Metal tile roof cleaning is washing grime, streaks and lichen off a coated metal tile roof. The metal roof code of practice allows a hose and soft brush, or low-pressure water blasting up to 20 MPa with the jet kept away from openings and laps. On this Auckland job our machine ran below 20 MPa.
Metal tile roof cleaning is washing the dirt, streaks and lichen off a roof made of metal tiles without harming its coating. On this Auckland job, My Homes Roofing Expert cleaned a pale green metal tile roof with a high-pressure cleaning machine run below 20 MPa. Here is what we found, what the metal roof code of practice says about water blasting, and what to ask before anyone cleans your roof.
What is metal tile roof cleaning?
Metal tile roof cleaning is the wash itself: removing grime, black streaks, lichen and other growth from the coated surface of a metal tile roof. It is part of our roof cleaning service in Auckland, which also covers concrete tile and long-run roofs.
On a coated metal roof, how the wash is done matters more than the name of the machine. NZ Metal Roofing Manufacturers publishes the NZ Metal Roof and Wall Cladding Code of Practice. Its washing clause says regular washing of pre-painted and metallic-coated roofing “increases durability by reducing attacks from airborne salts and pollutants.”
What did we find on this metal tile roof?
The roof is pale green metal tile. Black streaks ran down the rows of tiles, and round, pale lichen spots had formed, most of them near the edge of the roof and on its lower section.
The roof’s age and the tile maker are not recorded for this job, so we do not guess at them. Lichen is more than a stain, and the code of practice explains why.
“Lichen and mould retain moisture, are acidic, and have tiny roots that can penetrate a paint coating.”
NZ Metal Roof and Wall Cladding Code of Practice, 16.7.1 Lichen and Mould
The same clause says removal is necessary to prevent damage to the coating, but that recolonisation is very likely.
Can you water blast a metal tile roof?
Yes, within limits. The code of practice gives two ways to wash pre-painted and metallic-coated roofing, and only one of them uses a water blaster.
“Washing may be carried out with a hose and a soft bristle brush, using fresh water or a 10% solution of household detergent and water followed by a thorough rinse with clean water. Alternatively, low-pressure water blasting can be used at pressures up to 20 MPa, with the jet directed away from openings and sheet laps.”
NZ Metal Roof and Wall Cladding Code of Practice, 16.7 Washing
20 MPa is about 2,900 psi, if your cleaner’s machine is labelled in psi. The code calls water blasting up to that limit “low-pressure”. More than that, or a jet aimed into the laps and openings, is outside what the code describes.
The code also warns that “stronger concentrations of cleaners than those recommended can damage coating surfaces”. It says to clean from top to bottom and rinse straight away with fresh water, “avoiding over-cleaning or scrubbing, which can damage painted surfaces.”
How did we clean this metal tile roof?
We cleaned this roof with a high-pressure cleaning machine, run below 20 MPa, inside the code’s limit for low-pressure water blasting. “High-pressure” describes the machine. The pressure on the roof is what the code limits, and that is the number to ask about.
The black streaks and lichen spots are gone from view, and the roof reads as an even pale green. The exact pressure and the nozzle were not recorded, so we give no figure beyond “below 20 MPa”. You can see the two photos side by side in our metal tile roof cleaning case study.
Is soft washing better than water blasting for metal tiles?
It depends on the coating. The code lists the hose and soft bristle brush first and low-pressure water blasting as its alternative. Our professional roof cleaning guide describes soft washing as the safest method for painted metal roofs, and notes that high pressure can lift or damage roof paint if the coating is already beginning to fail.
| Method | What the code of practice says | Where to be careful |
|---|---|---|
| Hose and soft bristle brush | Fresh water or a 10% solution of household detergent, then a thorough rinse with clean water | Avoid over-cleaning or scrubbing painted surfaces |
| Low-pressure water blasting | Pressures up to 20 MPa, with the jet directed away from openings and sheet laps | Ask the cleaner what pressure the machine runs at |
| Lichen removal | Wash down, apply a 2% sodium hypochlorite solution by low-pressure spray, broom or brush, leave 5 minutes, then rinse | Read the product label for its active ingredient |
What should you ask before anyone cleans your metal tile roof?
- What pressure will the machine run at, in MPa or psi? The code’s limit for low-pressure water blasting is 20 MPa.
- Will the jet be kept away from the sheet laps and openings?
- Which cleaner will be used, and at what strength? Stronger concentrations than recommended can damage the coating.
- Will the roof be cleaned from top to bottom and rinsed straight away with fresh water?
- Do you collect rainwater? The code says that if the runoff is used for drinking water, roof outlets must be disconnected before the roof is washed with detergents.
- Who will be on the roof, and how will they work safely at height?
On the last point, WorkSafe New Zealand’s working-at-height guidance explains the risks. A metal tile roof is still work at height, however gentle the pitch looks.
How often should a metal tile roof be washed?
Most exposed metal roofing, the code says, is “adequately cleared of dirt by rain washing”. The parts rain does not reach are different. Its scheduled maintenance section says these unwashed areas should generally be washed every 3 months in severe environments and every 6 months in moderate ones.
Where lichen has been treated, the washing clause adds that “regular inspection should be undertaken to curtail spread of re-growth.” Tile shape matters too. As our metal tile vs long-run comparison explains, tile geometry creates more debris-catching edges than sheet does, so check the lower rows and edges first.
How much does metal tile roof cleaning cost in Auckland?
On our published prices, roof cleaning in Auckland costs $300–$500 for a roof up to 100 m², $500–$800 for 100–180 m² and $800–$1,400 for larger roofs. We confirm the price for your roof once we have seen its size, its access and how much growth is on it.
Book a free roof inspection or call 022 501 9921, and ask us which method suits your roof’s coating.
What Questions Do Auckland Homeowners Ask?
Can you water blast a metal tile roof?
The NZ Metal Roof and Wall Cladding Code of Practice allows low-pressure water blasting at pressures up to 20 MPa, with the jet directed away from openings and sheet laps. Its other method is a hose and a soft bristle brush with fresh water or a 10% household detergent solution. On our Auckland job the machine ran below 20 MPa.
What pressure should be used to clean a metal tile roof?
The code of practice sets 20 MPa, about 2,900 psi, as the upper limit for what it calls low-pressure water blasting on pre-painted and metallic-coated roofing. A roof whose coating is already failing may need less, or a soft wash instead. Ask your cleaner what pressure their machine will run at, and where the jet will point, before they start.
Does lichen damage a metal tile roof?
It can. The metal roof code of practice says lichen and mould retain moisture, are acidic and have tiny roots that can penetrate a paint coating, so removal is necessary to protect the coating. It also warns that regrowth is very likely, and that a treated roof should be inspected regularly to curtail its spread.
Should I clean my metal tile roof myself?
We recommend against it. Cleaning a metal tile roof is work at height, and WorkSafe New Zealand’s working-at-height guidance explains the risks. If you hire someone, ask what pressure and what cleaner they will use, and whether the downpipes to a drinking-water tank will be disconnected before any detergent goes on the roof.
How much does metal tile roof cleaning cost in Auckland?
Our published roof cleaning prices in Auckland are $300–$500 for a roof up to 100 m², $500–$800 for 100–180 m² and $800–$1,400 for larger roofs. The price for your roof is confirmed once we have seen its size, its access and how much growth is on it, and the roof inspection that comes first is free.
Topics covered
- can you water blast a metal tile roof
- metal tile roof washing
- clean a metal tile roof
- lichen on a metal roof
- metal roof cleaning Auckland