Roof Lichen and Black Mould Treatment
In the ACT's unique climate, roofs often develop biological growth due to a combination of frosty winters and shaded southern aspects. Biological growth like lichen, moss, and black mould are more than just cosmetic issues—they are active threats to the structural integrity of your roof. This guide outlines the professional approach to remediation and why specialized treatment is essential.
Understanding the Threat:
Lichen and Black Mould Lichen is a composite organism that attaches to roof surfaces with root-like structures (rhizines). Black mould (Gloeocapsa magma) is a bacteria that feeds on the limestone present in roof tiles.
Professional Chemical Treatment (The "Soft Wash" Approach)
This method involves applying a specialized, biodegradable solution (Alkyl Dimethyl Benzyl Ammonium Chloride) that kills the organisms at the root without damaging the roof surface.
● Pros: ○ Root-Level Elimination: Unlike pressure washing, which only removes the surface "head," chemical treatments penetrate the pores of the tile to kill the entire organism [1].
○ Surface Preservation: Does not use high pressure, which can strip the protective glazing off tiles. ○ Residual Protection: The treatment leaves an invisible barrier that inhibits the regrowth of spores for 12 to 24 months.
○ Structural Longevity: Prevents lichen from "prying" into tiles, which eventually leads to micro-cracks and water ingress. Especially important for terra-cotta.
○ Enhanced Curb Appeal: Effectively removes the dark streaks and green patches, restoring the roof's original color.
● Cons: ○ Weather Dependent: Cannot be applied during heavy rain or high winds to ensure the solution stays on the target surface.
○ Gradual Results: While mould dies quickly, lichen may take several weeks to fully dehydrate and weather away naturally after the treatment.
Alternative: High-Pressure Water Blasting
Using high-pressure water to physically blast the growth off the roof.
● Pros: ○ Provides an "instant" clean appearance.
● Cons: ○ Surface Damage: The force required to remove lichen often damages the underlying tile or metal coating, shortening the roof's lifespan [2].
○ Spore Dispersion: High speed water jets actually spread the black mould spores around, they settle back onto the roof and the growth process starts again the next day. This includes starting new growth on photovoltaic panels.
○ Incomplete Kill: Often leaves the lichen "roots" behind in the pores, leading to rapid regrowth within 6–12 months.
○ Water Incursion: Risk of forcing water under tiles or flashing, leading to internal ceiling damage.
References
1. Building Science Digest: Biological growth on roofing materials increases moisture retention, leading to premature substrate failure.
2. Roofing Manufacturers' Association (RMA): High-pressure washing is generally discouraged as it can void warranties and cause accelerated granule loss.