What Are The Black Streaks On My Roof?

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Thanks to its many benefits including ease of installation, relatively low up-front cost, and excellent waterproofing ability, asphalt shingle is today one of the most popular materials in North America. Unfortunately, over time, roofing materials, especially asphalt shingles, tend to develop ugly black streaks and dark spots, turning them into an eyesore. To deal with this problem effectively, you should know that the dark streaks ruining your beautiful roof are not necessarily soot, dirt, or even mold. Instead, they are more likely to be a type of algae called gloeocapsa magma. Keeping this in mind, here is some more information on this topic.
An Overview of Gloeocapsa Magma

Simply put, Gloeocapsa magma is pervasive and prevalent type of blue-green algae that invades asphalt shingles, eventually causing them to discolor. To protect itself from the sun’s ultraviolet rays, gloeocapsa magma forms a dark protective outer coating and the in the process, creates the ugly dark streaks on your roof.

Ideal Conditions for Gloeocapsa Magma Growth

Similar to other alga, gloeocapsa magma thrives in a moisture-rich and shaded environment. Typically, the alga first attacks the side of a roof that receives the least amount of sunlight due to either heavy tree coverage or geography. In the US, for instance, the north-facing side tends to receive less sunlight throughout the year compared to the south-facing side. Still, it typically takes two or more months for the ugly black streaks to become visible to the naked eye. On the nutrition front, the calcium carbonate contained in asphalt shingles provides the algae with the nutrients it requires.

How Gloeocapsa Magma Spreads

At this point, it is important to note that gloeocapsa magma is airborne algae, meaning its spores travel from one point to another through the air. Because of this, only a few spores of gloeocapsa magma can eventually damage many roofs in a single neighborhood. Besides the wind, people and animals can also spread these spores.

The Role of Weather in Algae Growth on Asphalt Shingles

In general, humid, warm, and coastal climates tend to create the ideal environments for the growth of gloeocapsa magma on shingles. However, with cheaper shingles becoming increasingly popular across the US, black streaks on roofs are now a common problem in virtually every part of the US, even in the northeast and Midwest states, which typically experience cold winter seasons and hot, humid summers. As previously mentioned, an algae invasion typically begins on the side of a house that receives the least amount of sunlight. This is because shaded areas generally retain more moisture, creating ideal environments for algae growth. Some of the other factors that can create a damp surface that encourages algae growth include overhanging trees and gutters that drain directly onto a roof.

Harmful Effects of Gloeocapsa magma

Gloeocapsa magma is not dangerous or harmful in itself. However, over time, it may damage your roofing shingles in several ways. For starters, it can cause the protective ultraviolet granules that cover your roofing shingles to come off. Secondly, by retaining excess moisture, the algae may cause your roof shingles to age prematurely. Thirdly, gloeocapsa magma can combine with airborne fungus, leading to the formation of lichen, which usually feeds on shingle filler. Moreover, unlike algae, lichen tends to develop roots, which not only make it harder to remove, but can also cause additional damage to your roof.


Gloeocapsa magma is a type of roof algae that mainly attacks asphalt shingles, leading to the formation of ugly dark spots or streaks. Typically, gloeocapsa magma prefers humid, warm, and coastal climates. However, the increasing use of calcium carbonate in roofing materials is driving the spread of the alga to areas that do not necessarily experience these climatic conditions.

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