Those black streaks on your roof are almost always algae. They are not soot, mold or a leak. The asphalt roofing industry names one main culprit, an airborne algae called Gloeocapsa magma. It loves warm, wet weather, and the Midlands has plenty. The streaks look bad. On their own they’re mostly a looks problem. But how you clean them can do real harm, so read this first.
What causes black streaks on a roof?
Algae, carried in by the wind. The Asphalt Roofing Manufacturers Association (ARMA) calls Gloeocapsa magma “probably the most prevalent” of the species involved. The spores land, settle and grow. The dark color is the algae’s own pigment.
ARMA notes the stains “may be mistaken for fungus growth, soot, dirt, moss, or tree droppings.” Moss is different. It grows in green clumps, usually in one spot. Algae runs in long streaks down the slope.
Why do the streaks run downhill? Rain carries the algae down from where it took hold. That’s why you often see it start near the ridge.
Why are black streaks so common in Columbia?
Our climate feeds it. Columbia is humid subtropical. Relative humidity here averages 70.2% over the year, with about 45 inches of rain (Wikipedia climate data, Columbia, SC). ARMA says algae stains are “most widespread in the Gulf States and along the Northwest and Eastern Seaboards,” and worst where it’s warm and humid.
Shade makes it worse. A north slope under pines stays damp for hours after a storm. That’s where you’ll see streaks first in Irmo, Lexington or Forest Acres.
Light colored shingles show it most. Dark roofs have it too. You just can’t see it as well.
Do black streaks damage your roof?
They usually don’t, at least not right away. ARMA treats it as discoloration, a looks issue. The bigger risk is a bad cleaning job. Scrubbing or blasting a shingle knocks off the granules. Those granules protect the shingle from the sun.
That said, streaks can hide other wear. If you see bare spots, curling or grit in the gutters, that’s not algae. That’s age. Our list of signs you need a new roof covers what to look for.
Can you pressure wash black streaks off a roof?
You shouldn’t. ARMA is blunt about it: “High pressure washing systems are likely to damage asphalt roofing and should not be used.” It also warns against scrubbing, since friction “may loosen and remove granules.”
The method ARMA describes is gentle. A mix of “one part chlorine bleach and one part water” sits on the roof “at least 15 minutes but for no more than 20 minutes,” then gets a light rinse. It also says the treatment makes the roof slippery and hazardous. ARMA “encourages the use of professionals.”
So please stay off the roof. A wet, bleached slope is no place to stand. Cover your plants, too. Bleach runoff kills shrubs.
Cleaning also wears off. ARMA says it “is only temporary, and discoloration will likely reoccur.” In our humidity, plan on it.
How do you keep black streaks from coming back?
The lasting fix is built into the shingle. Algae resistant shingles carry treated granules that hold algae off for years. CertainTeed, for example, backs its StreakFighter protection with algae coverage of 25 to 30 years.
If your roof is getting older, this matters. Our FAQ puts a shingle roof’s life at 15 to 40 years. If yours is near the end, a new algae resistant roof solves both problems at once. Our guide on how long a roof lasts helps you place where yours is.
A yearly look also helps. Our roof inspection checklist shows what to check from the ground.
Get a straight answer on your roof
The Red Shirt Guys Roofing is veteran-owned and has worked Midlands roofs for over twenty years. Call 803.243.3968 or request your free estimate. We’ll tell you if it’s just algae or something more.
FAQ
Are black streaks on my roof mold? They usually aren’t. They’re most often an airborne algae, Gloeocapsa magma. Moss grows in green clumps instead.
Will black streaks make my roof leak? They won’t on their own. Rough cleaning can strip granules, though. Missing granules shorten a shingle’s life.
Is it safe to clean my roof myself? We don’t recommend it. ARMA warns the cleaning makes a roof slippery and suggests using professionals.
How long does a roof cleaning last? It won’t last forever. ARMA says the streaks will likely come back. Algae resistant shingles are the long term answer.
Not sure what your roof needs?
Get a free, no-pressure roof inspection from The Red Shirt Guys.
Request a free inspectionor call 803-243-3968