Look at almost any roof in East Tennessee and you will see the same thing: black tiger-stripes running up the slopes from the eaves, usually worst on the shaded side of the house. It is not dirt. It is not age. It is a cyanobacteria commonly called gloeocapsa magma — an airborne algae that feeds on the limestone filler in asphalt shingles and thrives in humid, shaded climates.
Why It Spreads
The algae arrives as airborne spores that land on the north-facing, shaded side of a roof — the side that stays damp longest after rain and dew. From there it colonizes the shingles and creeps up the slopes, and rain slowly carries it down toward the eaves, gutter faces, and fascia. Once it takes hold, it spreads across the whole roofline season after season.
What Happens If You Ignore It
- A shorter roof life. The algae hold moisture against the shingles, accelerating granule loss and premature wear on a surface designed to shed water, not hold it.
- Higher energy bills. A dark, streaked roof absorbs more heat instead of reflecting it, making the attic and the rooms below work harder.
- Permanent staining. Left long enough, the streaks etch into the shingles so even a professional cleaning can only lighten them, not fully restore the original color.
- Lost value and curb appeal. Buyers, guests, and tenants read streaks as "this property is not maintained" — fair or not, and it shows up in every listing photo.
The Fix Is Cheaper Than the Damage
The only safe way to clean an asphalt shingle roof is soft washing: a low-pressure application of a solution that kills the algae at the root, followed by a gentle rinse. Pressure washing a roof strips the stain off along with the shingle granules — the exact coating that protects the roof in the first place. Because the organism is killed instead of just relocated, soft-washed roofs stay clean far longer.
For steep pitches and high rooflines no ladder crew should ever walk, drone-assisted cleaning delivers the same gentle treatment without anyone stepping on the shingles at all — and the whole job is documented with before-and-after photos.
The earlier the streaks are addressed, the less of the damage becomes permanent. If your roof has started striping, this is the cheapest moment it will ever be to fix.
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