Never water-blast a roof that could be asbestos
A home built before 1 January 2000 may contain asbestos, WorkSafe says, and you can't tell by looking. Businesses must not use water spray above 350 kPa on asbestos, and Health NZ's advice for asbestos-cement roofing is low-pressure water, a soft brush and hand tools only. If your roof is cement sheet, old shingle or decramastic, run it through our asbestos checker before anyone books a clean.
Why would a roof need cleaning?
Salt and grime on metal. On a COLORSTEEL roof, NZ Steel asks for nothing more than rain washing on the open sheets. The spots rain misses are different: under solar panels, around flues, beneath aerials and other sheltered areas need hand washing every 6 months in its Mild and Moderate categories, every 3 months in Severe and monthly closer to the surf. NZ Steel says washing those areas is a condition of the warranty.
Moss, mould and lichen. Growth holds moisture, and dead clumps wash into the spouting (gutters) and downpipes. The Insurance Council counts damage from blocked, overflowing gutters as gradual, which means it isn't covered.
Before painting. Paint won't hold on dirt, chalk or growth, so every roof painting job starts with a clean.
How hard can each roof be washed?
| Gentle enough | Too far | |
|---|---|---|
| COLORSTEEL and longrun | Water with a sponge or soft nylon brush, or water-blasting at no more than 2900 psi, says NZ Steel | Scrubbing lichen off, which NZ Steel says can't be done fully without damaging the paint |
| Concrete tiles | Treatment and a gentle clean by a professional | Pressure washers, which Monier strongly advises against. Its warranty excludes damage from cleaning with unsuitable chemicals or pressure |
| Clay tiles | Treatment and a light rinse | Heavy foot traffic. BRANZ's Level says clay tiles are easily damaged during maintenance work |
| Pressed metal tiles | The cleaning products the tile maker names | Water-blasting, which Gerard doesn't recommend on its tiles |
| Decramastic and asbestos cement | Biocide from a low-pressure sprayer with the surface kept wet, per WorkSafe | Any high-pressure spraying or blasting |
What happens on a roof clean
Identify the roof
The cleaner confirms what it's made of, and whether an older roof needs an asbestos test before work starts.
Protect water and garden
Lines to any rainwater tank are disconnected, as Roofing Industries and Resene both advise, and bleach-based products are kept off glass, aluminium joinery and plants.
Treat the growth
A moss and lichen treatment goes on and is given time to work: hours for moss, longer for lichen. Our page on moss and lichen treatment compares the options.
Rinse gently
Dead growth and grime are rinsed off. On metal roofing, Roofing Industries says a low-pressure water blaster up to 1000 psi may be used for the rinse.
Clear the spouting and check for damage
Everything that came off the roof ends up in the spouting and downpipes, so they're cleared at the end. A careful cleaner also photographs any cracked tiles or rust the growth was hiding.
Heritage homes and character areas
In Auckland's Historic Heritage Overlay, maintenance and repair is only a permitted activity if it avoids sand or water-blasting, among other conditions. Elsewhere, if your house is a scheduled heritage building, check the district plan before booking a clean.
Not sure what your roof needs? Tell us what it's made of and roughly how old the house is, and we'll pass it to one checked local roofer.
Questions to ask before you book a roof clean
- What's my roof made of, and does it need an asbestos test first?
- Which treatment will you use, and does the roofing maker accept it?
- Will you use a water-blaster at all, and at what pressure?
- How will you protect my water tank, garden, windows and joinery?
- What fall protection will you use?
- Who clears the spouting and downpipes afterwards?
- If a tile breaks during the job, how will it be replaced?
What you can do, and what needs a pro
You can
- Trim branches that shade the roof or drop leaves on it, or get an arborist for big trees
- Wash soffits and the undersides of spouting you can reach from the ground, which NZ Steel counts as maintenance
- Disconnect your rainwater tank before the roof is treated
- Check the roof's material with our asbestos checker before booking
Leave to a roofer
- Anything on the roof, including moss treatment
- Water-blasting of any kind
- Cleaning decramastic, clay or possible asbestos roofs
- Clearing spouting you can't reach safely
What we'd book
We'd book a treat-and-rinse clean rather than a hard water-blast, from someone who brings edge protection and names the product they'll use. On a coastal metal roof, we'd make washing the sheltered spots a regular habit, because that's what keeps the warranty alive. And if the roof is pre-2000 cement sheet or decramastic, we'd have it identified before anyone starts.
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.