Black Streaks on Your Roof: What They Are and How to Remove Them

The black streaks on your roof are algae, a species called Gloeocapsa magma, not dirt, soot or mold. You can lighten them with a low-pressure bleach-and-water rinse, never a pressure washer, and keep them from coming back with zinc or copper strips or algae-resistant shingles. If the roof is older, have it inspected before you clean it.

Updated October 2026 · Written by the Precision Exteriors team, Owens Corning Platinum Preferred Contractor, MHIC #166640, with offices in Cambridge and Salisbury.

What causes black streaks on roof shingles?

Gloeocapsa magma is the algae behind most dark roof stains. The Asphalt Roofing Manufacturers Association calls it the most common of several species that discolor shingles. It feeds on the limestone filler in asphalt shingles, and the dark color is the coating the algae grows to protect itself from sunlight.

Three things tell you it's algae and not something else:

• The stain starts near the ridge and runs downhill in drips, because rain carries the colony down the slope.
• It shows first on north-facing and shaded slopes, which stay damp longest after rain or dew.
• It spreads. Spores travel on the wind, so once one roof on a street streaks, the neighbors usually follow.

Damp, warm Eastern Shore summers suit it, and so does heavy shade over a roof.

Black spots are a different problem. Scattered dark spots, raised green patches or crusty gray-green rings are usually moss or lichen. Moss holds water against the shingle and lifts the edges, so it matters more than streaking and is worth a closer look.

Why does a newer roof get black streaks after a year or two?

There are three usual reasons:

1. The shingles had no algae protection. Many builder-grade and three-tab shingles are not algae-resistant. Check the wrapper, the invoice or your warranty paperwork for the product line.
2. There's a big source nearby. A streaked neighbor's roof or heavy tree cover can seed a new roof quickly, especially on a shaded slope.
3. It isn't algae. Dark marks under a tree line can be leaf tannin or debris staining that washes off with plain water.

If your shingles are an algae-resistant line, call the company that installed them before you clean anything. Owens Corning, for example, backs its Duration and Oakridge shingles with StreakGuard under a 25-year Algae Resistance Limited Warranty when Owens Corning hip and ridge shingles are used. A claim goes much better when the installer has seen the roof first.

How to remove black streaks from a roof

Soft washing is the method shingle makers accept. ARMA says the discoloration can be lightened with a solution of household bleach (sodium hypochlorite) and water, and that you should not use a power washer, brush or broom on the roof surface. Mixing ratios vary by manufacturer, so check your shingle maker's cleaning instructions before you mix anything.

The routine looks like this:

1. Wet down plants, shrubs and the lawn below the eaves first. Bleach runoff burns them.
2. Apply the solution at garden-hose pressure from the ground or a ladder, working on a cool, cloudy day so it doesn't dry too fast.
3. Let it sit for the time your shingle maker recommends, then rinse with low-pressure water.
4. Rinse the plants again.

Never walk on a wet, treated roof. If the roof is steep, two stories high or older, hire it out.

Why pressure washing ruins shingles

A pressure washer blasts the ceramic granules off the shingle surface. Those granules are what shield the asphalt from UV. The roof looks clean for a season, then the bare asphalt dries out, cracks and ages fast. High-pressure water also gets driven up under the shingle courses. If a cleaning crew shows up with a pressure washer and a surface cleaner, send them home.

How to stop black streaks from coming back

Cleaning removes the stain. These four steps keep it from returning, roughly in order of how well they work:

• Algae-resistant shingles. Owens Corning's StreakGuard shingles use copper-lined granules that help resist Gloeocapsa magma. Duration, which we install most often in Driftwood and Onyx Black, carries StreakGuard; our Driftwood roof replacement in Cambridge is one example.
• Zinc or copper strips below the ridge. Rain washes metal ions down the slope and suppresses algae. On long or steep slopes the effect fades lower down, so a second strip partway down may be needed.
• Trim back branches. More sun and air means the roof dries faster after rain and dew.
• Fix attic ventilation. A balanced attic keeps the deck drier from below and helps the whole roof last longer.

Do black streaks damage your roof?

Early on, mostly they're cosmetic. Over years the algae holds moisture on the shingle surface, and a damp shingle loses granules faster. If you see streaks along with curling, cracking or granules in the gutters, the streaks aren't the main problem; the roof may be near the end of its life. Our guide to when a roof should be replaced walks through those signs.

Fall is a good time to check

Once the leaves are down, shaded slopes are easier to see, and gutters full of leaves keep the lower edge of the roof wet. If the shingles are tired and streaked, we'll tell you whether cleaning is worth it or a replacement with algae-resistant shingles makes more sense.