What Are Those Ugly Black Streaks on My Roof?

If you've noticed dark streaks running down your roof—or your neighbor's—you're not alone. These unsightly marks are one of the most common aesthetic issues affecting roofs in North Alabama and throughout the humid Southeast. But what causes them? Are they harmful? And what can you do about them?

What Causes Black Streaks on Roofs?

Those dark streaks aren't dirt, mold, or soot. They're actually caused by a type of blue-green algae called Gloeocapsa magma. This algae thrives in warm, humid environments—making North Alabama's climate ideal for its growth.

Gloeocapsa magma feeds on the limestone filler and calcium carbonate commonly used as fillers in asphalt shingles. The dark color you see is actually a protective outer coating the algae develops to shield itself from UV rays. Over time, as the algae colony grows and spreads, these dark streaks become more prominent, often starting at the top of the roof and running downward with rainwater.

The problem is most visible on north-facing slopes and in shaded areas that stay damp longer, giving the algae more opportunity to thrive.

Is Roof Algae Harmful?

Here's the good news: roof algae is primarily a cosmetic issue. It won't immediately compromise your roof's structural integrity or cause leaks. However, there are reasons not to ignore it entirely:

Reduced Curb Appeal: Dark streaks are unsightly and can make even a relatively new roof look old and neglected. This can affect your home's resale value.

Accelerated Aging: Over time, algae can retain moisture against your shingles, potentially accelerating their deterioration. The algae can also cause granule loss as it feeds on the shingle material.

Heat Absorption: Dark algae streaks absorb more heat than lighter-colored shingles, potentially increasing attic temperatures and cooling costs.

Can You Clean Algae Off Your Roof?

Yes, roof algae can be cleaned, but it requires care. The Asphalt Roofing Manufacturers Association recommends a 50/50 mixture of water and liquid chlorine bleach (sodium hypochlorite) for removing algae. This solution should be applied carefully, avoiding damage to plants below.

Important warnings: Never use a pressure washer on asphalt shingles. The high pressure will blast away the protective granules and severely shorten your roof's lifespan. Always protect landscaping from bleach runoff. Consider hiring a professional roof cleaning service if you're not comfortable working on a roof.

Preventing Future Algae Growth

The most effective long-term solution is prevention. Modern shingles from manufacturers like IKO, Owens Corning, and GAF include algae-resistant (AR) technology. These shingles contain copper or zinc granules mixed into the regular granule coating. When it rains, trace amounts of these metals wash over the roof surface, inhibiting algae growth.

If your current roof doesn't have algae-resistant shingles, you can install zinc or copper strips along the ridge. As rainwater washes over these strips, it carries algae-inhibiting metal ions down the roof surface. While not as effective as AR shingles, strips can help slow algae growth.

Other preventive measures include trimming overhanging tree branches to increase sunlight and airflow, keeping gutters clean so water drains properly, and addressing any areas where moisture tends to accumulate.

When It's Time for a New Roof

If your roof is old enough to have significant algae staining, it may also be approaching the end of its useful life. Rather than investing in cleaning, it might make more sense to plan for replacement with modern, algae-resistant shingles.

At River City Roofing Solutions, we can assess your roof's condition and recommend whether cleaning, treatment, or replacement makes the most sense for your situation. Contact us for a free inspection.