Why Wellington roofs grow moss and gather salt
Damp air, most of all in winter
Mean 9 am humidity at Wellington Airport is 77.6% across the year, peaking at 81.1% in June and July. Up the Hutt Valley it's damper again: Wallaceville in Upper Hutt averages 43 fog days a year, the most in the region. Roofs that stay damp for long stretches are where moss and lichen get going.
Hills throw shade
Plenty of Wellington houses sit on hillsides, below a ridge or under tall trees, and a slope that rarely sees sun stays wet for days after rain. Those are the faces that go green first, and they need treating more often than the sunny side.
The wind carries salt, and spray drifts
Homes within 500 m of the coast or harbour are in exposure zone D. Spots the rain rarely washes, like overhangs and unlined soffits, collect salt and corrode faster, so a rinse there is part of looking after the roof. Wind matters for the treatment too: with an airport mean of 25.8 km/h, a calm day is worth waiting for so the spray lands on the roof, not the neighbour's car.
Older roofs: test before anyone washes
WorkSafe says homes built before 1 January 2000 may contain asbestos, and you can't tell by looking. Its safe work sheet for weathered asbestos cement and decramastic tile roofs says to apply biocide with a low-pressure sprayer, keep the surface wet, and never use high-pressure spraying or blasting.
How a good moss treatment goes
Identify the roof first
Iron, concrete tile, decramastic or possible asbestos cement each need a different approach. If there's any doubt, a sample gets tested before work starts.
Spray on a calm, dry day
Biocide goes on at low pressure when rain isn't forecast for a while, so it soaks in rather than washing straight into the spouting.
Let it work
Moss and lichen die back over the following weeks and the weather does most of the removal. Patience beats a pressure washer here.
Clear the spouting afterwards
Dead moss washes down, so spouting and downpipes need clearing once the roof has cleaned up. If you collect rainwater, disconnect the tank before treatment.
Rinse the sheltered spots near the coast
Near the harbour, a rinse of soffits, overhangs and under-flashings on the same visit helps a steel roof last.
Possible asbestos? Don't wash it, walk on it or scrub it
If your roof might be asbestos cement, don't water-blast, scrub, drill or walk on it. WorkSafe says if you're not sure, treat it as asbestos. If it gets disturbed by accident, stop, keep people and pets away, close doors and windows, and contact an asbestos contractor. Our asbestos guide explains how to tell.
What makes a roof clean bigger or smaller
Light lichen on a single-storey iron roof is a modest job. A steep or two-storey roof, thick moss, tiles or decramastic that need extra care, or asbestos precautions all add time.
Ask for a quote in writing that names the product, says how it goes on and at what pressure, and confirms whether clearing the spouting afterwards is part of it.
Worth asking before anyone starts
- Which product will you use, and how is it applied?
- Will anything be water-blasted, and at what pressure?
- How will you protect my garden, car and rainwater tank from overspray?
- Could my roof be asbestos, and how would you check?
- Is clearing the spouting afterwards included?
- Will this affect my roof or paint warranty?
What we'd do in Wellington
We'd book the treatment for a still, settled spell in summer, when January is the airport's driest month on average, and then leave it to work rather than pay for a blast-clean. Near the harbour, we'd add a rinse of the sheltered spots in the same visit. And if the house was built before 2000 and nobody knows what the roof is, we'd get it tested 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.