BLACK STREAKS, LICHEN
& ALGAE — ROOF GUIDE

Black Streaks — Gloeocapsa Magma

Most Common

The dark grey to black streaking that runs down from the ridge of Queensland roofs is caused by Gloeocapsa magma — a cyanobacterium (blue-green algae) that thrives in warm, humid conditions. It's by far the most common form of biological staining on roofs in Southeast Queensland.

The black colour is actually a UV-protective pigment the organism produces — a melanin-like shield that protects it from direct sunlight. This is why the streaking is most pronounced on the sun-exposed sections of the roof while the shaded areas often show green algae or lichen instead.

Gloeocapsa magma feeds on the calcium carbonate in concrete and terracotta tiles — it is literally consuming your roof tiles while it grows. On tank water properties, the toxins it produces wash into your rainwater system with every rainfall event.

Sodium hypochlorite solution at 2–4% applied with dwell time kills Gloeocapsa magma completely — including the melanin pigment that gives it its black colour. Results are visible within days as the dead organisms wash away in rainfall.

Pressure washing removes the visible streaking but does not kill the organism — regrowth reappears within months. Only chemical treatment kills it at a cellular level, producing results that last 12–24 months.

Green Growth — Algae & Moss

Very Common

Standard algae produces a thin, slimy green film on roof surfaces — most visible on shaded sections, the south-facing roof elevation and in areas where leaf matter accumulates. It spreads rapidly in humid conditions and is often the first biological growth to establish on a new roof surface.

Moss creates thicker, cushion-like clumps — green to dark green — most commonly found in roof valleys, under overhanging trees and in areas of persistent shade and moisture. Moss root systems (rhizoids) penetrate into tile surfaces and mortar, causing physical breakdown over time and creating pathways for water infiltration under tiles.

Both algae and moss on a roof are also fire hazards — dry accumulated moss is highly combustible and a significant risk in ember attack during bushfire events. Homes in South Burnett and Somerset should have roofs cleared of moss growth as a bushfire preparedness measure.

Algae responds rapidly to sodium hypochlorite treatment — visible greening clears within 24–48 hours of treatment as organisms die. Moss requires a slightly longer treatment and dwell period to penetrate the thicker growth structure.

Dead moss clumps may remain visible for 2–4 weeks after treatment before washing away in rainfall. This is normal — the organisms are dead and will not regrow. We advise customers to expect this and not to be alarmed when the roof appears to still have brown-coloured moss clumps immediately after treatment.

Lichen — The Difficult One

Hardest to Remove

Lichen is the grey, greenish-grey or orange-brown crusty growth that looks almost painted onto the roof surface. It is not a plant, not an algae and not a moss — it is a symbiotic organism formed from the partnership of fungi and algae living together. This dual nature makes it uniquely difficult to treat.

Lichen attaches to surfaces using a strong adhesive structure called a holdfast that physically penetrates into the substrate material — into the tile surface, into the mortar and into colorbond's paint coating. High pressure washing can remove the visible lichen body but leaves the holdfast intact, allowing full regrowth within weeks to months.

In Queensland's climate, lichen is most prevalent on properties with significant native tree coverage — South Burnett, Somerset and the leafier Redlands suburbs see the heaviest lichen growth rates. A heavily lichen-affected roof in these areas can go from clean to visibly affected within 12 months without treatment.

Sodium hypochlorite at 3–5% concentration with extended dwell time is the only treatment that kills lichen including the holdfast root system. The solution must penetrate deeply enough to reach and kill the fungal and algal components throughout the lichen structure.

After treatment, dead lichen may remain on the surface for 4–8 weeks before washing away in rainfall. During this period the lichen body turns from green/grey to brown/orange — indicating it is dead and decomposing. This is a normal and expected part of the treatment process, not a sign of incomplete cleaning. Results after full washout are typically 12–24 months of clean surface.