Tile and metal roofs fail in different ways, hold growth differently, and need different handling. The method that safely cleans a Colorbond roof can crack a concrete tile, and the approach that suits a tile roof can strip protective coating from metal.
If you are getting quotes, this distinction is worth understanding, because a contractor who treats both the same is telling you something about how carefully they work.
Tile roofs: the risk is what is underneath
Concrete and terracotta tiles are not sealed to each other. They overlap, and the roof relies on water running down the surface and off — not on the joints being watertight. That single fact drives everything.
- Water driven upward gets in. Pressure directed up the slope pushes water under the laps and into the cavity. It shows up as a ceiling stain weeks later.
- Tiles get brittle with age as the surface coating weathers away. Walking on them, let alone hitting them with pressure, breaks them.
- Ridge capping is mortar. Pressure strips it out. Re-bedding a ridge line costs many times more than the clean did.
- The surface is porous. Older concrete tiles hold moisture and grow lichen with roots into the surface, which is why chemistry rather than force is the answer.
So: tile roofs are soft washed, almost without exception.
Metal roofs: the risk is the coating
Colorbond and zincalume have a factory-applied coating providing corrosion resistance and colour. The steel underneath is unremarkable — the coating is the entire product.
- Abrasives are out. Anything that scours removes coating and creates conditions for corrosion.
- Harsh chemistry is out. Strongly alkaline or acidic cleaners can dull or discolour the finish permanently.
- Moderate pressure is tolerable, more so than on tile — but on the flat pans rather than driven into seams and fixings.
- Dissimilar metals matter. Run-off carrying copper or lead onto zincalume accelerates corrosion.
Metal roofs are usually a gentle wash with a mild solution — closer to washing a car than cleaning a driveway.
Growth behaves differently on each
On tile, lichen and moss get physical purchase in the porous surface and the gaps between tiles. They root in. Removal without killing them first is temporary.
On metal, growth mostly sits on the surface, concentrating where water lingers — low-pitch sections, behind solar panels, in the pans. Easier to remove, but those are also the places standing water does long-term damage, so it is worth clearing properly.
Frequency
Both benefit from attention before the wet season rather than after it. Tile roofs in shaded, treed Brisbane suburbs generally need it more often than metal, because the surface holds moisture longer.
The practical trigger is visible growth spreading, not a fixed interval. A roof that has stayed clean for three years does not need cleaning because a calendar says so.
What to ask
Ask which method they will use and why, and listen for whether the answer changes with your roof type. Ask what happens to the run-off. And on a tile roof, ask specifically what they do about ridge capping — anyone who has cleaned many tile roofs will have an immediate answer, because it is what most often gets damaged.
Not sure which you have, or what condition it is in? Read more about roof cleaning or request a free quote.
