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Black Streaks on Your Roof: What They Are and Whether Soft Washing Is Safe for Shingles

Those dark streaks are almost always algae, and they're especially common along the Gulf Coast. Here's what roofing manufacturers say about them and how to clean them without hurting your shingles.

Updated October 19, 2026 · About 7 min read · By ACE – Advanced Cleaning Experts

The short version

  • Black streaks are usually algae, most often Gloeocapsa magma, not dirt or soot.
  • The roofing industry says algae is mostly a cosmetic problem on asphalt shingles. Moss is a bigger concern.
  • Never pressure wash a shingle roof. Shingle makers call for a gentle, low-pressure chemical clean and a low-pressure rinse.
  • Cleaning is temporary. Streaks can come back, and the long-term fix is algae-resistant shingles when you reroof.

What those streaks actually are

The Asphalt Roofing Manufacturers Association (ARMA) says the brown-to-black discoloration on many roofs is algae. It's often mistaken for fungus, soot, dirt, moss or tree droppings. Gloeocapsa magma is probably the most common of several species involved (ARMA).

The algae makes a dark pigmented sheath that protects it from the sun's ultraviolet rays, and that pigment is the color you see. It usually starts as small spots and then turns into streaks across the roof plane. By the time you notice it, it has likely been there for several months or longer. The spores travel on the wind or on animals, which is why whole neighborhoods often show the same streaks (ARMA).

How roof algae streaks form Algae spores arrive on the wind and settle on damp shingles. The algae grows a dark protective pigment, starting as small spots, then spreading into streaks down the slope where moisture lingers. 1. Spores arrive on wind or animals 2. Small dark spots a pigment shields the algae from sunlight 3. Spots spread into streaks down the slope Warm, humid weather and damp, shaded areas help it grow. Often visible months after it starts.
Simplified diagram based on ARMA's description of roof algae.

Why it's so common here

ARMA says algae discoloration is most widespread in the Gulf States and along the Northwest and Eastern seaboards, and it grows best in warm, humid conditions (ARMA). It's also easier to see on lighter-colored shingles than on dark ones.

Does algae damage shingles?

According to ARMA, there is no scientific evidence that algae damages asphalt shingles, although it clearly affects how the roof looks. The exception is highly reflective "cool" roofs, where algae will reduce how well the roof reflects heat over time (ARMA).

Moss is different. ARMA notes that moss can lift or curl the leading edges of shingles, raising the risk of blow-off in high winds, and heavy buildup can lead to moisture damage or leaks (ARMA). Thick, green, raised growth deserves attention sooner than flat black streaks do.

Why a roof should never be pressure washed

Here the industry is unusually blunt. ARMA says: "DO NOT use a power washer or any type of brush or broom to clean algae from the roof surface," and warns that high-pressure systems are likely to damage asphalt roofing (ARMA). Its maintenance guide adds that pressure washing will cause granule loss and very likely premature failure of the roof (ARMA). GAF's technical bulletin on algae staining also says not to power wash (GAF).

Those granules aren't just decoration. Consumer Reports notes that they help extend the shingles' lifespan, and that a high-pressure stream can also loosen shingles (Consumer Reports).

What shingle makers recommend instead

Both ARMA and GAF describe the same basic approach (ARMA, GAF):

  1. A diluted solution based on household bleach (sodium hypochlorite) is gently applied to the roof. No scrubbing.
  2. It's left to work for a limited time set by the manufacturer.
  3. The roof is rinsed with low-pressure water.
  4. Landscaping and the rest of the building are protected throughout.

The exact mix and timing vary by manufacturer, and ARMA says other chemicals or methods shouldn't be used without the shingle manufacturer's approval (ARMA). That's why we don't publish a recipe here: follow the directions from the company that made your shingles. This low-pressure, solution-based approach is what the industry calls a soft wash.

What about my roof warranty?

We can't tell you how a particular warranty treats cleaning; only the manufacturer can. Before any cleaning, look up your shingle brand's care guidance and warranty terms. Following the manufacturer's own published method is the safest route, and anyone you hire should be able to explain how their method lines up with it.

Keeping the streaks from coming back

ARMA is clear that cleaning is only temporary and the discoloration may come back (ARMA). Its suggestions for slowing regrowth (ARMA):

  • Trim branches so more sun reaches the roof and less debris collects.
  • Clear debris regularly with a leaf blower or another non-abrasive method, blowing down the slope.
  • Keep gutters clean, and don't let an upper roof's downspout drain straight onto a lower roof.
  • When it's time to reroof, consider algae-resistant shingles. ARMA doesn't recommend adding zinc or copper strips to an existing roof, because installing them can cause leaks or wind damage.

Safety note: Roof work is dangerous. ARMA warns that the cleaning process makes the roof slippery and recommends that only trained professionals walk or work on roofs. It encourages using professionals for shingle cleaning (ARMA). Consumer Reports also advises never using a pressure washer while on a ladder, because the kickback can throw you off balance (Consumer Reports).

ACE and roof jobs

ACE takes roof jobs, including tile and metal, when the roof can be cleaned safely. Ask us. On shingles, that means a low-pressure soft wash with an X-Jet soft wash system, never high pressure. If you know your shingle brand, tell us when you ask for a quote so we can check its care guidance together.

Example photo: dark gray asphalt shingle roof on a brick and siding house
Stock example photo of an asphalt shingle roof, not an ACE job.

Quick answers

What causes black streaks on a roof?

Usually algae, most often Gloeocapsa magma. Its spores travel on the wind, settle on damp roofs and produce a dark pigment that shows up as spots and then streaks. ARMA says roof algae is most widespread in the Gulf States.

Do black streaks damage shingles?

ARMA says there is no scientific evidence that algae damages asphalt shingles, though it affects how the roof looks and can reduce the performance of reflective cool roofs. Moss is different and can lift shingle edges.

Can you pressure wash a roof?

No. ARMA and GAF both say not to use a pressure washer on asphalt shingles. High pressure can strip the protective granules and shorten the life of the roof.

Is soft washing safe for shingles?

Shingle makers' own cleaning guidance uses a gentle, low-pressure application of a diluted household bleach (sodium hypochlorite) solution followed by a low-pressure rinse. Follow your shingle manufacturer's directions, and don't use other chemicals without its approval.

Will cleaning void my roof warranty?

It depends on your shingle manufacturer and your warranty, so check both before any cleaning. Following the manufacturer's own published cleaning guidance is the safest route.

Will the streaks come back?

Usually, in time. ARMA notes that cleaning is only temporary and the discoloration may recur. Algae-resistant shingles are an option when you replace the roof.

Sources

  1. Algae Discoloration of Roofs, Asphalt Roofing Manufacturers Association (ARMA). https://www.asphaltroofing.org/algae-discoloration-of-roofs/
  2. Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems, Asphalt Roofing Manufacturers Association (ARMA). https://www.asphaltroofing.org/algae-moss-prevention-cleaning-asphalt-roofing-systems/
  3. Technical Bulletin TAB-R-102: Algae Staining on Shingled Roof Surfaces, GAF. https://www.gaf.com/en-us/document-library/documents/technical-bulletins-&-notes/r-102-algae-staining-on-shingled-roof-surfaces.pdf
  4. Can You Pressure-Wash It?, Consumer Reports. https://www.consumerreports.org/home-garden/pressure-washers/surfaces-safe-to-clean-with-a-pressure-washer-a5445180933/

Links checked October 19, 2026. This page is general information, not a substitute for your manufacturer's instructions or a product label.