Drive through any Greenville subdivision and you will see roofs with long dark streaks running down from the ridge, especially on the north and shaded slopes. Homeowners searching for black streaks on a roof, roof algae removal or how to clean a roof in South Carolina want to know whether it is damaging the roof, whether it is worth paying to remove and whether it is a sign the roof is done.
The honest answer is that the streaks themselves are rarely what ends a roof, but they often appear on roofs that are aging for other reasons.
What causes black streaks on a roof?
The usual culprit is Gloeocapsa magma, a cyanobacterium (blue-green algae) that feeds on the limestone filler in asphalt shingles and protects itself with a dark coating. The Asphalt Roofing Manufacturers Association technical bulletin on algae discoloration, cited by IIBEC, describes it. Humidity and shade make it worse, and it spreads down the roof with the rain, which is why the streaks run from the ridge. Southeastern roofs, including Greenville's, are more prone to it than dry-climate roofs, and the north slope is typically the first to show it.
Is roof algae harmful?
Mostly it is cosmetic, though it can raise surface temperature by darkening the roof and is a reminder that shingles are holding moisture. Streaking does not by itself mean the roof has failed. What matters is what is under the stain: granule loss, curling, cracking and leaks. Read signs you need a new roof to see which signs are cosmetic and which are not.
How to remove black streaks from a roof
Roofing sources differ on method, and many of them sell cleaning. The common advice is a low-pressure soft wash with a shingle-safe algaecide, and to avoid high-pressure washing, which can strip granules and shorten the roof's life. Check your shingle manufacturer's guidance on cleaning, because the wrong method can affect the warranty. Cleaning an old roof does not fix it: if the roof is past 20 years, spend the money on an inspection first.
How to prevent it: algae-resistant shingles and metal strips
Algae-resistant shingles contain copper granules that inhibit growth, and a strip of copper or zinc along the ridge releases algae-inhibiting metal ions with each rain. Several roofing sources describe both approaches; ask the roofer about algae-resistant options when you replace a roof. Metal roofs do not support the same growth, see the metal roof cost guide.
When streaks mean it is time to replace
If the roof is past 20 years old and also shows granules in the gutters, curled or cracked shingles or interior stains, the streaks are a symptom of an aging roof. Use the repair or replace guide, or book a free inspection and get the answer in writing with photos.
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Roof Algae Questions
Are black streaks on my roof mold?
Usually not. They are most often algae (Gloeocapsa magma), a cyanobacterium. An inspection can confirm.
Will algae damage my roof?
It is mostly cosmetic, but it can accompany an aging roof. Judge the roof by granule loss, curling and leaks, not by stains.
Can I pressure wash my roof?
High-pressure washing can remove granules and shorten shingle life. Roofing sources recommend low-pressure soft washing; check your manufacturer's guidance.
How do I stop black streaks from coming back?
Algae-resistant shingles, copper or zinc strips at the ridge and trimming shade-producing limbs all help.
Sources
Cause and prevention are drawn from the cited technical bulletin and secondary roofing sources, several of which sell cleaning. We quote no cleaning price because we have no sourced Greenville range.
- IIBEC, Algae Discolorations of Roofs (ARMA technical bulletin). Gloeocapsa magma and its effect on asphalt shingles
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