Black streaks on your roof are almost always a blue green algae called Gloeocapsa magma, not dirt, mold or burn marks. It grows on the shaded, slow drying parts of a roof and is mainly a cosmetic problem, not a sign your shingles are failing. You can clean it safely with a gentle wash, and you can keep it from coming back. The one thing you should never do is power wash it.
What causes black streaks on a roof?
Gloeocapsa magma is a cyanobacterium whose spores travel on wind and on birds and squirrels. Once it lands on a roof that stays damp, it spreads. Over time the colony builds a dark protective coating that shields it from UV light. That coating is what you see as black or dark green streaks running down the slope.
The streaks follow water. Rain carries the algae down the roof, which is why you get long vertical stripes rather than an even stain. They are often worst just below the ridge and spread downward over several years.
Older shingles made with limestone filler were especially prone to streaking, and on light colored shingles the stain is far more obvious. A gray or tan roof will show it years before a dark one does, even though the algae is growing on both.
Why do north facing and shaded roofs in western PA get it worst?
Algae needs moisture and time. Western Pennsylvania gives it plenty of both. We get humid summers, wet springs and a lot of overcast days, and many homes sit on wooded hillside lots with big oaks and maples hanging over the roof.
- North facing slopes get the least direct sun, so dew and rain sit longest.
- Roofs under trees stay shaded and damp, and leaf litter holds water against the shingles.
- Roofs below a higher roof or beside a taller house stay wet after the rest of the neighborhood dries out.
- Low slope sections drain slower and dry slower.
If the south side of your roof looks clean and the north side is striped, you are almost certainly looking at algae.
How is algae different from moss and lichen?
| Algae | Moss | Lichen | |
|---|---|---|---|
| Looks like | Flat black or dark green streaks | Green, fuzzy, raised clumps | Crusty, gray green or pale patches, often round |
| Where it grows | Shaded slopes, down from the ridge | Shaded edges, gaps between shingle tabs, under trees | Shingle surfaces, sometimes with algae nearby |
| Damage risk | Mostly cosmetic | Real risk; holds water and lifts shingle edges | Moderate; roots into the surface and removing it can pull granules |
Moss is the one to take seriously. It grows thick, acts like a sponge, and can lift shingle tabs so wind driven rain gets underneath. In freeze thaw weather, that trapped water expands and works the gap wider every winter. Lichen grips the shingle surface tightly and often leaves a ghost mark after it dies.
Do black streaks damage your shingles?
Algae alone does little structural harm. It sits on the surface and does not eat through the asphalt or the mat. The bigger concerns are indirect.
Dark streaks absorb more heat, which can add some heat load on the roof and attic in summer. Heavy growth holds more moisture on the surface and can make it easier for moss and lichen to take hold. And a streaked roof hurts curb appeal, which matters if you plan to sell.
The real danger is what people do to remove it. Pressure washing, scrubbing, or harsh chemicals strip the granules that protect the asphalt from UV light. That damage is permanent and can shorten the life of the roof by years.
How do you clean black streaks off a roof safely?
Follow the shingle manufacturer’s guidance. The Asphalt Roofing Manufacturers Association and most manufacturers recommend a low pressure wash with a diluted solution of household bleach and water, applied gently, allowed to work, then rinsed with plain water at garden hose pressure.
- Wet surrounding plants and lawn first, and cover them, since bleach runoff will burn them.
- Apply the solution with a low pressure sprayer, working from the bottom up so you can see where you have been.
- Let it sit for the time the product label recommends, without letting it dry.
- Rinse gently with a garden hose. Never use a pressure washer.
- Do not scrub. Brushing knocks granules loose.
Some commercial roof cleaning products are formulated to be gentler on plants and gutters. Whatever you use, check that it is approved for asphalt shingles. And keep in mind that a wet, slick roof is dangerous. On a steep or two story roof, this is a job for someone with proper fall protection, not a ladder and a garden sprayer.
Streaks may not vanish right away. Dead algae often keeps fading over several weeks of rain.
How do you keep black streaks from coming back?
Zinc or copper strips. A strip of zinc or copper installed just below the ridge releases metal ions every time it rains, and those ions wash down the roof and discourage algae growth. Copper is generally more effective than zinc. The protection fades the farther you get from the strip, so on long roof runs the lower section may still streak.
Trim back trees. More sun and airflow means the roof dries faster after rain. It also cuts down on the leaf litter that clogs gutters and holds moisture on the roof.
Choose algae resistant shingles. If you are replacing your roof anyway, this is the simplest long term answer. Many Owens Corning shingles include StreakGuard Algae Resistance Protection, which uses copper containing granules mixed into the shingle surface to inhibit the algae that causes black streaks. Check the specific product line for its algae resistance warranty terms, because they differ by shingle. You can read more on the Owens Corning website, or see how we install them on our asphalt roofing page.
When do streaks hide real roof wear?
Algae can mask problems that look similar from the ground. Have the roof looked at if you notice any of these alongside the streaks:
- Bald patches where granules are gone and the dark asphalt shows through
- Curling, cupping or cracked shingles
- Heavy granule buildup in the gutters or at the bottom of downspouts
- Thick moss, especially along the eaves or between tabs
- Stains on the attic sheathing or ceilings inside
On a roof that is 20 or more years old, streaks often show up right as the shingles reach the end of their life anyway. In that case, cleaning is money spent on a roof you will replace soon. Our guide on architectural vs 3 tab shingles covers what to expect from a new roof.
Frequently Asked Questions
Are black streaks on a roof mold?
No. They are almost always Gloeocapsa magma, a type of algae (technically a cyanobacterium). It is not the same as the mold that grows indoors, and it does not mean there is mold in your attic.
Will homeowners insurance pay to clean roof algae?
Generally no. Algae is considered a maintenance issue, not sudden damage. Insurance claims apply to events like wind, hail or falling trees.
Can I pressure wash black streaks off my roof?
You should not. Pressure washing blasts away the protective granules, can force water under shingles, and may void the manufacturer warranty. Use a low pressure wash approved for asphalt shingles instead.
How long does a roof cleaning last?
Without prevention, algae usually comes back within a few years, especially on shaded north slopes. Zinc or copper strips and better sun exposure slow it down considerably.
Do dark shingles prevent black streaks?
Dark shingles hide streaks better but do not stop algae from growing. Algae resistant shingles are what actually inhibit it.
If black streaks have you wondering whether your roof is wearing out, call Bella at (724) 515 5163 for a free roof inspection. We will tell you whether it is cosmetic or a sign the shingles are near the end, and walk you through options on our roofing services page. Fall schedules fill quickly before winter, so it is a good time to get on the list.