Black streaks on roof tiles aren't dirt and they're not age — they're a living organism called Gloeocapsa magma. Understanding what it is explains why pressure washing doesn't fix it, and soft washing does.
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If you've noticed dark black or dark grey streaks running down the tiles of your roof — often following the direction of water flow from the ridge — you're looking at Gloeocapsa magma, a cyanobacterium that is responsible for the majority of dark roof staining across Queensland and throughout Australia's humid east coast. Understanding what Gloeocapsa magma is helps explain why some cleaning methods fix the problem and others don't — and why the streaking keeps returning after pressure washing.
Gloeocapsa magma is a photosynthetic cyanobacterium — a microorganism that can both photosynthesize and produce energy from water. It feeds on calcium carbonate, which is a primary filler material in concrete roof tiles, and thrives in the warm, humid conditions of Queensland's subtropical climate. As it grows and ages, Gloeocapsa magma produces a dark pigmented outer sheath to protect itself from UV radiation — which is what creates the characteristic dark streaking rather than the green or grey of other roof growths.
Gloeocapsa magma reproduces via airborne spores that travel on wind and settle on roof surfaces. Once a colony establishes on one section of a roof, rainwater running across the roof surface carries both living cells and spores down the roof in the direction of water flow — creating the characteristic streaking pattern. This is also why roofs with a north or east facing aspect (where water runs toward street-visible sections) often develop visible streaking earlier than other aspects.
Pressure washing removes the dark pigmented outer layer of the Gloeocapsa magma colony from the tile surface — which is why the roof looks clean immediately after pressure washing. But the underlying colony survives the pressure blast, and the removal process scatters the spores to previously unaffected areas of the roof. The colony typically re-establishes and produces visible dark streaking again within 6 to 12 months.
Soft washing applies a biocidal solution containing sodium hypochlorite at the correct concentration to kill Gloeocapsa magma at the cellular level — including the dark pigmented sheath that causes the streaking and the spores that cause regrowth. The dead matter is rinsed away and the roof stays clean for 12 to 24 months because the organism has been killed rather than displaced.
Almost certainly Gloeocapsa magma — a cyanobacterium that produces a dark pigmented sheath as it grows, creating the characteristic dark streaking. It's extremely common in Queensland's humid climate.
To your roof, yes — over time it contributes to tile surface degradation. It's not known to be directly harmful to humans in the context of roof staining.
No — Gloeocapsa magma is a living colony that grows and spreads. Without treatment, the staining will progressively worsen.
Professional soft washing with a biocidal solution that kills Gloeocapsa magma at the cellular level. Results last 12 to 24 months. Starting from $799 — free quote provided.
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