How to remove algae from roof tiles

Algae is a significant cause of the dark streaking and staining on residential roof tiles across London and is frequently misidentified as general dirt or pollution. Understanding what algae is and how it differs from moss is important because the removal approach differs meaningfully. Algae is a broad category of photosynthetic microorganisms that grow in biofilm colonies directly on the tile surface. They do not have the root-like rhizoid anchoring structures that moss uses, but they do embed their spores into the micropore structure of porous tile surfaces. This means that physically removing the visible algae staining by washing or scrubbing does not kill the organism. Spores embedded in the tile surface survive washing and recommence growth rapidly after treatment, which is why roofs that have been pressure washed to remove algae staining frequently show significant regrowth within weeks or months of cleaning.

How to tell whether you have algae, moss or lichen

The correct treatment for algae on residential roof tiles

Biocidal softwashing is the most effective and longest-lasting treatment for roof algae. The biocidal solution is applied at low pressure across the entire roof surface, including all slopes and the ridge. The chemical attacks the cell walls of the algae colony, killing the biofilm and its spores at a molecular level. The dead algae staining fades from the tile surface over the following weeks as rain washes the dead material away. The tile surface is left clean and protected by the residual biocidal action, which inhibits the germination of new spores landing on the surface.

For roofs where an immediate visible result is required on the day of treatment, DOFF steam cleaning kills algae on contact through heat and produces a clean result immediately. Biocidal softwashing produces a more gradual visible improvement over several weeks as the dead material weathers away, but provides longer-lasting protection through the residual biocidal activity.

The spore persistence problem

The reason algae frequently returns within months of cleaning on some roofs is that the treatment used removed the visible biofilm without killing the spores embedded in the tile surface. Pressure washing blasts away the surface colony but has no biocidal effect on the spores in the tile pores. Once the wash is complete, the spores begin to grow again in the same environment that supported the original colony. Growth recommences within days and is visible again within weeks to months. Biocide application changes this outcome because the chemistry penetrates the tile surface and kills the spores where they sit, in the micropore structure of the tile. Without viable spores to regenerate from, the tile surface remains clean for significantly longer, typically 2 to 3 years in London's climate with a post-treatment biocide application.