March 22, 2024

Understanding Roof Algae: Causes, Effects, and Removal

By Preeti Shenoy

Roof algae is a common concern for homeowners, detracting from a home’s aesthetic appeal and potentially damaging roofing materials over time. This blog delves into the nature of roof algae, exploring its causes, impacts, and the most effective strategies for removal and prevention, with insights from experts in the field.

What is Roof Algae?

Roof algae, primarily Gloeocapsa magma, is a type of cyanobacteria that thrives in warm, humid conditions. It’s known for its distinctive black or dark green streaks, which can be mistaken for dirt or soot but are actually colonies of the organism. These streaks are most prevalent on the north and west sides of roofs, where moisture and shade are more abundant.

Causes of Algae Growth on Roofs

Algae spores are airborne and can land on your roof from a nearby tree or another house. Once established, the algae feed on the limestone filler in asphalt shingles and retain moisture, creating an ideal environment for further growth. Factors contributing to algae growth include:

  • High humidity and moisture levels
  • Lack of direct sunlight
  • Poor roof ventilation
  • Proximity to bodies of water
  • Roof Cleaning

The Impact of Algae on Roofing Materials

While algae itself doesn’t cause significant damage to roofing materials, the conditions it creates do. The moisture retained by algae growth can lead to the deterioration of shingles, causing them to warp, crack, or loosen. Over time, this can result in leaks, water damage, and reduced roof lifespan. Additionally, the dark colouration of algae can absorb heat, increasing cooling costs during warmer months.

Clint Smith from Just Clean Property Care emphasises the importance of addressing algae as soon as it’s noticed. “Not only does roof algae look unsightly, but the longer it’s left untreated, the more it can contribute to the degradation of your roof, potentially leading to costly repairs down the line,” he states.

Effective Removal Methods

The most effective method for removing roof algae involves a gentle cleaning solution applied with low pressure. A mixture of water, bleach, and trisodium phosphate (TSP) can be effective for killing algae and restoring the appearance of your roof. It’s crucial to avoid high-pressure washing, as this can damage shingles and dislodge granules that protect the roof from UV rays.

Expert Recommendations for Treatment and Prevention

Experts agree that prevention is the best strategy for dealing with roof algae. Some recommendations include:

  • Install Zinc or Copper Strips: Metal strips placed at the roof’s peak can release ions when wet, creating an environment that’s hostile to algae growth.
  • Ensure Proper Ventilation: Good attic ventilation can reduce moisture buildup on the roof, discouraging algae growth.
  • Regular Maintenance: Keep gutters clean and trim overhanging branches to reduce shade and moisture on the roof.
  • For treatment, Smith advises homeowners to seek professional help for the safest and most effective results. “Professional roof cleaners have the tools and experience to remove algae without damaging your roof. They can also offer tailored solutions for preventing future growth,” he recommends.

Long-Term Prevention

After cleaning, consider preventative measures to keep algae at bay. Regular roof inspections can help identify potential issues early, and treatments with preventive solutions can create an inhospitable environment for algae. Choosing algae-resistant roofing materials for new installations or replacements can also significantly reduce the risk of algae development.


Understanding and addressing roof algae is crucial for maintaining the health and appearance of your roof. By recognising the causes and implementing effective removal and prevention strategies, homeowners can protect their roofs from the damaging effects of algae. With expert advice and professional services, it’s possible to extend the life of your roof and keep it looking its best for years to come.