Why moss takes hold on Christchurch roofs
Fog and a weak winter sun
June gets the weakest sunshine of the year, and the airport records fog on about 49 days annually. A south-facing slope, or one under trees, barely dries out between showers in winter, which suits moss and lichen nicely.
Frost and July's rain days
There are around 34 frosts a year, and July brings the most rain days, 13 on average. Moss soaks that moisture up and holds it against the roof long after the weather clears.
Concrete tiles and decramastic
Heavy tiles cover a big share of the 1930 to 1979 houses in BRANZ's survey, and it counted 49 metal tile roofs among its 314 homes. Lichen loves rough concrete and stone chip. Both are easy to damage with too much pressure, and a water blaster can strip the chip straight off decramastic tiles.
Salt on the coast
In the Zone D band near the sea, the harbour and the estuary, salt builds up on parts of the roof rain doesn't rinse, such as overhangs. BRANZ testing on a harbourside building found sheltered spots collected about 3 to 8 times more salt than exposed ones, and the metal roofing code says a little ongoing maintenance, like washing those areas, is part of keeping the warranty.
Asbestos and brittle roofs: low pressure only
Homes built before 2000 may have asbestos cement roofing, and you can't tell by looking. For weathered asbestos cement and decramastic tile roofs, WorkSafe's safe work sheet says to apply biocide with a low-pressure sprayer, keep the surface wet, and never use high-pressure spraying or blasting. Businesses can't use water above 350 kPa on asbestos at all. Fibrolite and badly rusted iron are brittle, so nobody should stand on them.
How a moss treatment works
A roofer sprays a biocide (a moss and lichen killer) over the affected areas. The growth dies over the following weeks and months, and rain gradually washes it away. In a dry Christchurch summer that can take a while, so some roofers come back with a soft brush.
A full wash is usually only worth it before roof painting, when the surface has to be bare. Ask what product they're using, how they'll protect plants and paving, and disconnect any rainwater tank before spraying starts.
What makes a roof clean bigger or smaller
Roof size, pitch and height set most of it. Thick, long-established growth may need a second treatment or a soft brush-off, and a roof that could be asbestos cement or older decramastic needs low-pressure methods and extra care. A good quote names the product, the method and pressure, how runoff is handled, and whether a spouting clean and any follow-up visit are included.
What you can do, and what to leave
You can
- Look at the roof from the ground or an upstairs window to see where growth is worst.
- Cut back branches that shade the roof or drop leaves on it, working from the ground.
- Keep the spouting clear so dead moss has somewhere to go.
- On a low single-storey roof, some products go on with a hose-end sprayer from the ground. Read the label and keep it off plants.
Leave to a roofer
- Anything that means standing on the roof.
- Two-storey houses and steep pitches.
- Asbestos cement, fibrolite or older decramastic roofs.
- A pre-paint wash, where the surface has to be properly prepared.
Questions for a Christchurch roof cleaner
- What pressure will you use, and how do you handle decramastic or asbestos cement?
- Which biocide, and where does the runoff go?
- Does the job mean standing on the roof, and what fall protection will you use?
- How long until the moss dies off, and is a follow-up visit included?
- Will you clear the spouting afterwards?
- Did you spot anything on the roof that should be fixed before it's cleaned?
If it were our Christchurch roof
We'd treat in late summer or early autumn, while the roof is dry and before the frosty, foggy months, then let the weather finish the job. We wouldn't let anyone near a decramastic or pre-2000 roof with a water blaster. And if the roof needs painting soon anyway, we'd roll the clean into that job rather than pay for it twice.
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.