Stubborn Moss and Lichen Growth on Roof Tiles

If you seal or paint a roof without completely eradicating moss and lichen, the coating will fail within a single season. These biological organisms are not just surface imperfections, they anchor themselves into your roof tiles with microscopic roots. If trapped beneath a paint film, they will continue to expand as they search for moisture, physically rupturing the bond between the paint and the tile. To prevent this, you must use a specialised biocide cleaner that kills the spores at the root level before any coating is applied.

• Structural Damage: Moss holds moisture against the tile, which leads to freeze-thaw cracking and eventual leaks.

• Adhesion Barrier: Lichen creates a waxy, water-repellent biofilm that prevents paint from "wetting out" and bonding to the tile surface.

• The Biocide Solution: Unlike pressure washing alone, a biocide chemically neutralizes the spores, ensuring the roof remains biologically sterile before it is sealed.

The Challenge of the "White Spot": Why Lichen is So Stubborn

Moss is relatively easy to remove with mechanical scraping, but lichen - those small, crusty white or yellow circular spots - is a far more aggressive colonizer. Lichen is a symbiotic relationship between algae and fungus. It produces an acid that actually dissolves the minerals in your roof tiles to create a grip.

If you simply pressure wash the roof, you might remove the visible lichen "shield," but the microscopic roots remain embedded in the tile’s pores. If you then apply a roof paint or sealer, you have provided these roots with a protective canopy. 

The Role of Biocide Cleaners

A biocide is a chemical wash designed to kill organic growth without the need for high-pressure water, which can often damage the surface of older tiles. Biocides work by disrupting the cellular walls of the moss and lichen.

Understanding Biocide Strengths

Not all roof tiles are made of the same material, and using a biocide that is too aggressive can lead to surface degradation or "burning" of the substrate.

The Correct Preparation Workflow

To ensure your roof paint or sealer lasts for its full rated lifespan, you must follow this biological decontamination process:

1. Mechanical Scraping

Remove the bulk of the "heavy" moss cushions using a hand scraper. This allows the biocide to reach the tile surface directly rather than being wasted by soaking into the moss.

2. The Biocide Application

Apply your chosen biocide using a low-pressure sprayer. Ensure the roof is dry before application so the tiles are "thirsty" and pull the chemical deep into their capillaries.

• The Kill Time: Most professional biocides require a "dwell time" of 24 to 72 hours to fully kill the lichen rhizines. You will see the white spots turn grey or black as they die.

3. The Neutral Rinse

After the biocide has done its work, the roof must be rinsed thoroughly. This removes the dead biological matter and any chemical residue. If you paint over the biocide residue, it can interfere with the paint’s chemistry, leading to a soft or tacky finish.

Honest Trade-offs: Biocide vs. Pressure Washing

The "Instant" Fallacy: Pressure washing gives you an instant "clean" look, but it often does more harm than good by stripping the protective "sand" finish off concrete tiles and driving water into the roof space. A biocide takes longer to work but provides a much deeper, long-lasting clean.

Environmental Care: Biocides are powerful chemicals. You must ensure that all runoff is diverted away from garden ponds and that your gutters are disconnected from any rainwater harvesting tanks during the cleaning process.

The Survival Window: Even after a biocide treatment, if a roof is left unpainted for several months, new spores will land and begin to grow. For the best results, you should aim to seal or paint the roof within 7 to 14 days of the biocide rinse.