Why Cairns roofs grow things
Warm and humid, even in winter
Mean maximums in Cairns run from 26.2°C in July to 31.8°C in December, and afternoon humidity averages 62% across the year, peaking at 71% in February. Wikipedia sums it up as hot, humid summers and very warm winters. Mould, algae and lichen don't get a cold season to slow them down.
A roof that rarely dries
With just under 2,000 mm of rain a year and a wet season from November to May, shaded parts of a roof can stay damp for weeks. That's where growth starts, usually under trees and on the side that gets least sun.
Salt on the beachside roofs
Along the Northern Beaches, from Holloways Beach to Palm Cove, salt settles on roofs too. BlueScope's maintenance bulletin says sheltered spots that rain can't wash, like the underside of eave gutters, should be hosed every three months in coastal salt spray areas.
How each roof type should be cleaned
| Roof | Do | Don't | |
|---|---|---|---|
| Concrete or terracotta tiles | Use a professional roof cleaner and a soft-wash approach | Use a pressure washer, which Monier says risks damaging tiles | |
| Colorbond | Wash with mild household detergent, a soft broom and fresh water | Use abrasive or solvent cleaners, or hard bore water | |
| Bare Zincalume | Rinse with fresh water and a soft broom | Use sugar soap, which BlueScope says shouldn't go on bare Zincalume | |
| Asbestos cement (old fibro) | Get it tested, then consider a fungicide and sealant | Water-blast it, which is illegal in Queensland |
Fibro roofs and the $10,000 fine
Queensland homeowners can be fined up to $10,000, plus clean-up costs, for water-blasting an asbestos cement roof. A house built before 1990 is likely to contain some asbestos, and you can't tell by looking. ASSEA adds that storms and floods can damage asbestos materials, so don't hose down a damaged fibro roof after a cyclone either.
Is the green stuff actually doing harm?
On tiles, mostly not. Monier says moss and lichen grow more on terracotta, especially unglazed, but only on the surface, without affecting the tile. The bigger risk is growth holding water in valleys and gutters, or hiding a cracked tile. On metal, grime and salt left sitting on the coating are the bigger worry.
If you're planning to paint, a clean is the first step. If you're not, cleaning every few years and treating it as a chance to inspect is a sensible middle path. Our guide to moss and lichen removal goes into treatments.
Ask before anyone cleans your roof
- What method and pressure will you use on my roof type?
- Which chemicals, and are they safe for my rainwater tank and garden?
- Will you check whether my roof contains asbestos first?
- Will you photograph any broken tiles, rust or loose fixings you find?
- What happens if a tile cracks while you're working?
If it were our roof
We'd book a soft wash in the dry season, ask for photos of anything broken, and skip a clean that's only cosmetic on a roof that's otherwise fine. In Cairns the growth will come back, so spend on the roof's condition first and its looks second.
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.
Let us know your suburb and the roof type. We'll pass your job to one local roofer who cleans roofs there, where we have one. More on our national roof cleaning page and the Cairns hub.