Moss, lichen and your Colorbond roof: what the maker actually recommends
BlueScope — the maker of Colorbond — says it plainly: regular washing with clean fresh water enhances the service life of the coating, sheltered areas need washing every six months at minimum and every three months in coastal areas, and abrasive or solvent cleaning should never touch the surface. Moss and lichen make that maintenance urgent, because they hold moisture against the roof permanently.
What moss and lichen actually do up there
Moss and lichen are not a cosmetic problem that happens to be green. Both work by holding water — a wet sponge fixed to your roof that keeps the surface damp long after the rest has dried. On tiles, that constant moisture works into the surface and the pointing. On coated steel, organic growth and the debris it traps create exactly the damp, sheltered pockets where coating trouble starts.
Lichen is the stubborn one. It bonds to the surface, so blasting it off takes the finish with it. The correct removal is chemical and patient: a soft-wash treatment that kills the growth, then lets weather and gentle rinsing carry it away — the same logic the manufacturers apply to every coated surface.
The maker’s own maintenance rules
BlueScope publishes the rulebook for Colorbond in Technical Bulletin TB-4, and three lines in it matter to every steel-roofed home in Sydney. First: simple regular washing with clean fresh water enhances the service life of the product. Second: the sheltered areas rain never rinses — under eaves, fascia, the underside of gutters — should be washed at least every six months, and at least every three months in coastal areas where marine salt is prevalent. Third: never abrasive scourers, never solvents.
Read that back against how most roofs are treated — blasted with a pressure lance once a decade, or never touched at all — and the gap between manufacturer guidance and common practice is the whole story of why coated roofs age badly near the water.
The Sydney factor: salt on steel
The three-month coastal cadence is not caution for its own sake. BlueScope’s bulletin explains the mechanism: dust and dirt in unwashed areas absorb condensation, and near the coast that build-up includes marine salts — compounds that can initiate corrosion where they sit. A harbour-side roofline collects that mix continuously. The fix is almost embarrassingly simple: fresh water, regularly, before the build-up matures.
That is the shape of proper roof care — soft wash for the growth, fresh-water rinses on the schedule the maker publishes, and photographs so you can see the state of a surface you otherwise never look at. It is also, not coincidentally, how a roof stays inside the spirit of its warranty rather than arguing with it.