Why Sunshine Coast Roofs Turn Black: Salt Air, Algae and How to Stop It

It's not dirt, and it's not mould

Most people assume the black streaking on an ageing roof is just dirt, soot or general grime. It isn't. It's a cyanobacteria — Gloeocapsa magma — and it's genuinely alive, spreading across the roof surface and feeding on the calcium carbonate in concrete and terracotta tiles. On metal roofs it behaves a bit differently, growing in the organic film of dust, pollen and bird droppings that builds up on the coating over time, but the end result looks similar: dark streaking that starts on the shaded, south-facing side of the roof and spreads from there.

Why it's worse on the Sunshine Coast specifically

This algae needs two things to thrive: moisture and warmth, both of which the Sunshine Coast has in abundance for most of the year. But the specific thing that makes this region worse than, say, inland Queensland is the salt-laden air coming off the ocean. Salt in the air helps the algae retain moisture on the roof surface even after the tiles themselves look dry, which is exactly the condition it needs to keep spreading. We see it hit hardest in exposed coastal pockets — Mooloolaba, Maroochydore and Caloundra roofs facing the ocean streak up noticeably faster than equivalent roofs 10–15km inland.

Tree cover changes the picture too. In hillier, more densely treed suburbs like Buderim, Doonan and parts of Palmwoods, roofs don't get the same salt exposure but they do get constant shade and leaf litter, which keeps moisture sitting on the tiles for longer each day. Different cause, same result: black streaking, just concentrated on the shaded side of the roof rather than the ocean-facing side.

Does it actually damage the roof, or is it just cosmetic?

Both, but the damage side matters more than most homeowners realise. The algae and the moisture it holds against the tile surface do three things over time: they lift and loosen tile glaze, they hold water in valleys and joints longer than a clean roof would, and on a long enough timeline they contribute to premature tile cracking and metal roof corrosion. None of that happens overnight, but a roof that's been visibly black for several years is generally further down that path than it looks from the ground.

What actually works — and what doesn't

High-pressure blasting looks dramatic and satisfying, but on tile roofs it's the wrong tool. It strips the protective glaze off the tile surface, which means the roof gets dirty again faster next time, not slower. The right approach is a soft wash: a biodegradable chemical treatment that kills the algae at the root, followed by a low-pressure rinse to clear it away. Metal and Colorbond roofs get a slightly different, more controlled wash that lifts oxidisation and grime without damaging the coating.

The step most people skip is the after-treatment — a longer-acting biocide applied once the roof is clean. Skip it and a roof in a humid, salt-exposed spot can be visibly green or black again within a year. With it, most Sunshine Coast roofs stay clean for 2–4 years before needing another full treatment.

What to do about it

If your roof has started to streak, the earlier it's treated the less work (and cost) it takes to bring it back, and the less time the algae has had to actually affect the tile or coating underneath. See our roof washing page for what's included in a standard clean, or get in touch for a free quote — we'll tell you honestly whether your roof needs a full clean now or can wait another season.