Those dark streaks running down your roof are not dirt, and they are not staining from rust or run-off. They are a living organism — a hardy blue-green algae called Gloeocapsa magma — and understanding that changes how you deal with them.
It matters because the two instincts most people have, scrubbing and pressure washing, are the two most likely to damage the roof while leaving the problem in place.
Why it streaks downward
The algae feeds partly on limestone filler used in many roof tiles. It establishes near the ridge, where moisture lingers longest, then spreads downhill as rain washes spores down the slope. That is why the pattern is always vertical and always worse below the ridge.
It thrives in exactly Brisbane’s conditions: humidity, warmth, and surfaces that stay damp in shade. North-facing sections dry faster and stay cleaner. The south side and anything under trees goes first.
Why it is not only cosmetic
The dark colouring absorbs heat, raising roof surface temperature. The algae also holds moisture against the surface, and sustained dampness accelerates the breakdown of tile coatings and ridge capping mortar.
None of that is catastrophic in a year. Over a decade of untreated growth it shortens the life of the roof surface, which is a genuine cost.
Why pressure washing is the wrong tool
High pressure on a roof does three things you do not want. It can crack or displace tiles. It drives water up under the laps, where it has no path out except into the roof cavity. And it strips mortar from ridge capping, which keeps the ridges watertight.
It also does not solve the problem. Pressure removes the visible layer; the root structure remains, and in a humid climate it regrows within months. You get the damage and the streaks come back.
What actually works
A soft wash. Low pressure, roughly garden-hose level, with a solution that kills the organism rather than knocking it off. Applied, given time to work, then rinsed.
Two things follow. The roof often looks patchy for days or weeks afterwards — dead algae weathers off gradually rather than vanishing. And because the organism is dead rather than displaced, regrowth takes considerably longer.
Full comparison in what Brisbane homeowners should know about roof cleaning.
What about the metal strips people mention?
Zinc or copper strips near the ridge release trace metal ions when it rains, inhibiting growth below them. The effect is real but modest, and it only protects what is directly downhill.
Treat it as a way to slow regrowth after a proper clean, not as a treatment for a roof that is already streaked.
What you can do yourself, honestly
Very little, safely. Roof work involves height, a slippery surface, and tiles that may be brittle. Cleaning solution on a wet roof makes it slipperier still, and the roofs that most need attention are those damp and shaded for years.
What you can usefully do from the ground: keep gutters clear, cut back branches shading the roof, and note which sections stay wet longest.
For the rest, request a free quote. We will tell you whether the streaking is superficial or whether the surface underneath needs a roofer first.
