Most homeowners assume black streaks mean their roof is failing. In almost every case, it's algae, not a structural problem, and it's removable without ever putting a pressure washer near your shingles.
Black streaks, green moss, and white lichen are three different organisms with three different growth patterns, and they don't all get treated the same way.
Those dark, wavy streaks running down from the ridge are almost always a specific type of blue-green algae, Gloeocapsa magma, not soot, mold, or dirt. It spreads by airborne spores and feeds on the limestone filler used in most asphalt shingles, which is why it appears on shingle roofs across entire neighborhoods at once, spreading from roof to roof.
It shows up worst on the north-facing slope of a roof, the side that stays shaded and damp the longest after rain. Left alone, it spreads and darkens over years; it does not go away on its own, and shingle color alone won't hide it once it's established.
Moss is a plant, not algae, and it's a bigger physical concern than black streaks. Moss holds moisture directly against the shingle surface and, as it grows, can work its way under shingle edges and tabs, lifting them slightly. That's where moss stops being cosmetic and starts being a maintenance issue: lifted shingles are more vulnerable to wind and water intrusion.
Moss thrives in the same shaded, moisture-retentive spots as algae, often near overhanging tree limbs, valleys, and north-facing slopes with poor sun exposure.
Lichen is a combined organism, part fungus and part algae, that anchors itself to the shingle surface with root-like structures. Of the three, lichen is the most tightly bonded to the shingle and the most stubborn to remove, which is also why it's the one most likely to tempt someone toward high pressure or scraping, both of which damage the shingle granules underneath.
A proper soft-wash solution is formulated to break that bond chemically rather than needing to be forced off mechanically, which is the only way to remove lichen without damaging the roofing material it's attached to.
Click below to see what a pressure washer actually does to an asphalt shingle.
We check shingle condition and identify algae, moss, and lichen growth before starting.
A roof-safe cleaning solution is applied at low pressure to break down growth at the source.
A low-pressure rinse clears away the loosened algae and moss without disturbing the granules.
We confirm gutters and landscaping are clear and you're happy with the results.
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Left in place, algae and moss retain moisture against the shingle and accelerate the granule loss that ages a roof beyond its years.
Dark algae staining absorbs more heat than clean shingles, which can modestly increase attic and roof surface temperatures in summer.
Some insurers note visible algae or moss during roof condition assessments. Addressing it proactively keeps the roof's documented condition current.
The roof is one of the largest visible surfaces on a home. Black streaks are one of the most common things buyers and appraisers notice first.
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Also cleaning: House Washing · Gutter Cleaning