What the Black Streaks Are
The black streaks on a roof have a common cause, and a Coatesville homeowner benefits from understanding it. Here is the picture.
Usually Algae
The dark streaks are usually a type of algae that grows on roofs, appearing as black or dark green streaks. They are usually algae. It grows on roofs. It appears dark. It streaks downward. It is common.
It Spreads Over Time
Algae spreads gradually across the roof over time, so streaks can grow if left alone. Algae spreads gradually. It grows over time. Streaks expand. They worsen if ignored. It is progressive.
It Feeds on Shingle Materials
Roof algae can feed on materials in the shingles, which is why it takes hold and spreads on a roof. It feeds on shingle materials. That is why it takes hold. It spreads from there. It is suited to roofs. It is persistent.
Not the Same as Moss
Algae streaks are different from moss, which is thicker and green, so the two are distinct growths. Algae is not moss. Moss is thicker. Moss is green. They are distinct. They differ in treatment.
What the Streaks Are, in Short
The black streaks on a roof are usually a type of algae that grows on roofs and appears as black or dark green streaks, which spread gradually over time and can feed on materials in the shingles, and which are different from moss, a thicker, green growth, so the two are distinct.
Get Your Streaks Identified
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One thing worth being clear about for Coatesville homeowners is that those unsightly dark streaks running down a roof are, in the great majority of cases, a type of algae rather than dirt or general staining. This algae grows on roofs and shows up as black or dark green streaks, and it tends to spread gradually across the roof over time, so streaks that start small can grow larger if they are simply left alone. Part of the reason it takes hold so readily is that it can feed on materials in the shingles, which gives it a foothold and lets it spread. It is worth knowing that this algae is not the same thing as moss, which is a thicker, green growth, so the two are distinct and are treated differently. As for what causes it, the common thread is moisture, since algae thrives wherever a roof stays damp. Shaded areas of a roof, which dry out slowly, are more prone to it, and that is especially true under overhanging trees that both cast shade and drop debris. Humid climates encourage algae as well, because the moisture in the air supports its growth. A very common pattern is for north-facing slopes, which receive less direct sun and therefore stay damp longer, to show noticeably more algae than the sunnier sides of the same roof. None of this means the situation is dire, but it does explain why the streaks appear where they do.