A dirty roof is the fastest way to age a house from the street, and every Sydney summer brings a wave of homeowners onto ladders with a pressure washer. It’s the worst thing you can do to a tile roof, and it’s worth understanding why before you or a cheap operator does permanent damage.
What that black and green streaking actually is
The staining on most Sydney roofs isn’t dirt. It’s living growth — lichen, moss and gloeocapsa algae that root into the porous surface of concrete and terracotta tiles. On the shaded southern sides and under tree cover, it holds moisture against the tile and slowly breaks down the protective coating. Blasting it off the top leaves the roots behind, so it grows straight back.
Why high pressure wrecks tiles
Concrete and terracotta tiles have a factory glaze or coating that sheds water. A high-pressure lance strips that coating, opens the surface up, and drives water under the tiles and into the roof cavity. The result is a roof that looks clean for a season, then weathers faster than ever and often leaks. Most tile manufacturers explicitly void their warranty if the roof has been high-pressure cleaned.
How a roof should be cleaned
The correct method is soft washing: low pressure combined with the right treatment that kills the growth at the root. The biological staining is treated and left to break down and weather off, rather than blasted. It’s slower to show its full result but it lasts far longer, protects the tile surface, and keeps the warranty intact. For metal and Colorbond roofs the same principle applies — gentle washing, correct chemistry, no gouging.
Do it safely or don’t do it
Roofs are the single most dangerous DIY cleaning job. Steep, wet, often two storeys up, and slippery once any solution is applied. This is genuinely a job to leave to insured professionals with harnessing and the right access equipment. Our roof cleaning service uses soft washing on every job, and it usually goes hand in hand with gutter cleaning while we’re up there.
The Prestige Standard approach
Surface-matched method for tile, terracotta or metal, growth killed at the root, a walkthrough before we pack up and a photo report on every job. Fully insured with $20 million public liability. Request a fixed written quote and we’ll respond within 24 hours.
