Why Auckland roofs get grubby
Humidity that rarely lets up
Mean 9 am humidity runs at 82% at Auckland Airport and 86% at Henderson River Park, climbing to 92% there in June and July. A roof that stays damp for days gives moss, lichen and mould an easy start, especially on shaded slopes and under trees.
The ranges stay wetter
The Waitākere Ranges average over 2,000 mm of rain a year, more than double the city, and Whenuapai is NIWA's most fog-prone Auckland site at 44 fog days a year. Bush-fringe homes out west usually need moss treatment more often than sunny isthmus roofs.
Salt hides in the sheltered spots
BRANZ testing near the harbour found sheltered parts of an Auckland building collected 3 to 8 times more salt than exposed ones. The metal roofing Code of Practice flags overhangs, unlined soffits and canopies as areas that corrode faster because rain rarely washes them.
Surf beaches need more washing
BRANZ says buildings closer to breaking surf need more frequent maintenance, especially washing off salt deposits. If you're near Piha, Muriwai, Te Henga or Karekare, a regular wash is part of looking after a steel roof, not a luxury.
Why the cleaning method matters
A moss treatment is a biocide sprayed onto the roof and left to kill the growth, which weathers off over the following weeks. It's gentle and suits most roofs. Water-blasting is faster, but on older roofs it can strip coatings, force water under laps and blast the stone chip off decramastic tiles.
Asbestos is the big one. WorkSafe says a home built before 2000 may contain asbestos, and asbestos cement roofing is one place it turns up. Its safe work sheet for weathered asbestos cement and decramastic roofs says to apply biocide with a low-pressure sprayer, keep the surface wet and never use high-pressure spraying or blasting. Businesses must not use water spray over 350 kPa on asbestos at all.
Heritage homes have their own limit. Under the Unitary Plan's Historic Heritage Overlay, maintenance only counts as a permitted activity if it avoids sand or water-blasting.
The size of the job comes down to roof size, pitch and height, how much growth there is and whether the roof needs slow, low-pressure work. A good written quote names the cleaning method and the product being used.
What you can do, and what to leave
You can
- Look over the roof from the ground or an upstairs window for green or black growth
- Trim branches you can reach safely from the ground
- Keep downpipe outlets clear at ground level and note any overflow
- Book a clean before any repaint, since coatings won't bond over moss
Leave to a roofer
- Anything done on the roof itself, including spraying moss treatment
- Fragile roofs: WorkSafe lists fibrolite (usually asbestos), some old masonry tiles and badly corroded metal among roofs not to walk on
- Washing salt off steel near the coast as the manufacturer's maintenance requires
- Spotting cracked tiles, rust or lifted flashings while they're up there
Before you book a roof clean
- Will you use a biocide, water-blasting or both, and at what pressure?
- Have you checked whether my roof could be asbestos cement?
- How will you stop run-off going into the garden or a rainwater tank?
- Will you wash the sheltered areas under eaves and canopies, not just the open roof?
- Does the roofing manufacturer set a washing routine for my area, and will you follow it?
Our call on cleaning an Auckland roof
On an older roof, or anything that might be asbestos, we'd choose a low-pressure moss treatment and a bit of patience over a water-blaster. On a newer steel roof near the harbour, we'd book regular washing of the sheltered areas as upkeep, since metal roofing needs some ongoing maintenance to keep its warranty. And if someone knocks on the door offering a quick water-blast, we'd pass. If you've already agreed on the doorstep, you can cancel within 5 working days of getting your copy of the agreement.
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.