What Causes Black Streaks on Your Roof (and How to Get Rid of Them)

If you’ve noticed dark streaks running down your roof, you’re in good company. Drive through almost any South Jersey neighborhood, from Cherry Hill to Margate, and you’ll see them on house after house. They usually start on the north side, run down from the ridge, and get darker every year.

The good news: those streaks are almost never dirt, mold from inside the house, or a sign your roof is failing. They’re algae. Below, we’ll explain what causes black streaks on a roof, whether they’re hurting your shingles, and how to get rid of them without doing more harm than good.

What causes black streaks on a roof?

The streaks are a colony of algae, most often a species called Gloeocapsa magma. The Asphalt Roofing Manufacturers Association calls it the most common cause of roof discoloration. It looks green at first, but it builds a dark outer coating to protect itself from the sun. That coating is the black you see.

A few things make South Jersey roofs especially prone to it:

• Humidity. Algae needs moisture, and humid summers keep roofs damp for hours after a rain or a heavy dew. That’s especially true near the shore and the bays.
• Shade. North-facing slopes and roofs under trees dry slowest, which is why streaks usually start there.
• Wind-carried spores. Algae spores travel on the wind and from roof to roof. Once one house on the block has it, the neighbors often follow.
• Water running down from the ridge. Rainwater carries the algae downhill, which is why it forms long vertical streaks instead of spots.

It isn’t a sign that your roof was installed badly, and it isn’t coming from inside your attic.

Are black streaks damaging your roof?

In most cases, roof algae is a cosmetic problem, not a structural one. It makes your roof look older than it is and can drag down curb appeal, which matters if you’re selling. It isn’t eating through your shingles.

That said, it’s worth keeping an eye on for a few reasons:

• Dark roofs absorb more heat. Heavy streaking can make a light-colored roof act like a darker one in summer.
• Algae often comes with moss and lichen. Those are a different story. Moss holds water against the roof and can lift shingle edges, which lets water in.
• The wrong cleaning method does real damage. Pressure washing a streaked roof strips the protective granules off the shingles and shortens their life. More roofs are hurt by bad cleaning than by the algae itself.

How to get rid of black streaks on your roof

The method shingle manufacturers recommend is simple: a bleach-and-water wash, applied gently and rinsed off. The ARMA cleaning guidance works like this:

1. Mix the solution. Use a 50/50 mix of laundry-strength liquid chlorine bleach and water. Some manufacturers call for a weaker mix, so check your shingle maker’s instructions first.
2. Protect everything below the roof. Wet down plants, grass, and siding before and after, and cover anything delicate. Bleach runoff will burn landscaping.
3. Spray it on gently. Use a garden sprayer, working from a ladder or the ground rather than walking on the roof.
4. Let it sit 15 to 20 minutes. Don’t let it dry on the roof.
5. Rinse with low-pressure water. A garden hose is enough.

The streaks may not vanish on the spot. They often keep fading over the next few rains as the dead algae washes away.

What not to do

• Don’t pressure wash. ARMA is blunt about it: a pressure washer “will cause granule loss and very likely premature failure” of asphalt shingles.
• Don’t scrub or brush the shingles. That knocks off granules too.
• Don’t walk on a wet, bleach-coated roof. It’s extremely slippery. This is the step where most DIY roof cleaning goes wrong.

If your roof is steep, two stories or more, or you’d rather not handle bleach at height, this is a job for a pro. Look for a “soft wash” roof cleaning service, which uses the same low-pressure approach.

How to keep black streaks from coming back

Cleaning kills the algae you can see, but spores will land again. These steps slow the return:

• Let the sun in. Trim branches that shade the roof. Sunlight and airflow help the roof dry faster after rain.
• Keep the roof clear. Blow off leaves, pine needles, and debris that hold moisture. A leaf blower is gentler than a broom.
• Keep gutters flowing. Clogged gutters keep the roof edge wet. Our gutter cleaning service can handle it before the fall leaves pile up.
• Choose algae-resistant shingles when you replace. Most major manufacturers now make shingles with copper-coated granules that inhibit algae growth. If you’re planning a new roof, ask for them. For shore homes especially, it’s one of the cheapest upgrades you can make.
• Consider zinc or copper strips on a new roof. Installed near the ridge, rain washes a small amount of metal down the roof, which discourages growth. ARMA advises adding them during a replacement, not retrofitting them onto an existing roof. Installing them later usually means exposed nails or breaking the shingles’ sealant bond, which can cause leaks.

When black streaks mean something more

Most streaks are just algae. But dark areas on a roof can have other causes, so call a roofer if you notice any of these:

• Dark patches that aren’t streaks. Bare, blotchy spots can mean the shingles have lost their granules and are wearing out.
• Thick green moss. Moss can lift shingles and hold water against the roof deck.
• Curling, cracking, or missing shingles anywhere near the stained area.
• Stains inside. Any water marks on top-floor ceilings or in the attic point to a leak, not algae.
• A roof over 20 years old with heavy staining. Cleaning may not be worth it if the roof is near the end of its life.

A quick roof inspection will tell you whether you’re looking at a cosmetic problem or something that needs repair.

Frequently asked questions

Are black streaks on a roof mold?

No. They’re almost always algae, usually Gloeocapsa magma. It grows on the outside of the shingles and has nothing to do with mold inside your home.

Is roof algae harmful to my family?

Roof algae isn’t known to be a health risk to the people living in the house. It’s mainly a cosmetic problem, though it’s worth cleaning to protect your home’s appearance and value.

Can I pressure wash black streaks off my roof?

No. Pressure washing strips the protective granules from asphalt shingles and can shorten the life of the roof. Use a gentle bleach-and-water wash and a low-pressure rinse instead.

How long does it take for black streaks to come back?

It depends on shade, humidity, and your shingles, but streaks commonly return within a few years after cleaning. Trimming trees and keeping the roof clear slow it down. Algae-resistant shingles are the longest-lasting fix.

Do I need a new roof to get rid of black streaks?

Not usually. A proper cleaning takes care of most streaks. If the roof is already near the end of its life, though, a replacement with algae-resistant shingles solves both problems at once.

Not sure what’s on your roof?

Loonstyn Roofing has been working on roofs across South Jersey and Philadelphia since 1976, from Margate and Longport to Hammonton and beyond. If you’re not sure whether those streaks are just algae or a sign of something bigger, we’ll take a look and give you a straight answer.

Call (215) 235-7000 or request your free estimate online.