Why some roofs grow so much of it
New Zealand Steel and Dimond both say temperature, dust and rainfall can create a good environment for lichen, and that it can grow on almost any surface. Moss and mould turn up most where trees overhang the roof, Resene says.
The roof itself plays a part. Resene says concrete tiles become relatively porous as they weather and readily support moss and mould, and BRANZ's Level guidance says lichens and mosses grow on damp, weathered clay tiles.
So does where you live. Invercargill Aero averages 157 wet days a year and Blenheim just 79, according to NIWA, and West Auckland's Henderson station has a mean 9 am humidity of 86%. A shaded south-facing slope in a damp suburb will always grow more than a sunny roof in a dry one.
Does it actually harm the roof?
Mostly it's slow. Moss holds water against the surface, and clumps that wash off end up in the spouting and downpipes. That's where the real damage tends to start: insurers don't treat blocked spouting that overflows and damages walls as sudden damage, so it usually isn't covered.
It matters more if you're planning to paint. Resene says mould that isn't removed within a few months of appearing gets into the paint film and can grow through later coats, so a roof has to be properly cleaned before it's painted.
Upkeep is also part of keeping a warranty. Metal roofing needs a small amount of ongoing maintenance to meet its durability requirements and keep its warranty, and Gerard's maintenance table for its pressed metal tiles sets a chemical or moss treatment every 3 years.
Keep the water-blaster away from older roofs
If your home was built before 1 January 2000, the roof could be asbestos cement, and nobody should water-blast it. WorkSafe's method for weathered asbestos cement and decramastic tile roofs is biocide from a low-pressure sprayer, with the surface kept wet and no high-pressure spraying or blasting. Gerard also says water-blasting isn't recommended for cleaning growth off its tiles. Not sure what yours is made of? Start with is my roof asbestos?
Fast treatments or slow ones?
| Fast-acting wash | Slow spray-and-leave | |
|---|---|---|
| How it works | A bleach-based (hypochlorite) solution left on for hours, then the dead growth is scrubbed or rinsed off | Sprayed on and left alone. Wind and rain do the work, and it isn't rinsed off |
| How long it takes | Resene says 2 to 4 hours for moss and mould; lichen can take several days, with dead growth removed after 1 to 2 weeks | Wet & Forget NZ says algae goes within days, black mould takes 2 to 4 months and heavy lichen 6 to 12 months |
| Watch out for | It's a strong bleach: keep it off glass, aluminium joinery and plants, and never mix it with acids or ammonia | Divert rainwater tanks until at least 36 mm of rain has fallen |
| Not for | Gerard pressed metal tiles: the maker says never use bleach-based cleaners on them | Any roof the label doesn't list, so check it against yours |
| Suits | Cleaning before a roof is painted | Ongoing upkeep when there's no rush |
What to do about a mossy roof
Look from the ground
Stand back in the yard or across the street, with binoculars if you have them. Note which slopes are worst and whether the spouting is sprouting green too.
Work out what the roof is
COLORSTEEL, concrete tile, clay tile, decramastic or old cement sheet all need different handling. If it's pre-2000 cement sheet, treat it as possible asbestos until it's tested.
Deal with the cause
Trim back branches that shade or drop leaves on the roof. For big trees, an arborist is the right call. Less shade and less leaf litter means slower regrowth.
Match the treatment to the roof
Resene lists painted metal roofing such as COLORSTEEL and tile roofs among the surfaces its Moss & Mould Killer suits, while Gerard recommends its MossBoss or a quaternary ammonium wash for its pressed metal tiles. Lichen often needs scraping as well, which is roof work.
Protect your water supply
If you collect rainwater, disconnect the tank before any treatment. Roofing Industries and Resene both say to disconnect it when the roof is being washed.
Book someone with fall protection
WorkSafe says people working on single-storey homes need the same level of fall protection as on two-storey homes. A roof cleaner or roofer who brings edge protection or scaffolding is doing it properly.
Treatment, cleaning or painting?
A moss treatment on its own suits a roof that's sound and just looks tired. If the coating is failing underneath, cleaning is only the first step of roof painting, and the painter will want the roof properly cleaned and dried before the first coat.
Where cleaning reveals rust, cracked tiles or tired ridge capping, you're into a bigger decision. Our guide to restoration or re-roofing helps you weigh it up.
What you can do from the ground, and what needs a pro
You can
- Trim back overhanging branches you can reach safely, or call an arborist for big trees
- Treat a low shed or carport roof you can reach from the ground, following the label
- Keep downpipe outlets at ground level clear
- Disconnect your water tank before any roof treatment
- Photograph the roof from the ground each year to see if it's spreading
Leave to a roofer
- Anything that means being on the roof
- Scraping lichen and cleaning before painting
- Low-pressure cleaning on decramastic or possible asbestos roofs
- Clearing spouting (gutters) you can't reach from the ground
- Checking for damage the moss has been hiding
What we'd do about a mossy roof
We'd trim back whatever is shading the roof first, then have it treated with a product that suits the roofing, applied by someone with proper fall protection and no water-blaster in sight. If the roof is older than 2000 or decramastic, we'd ask about testing and the maker's method before anyone starts. Then we'd check the spouting every autumn.
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.