Black Streaks on Your Roof: What Causes Them and How to Fix It

Black streaks on your roof are caused by Gloeocapsa magma, a blue-green algae that feeds on the limestone filler in asphalt shingles. The dark streaks are the algae's protective UV coating, not dirt or mold. Algae growth is especially common on the Eastern Shore due to high humidity and warm temperatures. Black streaks don't immediately damage your roof, but they accelerate shingle deterioration over time by retaining moisture.

Prevention is better than treatment — Owens Corning Duration shingles with StreakGuard® algae protection contain copper granules that prevent algae growth for the life of the shingle.

What Causes Black Streaks on Roofs in Maryland?

Gloeocapsa magma algae. It's airborne and spreads from roof to roof. The Eastern Shore's warm, humid climate creates ideal growing conditions. North-facing and shaded roof slopes are affected first because they retain moisture longer. The algae feeds on calcium carbonate (limestone) in shingle filler. Over time, it creates dark streaks that spread and darken.

The Eastern Shore's average humidity of 70–80% makes algae growth more prevalent than inland areas. Homes surrounded by trees or near water are especially susceptible due to shading and moisture retention.

How Do I Remove Black Streaks from My Roof?

The safest method is a low-pressure wash with a 50/50 solution of water and household bleach (sodium hypochlorite). Never use a pressure washer — high pressure blasts off granules and dramatically shortens shingle life. Apply the solution, let it sit 15–20 minutes, and rinse gently with a garden hose.

For severe cases, hire a professional roof cleaning service ($300–$600 for a typical home). Or, if your roof is due for replacement, choose algae-resistant shingles and eliminate the problem permanently. See our roof replacement options.

How Do I Prevent Black Streaks from Coming Back?

Install shingles with algae-resistant granules — Owens Corning Duration shingles with StreakGuard contain copper granules that prevent algae growth. Zinc or copper strips installed along the ridge also help. Trim overhanging tree branches to increase sunlight and reduce moisture retention. Ensure proper attic ventilation to minimize roof surface moisture.

Our Shore Protection Tune-Up program includes algae inspection and prevention recommendations as part of the 25-point roof assessment.

Do Black Streaks Affect My Roof's Performance?

While primarily cosmetic at first, algae does affect performance over time. The dark streaks absorb more heat (increasing cooling costs by 5–10%), retain moisture that accelerates granule loss, and can reduce your roof's effective lifespan by 3–5 years. They also reduce curb appeal and can affect your home's value. Learn more about asphalt shingle options.

When Should I Replace Instead of Clean?

If your roof is over 15 years old and showing algae plus other signs of wear (granule loss, curling, cracking), replacement with algae-resistant shingles is more cost-effective than repeated cleaning. Call (443) 675-6485 for a free roof inspection.