Black Streaks on Your Roof: Algae and What to Do

Black streaks running down a shingle roof are usually blue-green algae, a type of bacteria called Gloeocapsa magma. GAF says this algae "does not damage your roof," but it makes a roof look older than it is. You can lighten the stains with a gentle, diluted bleach wash, but never a pressure washer. The stains tend to come back. The lasting fix is algae-resistant shingles when the roof is replaced.

Late summer is when a lot of people notice the streaks. Here is what they are, how to tell them from real roof problems, and how to deal with them without hurting the shingles.

What causes black streaks on a roof?

GAF's algae protection page explains that blue-green algae "travels on the wind and thrives in moist environments." That is why one house on a street often has it, and the neighbors soon do too.

Once the algae settles on a roof, it forms a dark outer shell to protect itself from the sun, according to a GAF article on algae stains. That shell is the black you see. The streaks follow the path rainwater takes down the roof.

The Asphalt Roofing Manufacturers Association (ARMA) says on its algae discoloration page that this growth happens in every U.S. region, "especially those subjected to warm, humid conditions." Cincinnati's humid summers fit that. Shaded slopes that stay damp longer often show it first.

Is it algae, moss, or something worse?

Before you clean anything, make sure you know what you are looking at.

Moss is the one to take seriously. GAF's moss article says moss roots "grow in between shingles" and can push them apart, "allowing water to penetrate and leak into your home." GAF recommends a professional for moss removal.

Does algae damage shingles?

The algae itself does not, according to GAF. The cleaning is where people do damage.

ARMA warns that high-pressure washing is "likely to damage asphalt roofing" and that scrubbing "may loosen and remove granules." Granules are the sand-like coating that shields the asphalt from the sun. Strip them off and the shingles age faster. A dirty roof that is intact is better than a clean roof with its granules blasted off.

How do you clean algae off a roof safely?

ARMA's guidance on algae cleaning boils down to these steps:

1. Use a diluted household bleach solution. Mix it the way your shingle maker directs. ARMA says not to use other chemicals or methods without the manufacturer's approval.
2. Protect the plants and the house. ARMA says to protect the building and the landscaping from overspray.
3. Apply gently. Spray it on with low pressure. Do not scrub.
4. Let it sit, but not too long. ARMA says to leave it on "at least 15 minutes but no more than 20 minutes."
5. Rinse gently with water. No power washer, brush, or broom.
6. Protect yourself. Wear eye protection and rubber gloves.

ARMA also points out that the solution makes the roof slippery and that walking on roofs is dangerous. It recommends a professional and suggests working from a ladder or walkboards where possible. My advice is plain: do not get on your roof to clean it. If you want it cleaned, hire someone with the right fall protection and ask them to follow the shingle maker's instructions.

Will the black streaks come back?

Usually, yes. ARMA says cleaning results are temporary and "the discoloration may recur." The algae is in the air around your neighborhood. A cleaned roof will be exposed again.

How do algae-resistant shingles work?

Algae-resistant shingles use copper in the granules. The copper slowly washes out with rain and keeps algae from taking hold. GAF says older copper granules release less copper over time. Its newer shingles use what it calls Time-Release Algae-Fighting Technology to release copper more steadily.

Here is what some common shingle lines offer. These are limited warranties against algae discoloration, and they have conditions.

Sources: GAF's warranty comparison guide and CertainTeed's Landmark page.

Read the fine print. GAF's shingle warranty covers blue-green algae only on shingles labeled with its StainGuard logos. It does not cover discoloration from moss, lichen, other algae, or organic material on the roof.

When are streaks a sign you need a new roof?

Streaks alone are not a reason to replace a roof. I tell homeowners that every time. But if the roof also has curling edges, bare spots, granules piling up in the gutters, or missing shingles, the roof may be near the end of its life anyway. In that case, choosing algae-resistant shingles for the next roof makes sense.

Algae is also a hint about moisture. A roof that stays damp may sit over an attic that is not venting well. Our guide to attic ventilation and summer heat explains what to check.

How Shamrock can help

We can tell you whether those streaks are just algae or a sign of wear. Our free roof inspection includes photos, so you can see what we see. If you are already planning a replacement, we can show you algae-resistant shingle options and what their warranties cover. Learn more about our roofing services.

FAQ

Can I pressure wash my roof? No. ARMA says high-pressure washing is likely to damage asphalt roofing. It strips granules and shortens the roof's life.

Does homeowners insurance pay to remove roof algae? Algae is not storm damage, so it is unlikely to be covered. Check your policy or ask your agent.

Does algae spread from house to house? It travels on the wind, according to GAF. If your neighbors have streaks, your roof has likely been exposed too.

Will a new roof get streaks? It can, unless the shingles are algae resistant. Ask what algae warranty comes with the shingles in your estimate.

Related guides

• Attic ventilation and summer heat
• Roof replacement in Cincinnati: signs, steps, and timeline
• Choosing a Cincinnati roofing contractor