What Those Dark Streaks on Your Roof Really Mean
When you look up at your roof and notice dark, streaky lines running down the shingles, it’s easy to assume it’s just dirt or shadows. But those streaks are actually a warning sign your roof is sending—and ignoring them can shorten the life of your roofing system.
At A Plus Roofing, we’ve seen this issue across Maine homes, especially in areas with dense tree cover and coastal humidity. Let’s take a closer look at what those streaks really are, what causes them, and how to fix (and prevent) them before they cause bigger problems.
1. The Real Culprit: Algae Growth
Those dark streaks are most commonly caused by a type of blue-green algae called Gloeocapsa magma. This airborne organism thrives in humid climates, feeding on the limestone filler used in many asphalt shingles.
Over time, the algae form colonies that appear as dark streaks or blotches, usually on the north-facing side of your roof, where shade and moisture linger longest. While algae itself doesn’t eat through shingles, it retains moisture—accelerating wear and creating ideal conditions for moss and lichen to take hold later.
2. Why It’s More Than a Cosmetic Problem
At first, the streaks might seem like a minor aesthetic issue. But here’s why you shouldn’t ignore them:
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Reduced energy efficiency: Algae-covered shingles absorb more heat, raising attic temperatures and driving up cooling costs in summer.
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Shortened roof lifespan: Prolonged moisture retention leads to shingle granule loss and early degradation.
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Curb appeal loss: Dark streaks make even newer roofs look aged and neglected, which can lower your home’s resale value.
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Potential for moss and lichen: Once algae establish, moss and lichen can follow, causing real damage by lifting shingles and breaking down protective layers.
3. Maine’s Perfect Conditions for Algae Growth
Maine’s combination of cool nights, morning dew, and wooded surroundings make it ideal territory for algae growth. Salt air along the coast and long winters with melting snow can further trap moisture in your roofing materials. That’s why even high-quality roofs can develop these streaks over time if they aren’t properly maintained.
4. The Right Way to Remove Dark Streaks
Power-washing might seem like a quick fix, but it’s one of the fastest ways to damage shingles and void your warranty. Instead, a professional roof cleaning uses soft-wash systems and specialized cleaning agents that kill algae without stripping protective granules.
At A Plus Roofing, our team uses manufacturer-approved cleaning solutions that:
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Eliminate algae, moss, and lichen at the root
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Restore your roof’s original color and performance
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Preserve shingle integrity for the long haul
5. How to Keep Streaks from Coming Back
Once your roof is clean, a few preventive steps can help keep it that way:
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Install zinc or copper strips near the ridge—when rainwater runs over the metal, it releases ions that naturally inhibit algae growth.
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Trim back overhanging branches to reduce shade and improve airflow.
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Clean your gutters regularly to keep water moving off the roof surface.
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Schedule annual roof inspections to spot early signs of moisture retention or streak formation.
6. When to Call the Pros
If you notice streaking that’s spreading or accompanied by moss, curling shingles, or granule loss, it’s time to have your roof inspected. What looks like algae could also be a sign of aging materials or hidden moisture issues beneath the surface.
With over 38 years of experience serving Maine homeowners, A Plus Roofing can tell you exactly what’s happening on your roof—and what steps will extend its lifespan safely and affordably.
Bottom Line
Those dark streaks aren’t just a cosmetic nuisance—they’re a sign your roof is under biological attack. But with proper care, professional cleaning, and a few preventive measures, you can keep your roof looking great and performing like new for decades to come.
Schedule Your Free Roof Inspection
Think your roof might be showing signs of algae or early damage?
Call A Plus Roofing today for a free inspection and honest assessment.
We’ll help you restore beauty, efficiency, and peace of mind—before small streaks become costly repairs.