Why Does My Roof Have Algae on it?

Published on: 7/26/2023

5 mins read

Roof algae, a common and unsightly issue, exists due to various environmental factors and its symbiotic relationship with other organisms. This paper explores the reasons behind its presence, the impact it has on roofs, and effective methods of prevention and remediation. By understanding the underlying causes, homeowners and building managers can take informed measures to combat this persistent problem.

  1. Introduction: Roof algae, specifically the blue-green variety known as Gloeocapsa magma, is a form of cyanobacteria commonly found on roofs across various regions. It thrives in humid and warm environments, resulting in the unsightly black or green streaks that mar the appearance of many buildings. Understanding the reasons for its existence is essential for developing effective strategies to prevent and mitigate its growth.
  2. Environmental Factors: Several environmental factors contribute to the existence of roof algae:

a) Humidity: High humidity levels create the ideal conditions for roof algae growth. As moisture accumulates on roofs, it provides the necessary moisture for the cyanobacteria to thrive.

b) Sunlight: Roof algae tend to grow faster in areas with less direct sunlight. Shaded spots on the roof trap moisture for more extended periods, encouraging their growth.

c) Temperature: Warm temperatures promote algae growth. In regions with mild winters, the algae can survive and reproduce throughout the year.

d) Airborne Spores: Roof algae can spread through airborne spores. When carried by the wind, these spores land on roofs, leading to new infestations.

Symbiotic Relationships: Roof algae's existence is also closely tied to symbiotic relationships with other organisms:

a) Lichens: Lichens, a composite organism consisting of a fungus and a photosynthetic partner (algae or cyanobacteria), often coexist with roof algae. These lichens aid algae's growth by providing a protective environment and access to essential nutrients.

b) Moss: Roof algae and moss often grow in conjunction. Moss provides shade and retains moisture, which favors the growth of algae. In return, algae create a suitable environment for moss by contributing to a more acidic surface.

Impact on Roofs: The presence of roof algae can have several adverse effects on roofing materials and structures:

a) Aesthetics: The most noticeable impact of roof algae is its negative effect on a building's appearance. Streaks and discoloration can significantly decrease curb appeal and property value.

b) Roof Degradation: As roof algae colonies expand, they create a protective layer that traps moisture. This trapped moisture can damage roofing materials over time, leading to premature deterioration, rot, and leaks.

c) Energy Efficiency: Algae growth on roofs can decrease energy efficiency. The dark pigmentation of the algae absorbs heat, leading to higher indoor temperatures and increased cooling costs during warm seasons.

d) Structural Damage: In extreme cases, unchecked algae growth can lead to structural damage, compromising the integrity of the roofing system.

Preventive Measures: Homeowners and building managers can take proactive steps to prevent roof algae growth:

a) Regular Maintenance: Regular roof inspections and maintenance can help identify and address potential algae growth early on.

b) Sunlight Exposure: Trimming back trees or nearby foliage can increase sunlight exposure on the roof, making it less favorable for algae growth.

c) Zinc or Copper Strips: Installing zinc or copper strips along the roof ridge can inhibit algae growth by releasing metal ions that deter their colonization.

d) Algae-Resistant Shingles: Opting for algae-resistant roofing materials can provide a long-term solution to combat algae growth.

Remediation: In cases where roof algae is already present, timely remediation is essential to prevent further damage:

a) Soft Washing: Soft washing involves using low-pressure water and environmentally friendly cleaning solutions to remove algae, lichen, and moss from the roof surface without causing damage.

b) Professional Cleaning: Hiring professional roof cleaners can ensure thorough removal of algae and prevent potential roof damage.

c) Preventive Coatings: Applying specialized algae-resistant coatings can prevent future algae growth and extend the life of the roof.


Roof algae exists due to a combination of environmental factors and its symbiotic relationship with other organisms. Its presence can significantly impact the aesthetics, durability, and energy efficiency of roofing materials. By understanding the underlying causes and implementing preventive measures and effective remediation techniques, property owners can maintain clean and durable roofs, ensuring the longevity of their investments.