
A look first
The roofer identifies the roofing, looks for damage under the growth, and decides if an older roof needs an asbestos test.
Tanks and gardens protected
Downpipes feeding a rainwater tank are disconnected. Resene says its Moss & Mould Killer is a strong bleach to keep off plants, glass and aluminium joinery.
The right strength
Resene's dilutions run from 1:4 for mild growth to 1:1 for severe growth or lichen. Heavy growth may need a second round.
Rinse, or leave it to the weather
With a fast wash, the dead growth is scrubbed and rinsed off. Lichen is stubborn: Resene's roofing guide says it needs scraping off, but NZ Steel warns physical removal damages COLORSTEEL's paint, so the method has to suit the roof.
Spouting cleared
Dead moss ends up in the spouting (gutters), so it's cleared at the end or on a return visit.
What about Wet and Forget?
It's a brand plenty of people ask about. Wet & Forget NZ says its products are mixed and bottled in Silverdale, and that its Moss & Algae Remover is mixed 1 part to 5 parts water and sprayed onto a dry surface. It isn't rinsed off, because rinsing would stop it working.
The trade-off is time. By the maker's own figures, black mould can take 2 to 4 months to go and heavy lichen 6 to 12 months. It's one option a roofer may suggest, not one we're recommending over others, and the label still needs checking against your roofing.
If you'd rather hand the whole job over, tell us about the roof and we'll pass it to one checked local roofer.
Your jobs, and the roofer's
You can
- Cut back branches that shade the roof, since shade and leaf litter feed regrowth
- Treat a garden shed or low carport from the ground, with a product labelled for that surface
- Divert your rainwater tank before treatment day
Leave to a roofer
- Spraying, scrubbing or scraping on the roof itself. WorkSafe says single-storey homes need the same fall protection as two-storey ones
- Choosing a product your roofing maker accepts
- Treating decramastic, clay or possible asbestos roofs
Our pick for a mossy roof
We'd match the product to the roofing first, then decide on speed. Painting soon? A fast wash that's rinsed off makes sense. If not, a spray-and-leave treatment avoids scrubbing and rinsing, as long as you can wait months for lichen to fade. Either way, we'd trim the trees first.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.