Why Are There Black Streaks on My Roof?

Why Are There Black Streaks on My Roof?

You take pride in your home’s appearance and then one day you notice ugly black streaks or splotches on your roof. Alarmed, you think, what’s that on my roof and where did it come from?

Here’s a quick rundown of what it is, what causes it, and how Wet & Forget Outdoor erases those unsightly asphalt shingle stains.

What is it?

Although it may look like mold or fungi, these unsightly black streaks or smear-like stains may indicate a type of roof algae, known in technical terms as, “Gloeocaspa Magma“. This blackish algae creeps it way across neighborhood roofs, gradually turning your home’s asphalt shingles an ugly dark brown or black.

Roof algae is most prevalent on north and west-facing shingles and in neighborhoods with lots of mature trees. Your home’s shingles may have curled corners and raised surface bumps, which can eventually result in broken shingles and a prematurely decayed roof.

What is it caused by?

In the 1980s roof shingles were made of asphalt plus a cotton rag and wood filler. Today’s new fiberglass shingles provide an ideal environment for algae growth because they contain a filler that includes limestone dust.

Unfortunately, roof algae’s favorite foods are limestone and lime-based products like cement. This algae is tough stuff! It has continued to migrate north and now flourishes in less humid environments.

Some credit its expansion to global warming. Whatever the cause, it’s now a growing problem worldwide.

algae on roof

Why is it growing on my roof?

Humidity, warmth, and a constant meal of limestone ensure that a bloom of roof algae will survive. Roof algae grips your shingles and holds on tight by secreting a glue-like substance.

In addition to roofs, it can grow on any moist or wet outdoor surface including walls, driveways, and sidewalks.

Why the dark streaks?

Roof algae contain a dark pigment that serves as a protective coating that shields it from the sun’s ultraviolet rays. This is what gives it the unattractive blackish color.

As the algae grows, gravity gradually pulls it down the roof creating the characteristic dark streaks. On light-colored roofs, these dark stains can raise indoor temperatures leading to an increase in your home’s cooling costs.

Is it harmful?

There’s a debate in the industry on whether the algae actually causes roof deterioration. Some say the act of constantly removing it with caustic chemicals, like bleach, may cause deterioration itself.

Some experts say that the algae may eat away at the shingle’s base causing the shingle to expand and contract with changes in temperature. This constant “movement” may shorten the life of your roof.

On cedar shakes and shingles, algae and moss are partners in crime. It’s believed that moss growth can develop from algae.

Moss, algae’s sidekick, is destructive to cedar shingles because it eats the wood, eventually causing rot. Some homeowners and condo associations require roof algae removal on a regular basis.

How can I prevent it from happening?

Gloeocaspa magma is an airborne strain. Its spores are spread from one roof to another by the wind.

These spores travel from roof to roof, eventually settling in and grabbing hold. Some experts suggest that overhanging trees should be trimmed to slow the algae’s growth, but this is only a temporary solution.

It’s recommended to clean your roof every 6 to 18 months to maintain your home’s appearance.

Why should I remove roof algae?

It can affect your homeowner’s insurance. Insurance companies may send out an inspector to look over your home to see if anything has occurred that will allow them to raise your rates or even to cancel your policy.

A dirty, algae-stained roof may give them the excuse they need.

Available Cleaning Methods:
Chlorine bleach

Applying a chlorine bleach solution can be an inexpensive cleaning method in the short term but it can cost you in the long run.

The downside to cleaning with bleach is that it may discolor or “bleach” your shingles. Bleach can also speed up the corrosion of metal roof flashings, gutters and downspouts.

A bleach solution that drips or is rinsed off can kill your landscaping, discolor awnings or affect nearby outdoor items. Bleach can be a hassle to clean with and is hazardous to the environment.

The effects of chlorine bleach are considering to be short-lived. True to its name, chlorine bleach “bleaches” roof algae growth.

You may think it’s gone, but it actually leaves a food source for newly arrived spores. As a result, the algae can come back quicker and thicker than before.

Oxygenated bleach

Oxygenated bleach (sodium percarbonate) comes in a solid form; usually a powder. It’s made up of hydrogen peroxide plus soda ash.

Oxygenated bleach’s cleaning action is activated by water. The negative to cleaning with oxygenated bleach is the shingles must be kept wet for up to 45 minutes to provide optimum cleaning. That’s a lot of time on the roof!

Sodium hydroxide (Lye)

Another choice to remove algae is sodium hydroxide (lye). Lye is gentler on the environment than bleach, but is nonetheless toxic and can easily burn the skin.

Extreme care must be taken by the homeowner and with anyone the lye solution may come in contact with. Sodium hydroxide can cause premature wear on asphalt shingles.

It’s so strong that it can dissolve roofing nails! It also requires heavy rinsing with water. Most lye-based products have been removed from the market due to their impact on the environment.

Trisodium Phosphate (TSP)

Trisodium Phosphate is a compound that was widely used as an ingredient in dishwashing powder and clothes detergent until the 1970s. Eventually, environmentalists discovered the damage that phosphates can do to the natural world.

Care must be taken as trisodium phosphate can damage metal and painted surfaces and also stain wood. Landscaping near your home may be affected by the runoff.

Metal Inhibitors

Roof algae stays away from galvanized steel, copper and zinc because these metals act as a natural biocide. Installing strips of copper or zinc along the roof ridge line may inhibit algae growth. When it rains, a tiny amount of copper or zinc will run down on the roof, washing the shingles and preventing the algae from grabbing hold.

Unfortunately, minute amounts of copper and zinc leaching from your roof isn’t good for the environment. The runoff will eventually finds its way into in creeks and ponds where it is toxic to aquatic life.

Metal runoff can also render rainwater unusable for landscape use.


Pressure-washing is not recommended for algae removal. High-pressure washing systems can degrade your roof’s mineral granular surface which protects your roof from the sun’s ultraviolet rays.

Your roof’s protective layer may be compromised by the use of high-pressure washing systems. In addition, the exterior surfaces of your home are not made to withstand the destructive forces of H20 under high pressure. Skylights and roof windows may be damaged.

The watertight seal around windows and doors may be broken, forcing water inside your home, damaging carpets and furniture. Water that is forced underneath vinyl siding can migrate into your walls causing mold problems.

A note: most shingle manufacturers will void the warranty on their products if they discover that you’ve used a pressure washer on your roof.

Why is Wet & Forget Outdoor the Best Way to Clean Roof Shingles?

“I tried this product on the north side of my roof about two months ago. There were visible dark streaks that made me think I needed to replace my roof to improve the appearance of my house. In about two weeks I noticed a significant improvement in the roof and a week or so later I would say 75% of the streaks were gone.

I sprayed on a second application and the roof looks better every week. Currently, 90% of the mold stains are gone and I have dropped any thoughts of replacing my roof any time in the near future. This is the best investment I’ve ever made in improving the appearance of my house.

If this product wasn’t outstanding, I would never take the time to post these comments.” -Jon B.  from Hessel, MI

Wet & Forget Outdoor is:
• An environmentally-friendly way to clean your roof.
• Easy to use; just dilute Wet & Forget solution with water, spray, and you’re done!
• A gentler way to clean. Wet & Forget has a pH value of 8, which is close to the neutral pH 7 value of water.
• Wet & Forget contains no hazardous bleach or acidic chemicals so it won’t harm your roof’s shingles.
• Wet & Forget cleans thoroughly so surfaces stay cleaner, longer.

Gloeocapsa Magma – that ugly “black stuff” on roofs is here to stay. Keep your shingles, tiles, and concrete surfaces clean and like new with Wet & Forget Outdoor.

Apply Wet & Forget Outdoor to your roof with a garden sprayer. Dilute 1 part Wet & Forget Outdoor to 5 parts water.

Make sure your roof is dry before application. For best results, apply Wet & Forget Outdoor on a cool, dry day with no chance of rain for four to five hours after application. The morning or evening is best.

This will ensure that the surface stays wet for an extended period while Wet & Forget Outdoor works on breaking down the growth. Be sure to saturate the area that is being cleaned. The product needs to soak through the growth and reach beneath the surface.

After the stains have disappeared, use Wet & Forget Outdoor as a regular maintenance tool. Re-apply at the first sign of new algae growth to keep those ugly stains from coming back.

No scrubbing necessary–just spray and forget it!

Listen to home improvement expert, Danny Lipford talk about Wet & Forget for your roof!

Stop by our website to learn more about our easy solutions to everyday problems, so you can spend less time cleaning and more time enjoying life!


  1. Bob Young

    Every year I wash my vinyl siding to remove unsightly green mold. It’s another of my “least favorite chores”. There must be a better way. Discovered Wet & Forget online. It sounded too good to be true but what did I have to lose? Found W&F at my local ace hardware store. Following the instructions on the bottle (cool, dry day; 1 part W&F to 5 parts water) and applying it with a handheld garden sprayer (just spray it on and let it soak), the results are amazing!!! within 24 hours the mold was DISAPPEARING. Seventy-two hours later it was completely gone!!! I’m a believer. What a great product!!!

  2. kEN bARRETT

    Guess I really messed up. I applied Wet And Forget on a hot sunny day. It started evaporating right away. I wasted a lot of money trying to take the black streaks of my roof. But a lesson learned by not paying attention!

    1. Wet And Forget

      Hi Ken,
      I’m sorry to hear this. My best advice would be to reapply Wet & Forget on a cooler day. I would love to say there may be a chance that the application took, but if you saw the product evaporating instantly on a hot roof, I don’t think you’re going to have great results. You can try giving the surface around a month or so to see if it lightens up from rainfall, but I really think you’re going to need to reapply. If you have any questions we’re always here to help.

  3. Susan

    I have a light red metal roof that has black algae / mold on it, primarily from the Oak trees in my yard. The roof is slanted and would be hard to climb on to apply your product. Any ideas? Could I use some kind of water hose With a sprayer? Thanks

    1. Wet And Forget

      Hi Susan,
      Thanks for commenting! Wet & Forget needs to be applied at a 1:5 mixing ratio. Unfortunately you cannot use a hose-end type sprayer because the spray will come out at a 1:9 ratio. There is a sprayer called a Trombone Sprayer that will get you a good distance from the ground and works well with Wet & Forget.

      1. Bill

        Can you pleas give me more information on the trombone sprayer. Manufacturer, Model? I also need to spray the roof from the gutter line and need to go at least 30+ feet.

        1. Wet And Forget

          Hi Bill,
          Here’s a link to trombone sprayers listed on Amazon. I don’t think you’re going to be able to get 30+ feet from the ground with a trombone sprayer. You can also look into agricultural diaphragm pump sprayers where you would be pulling directly from a premixed large tank if you have a very large area to apply to.

  4. Joe Karst

    How would Wet And Forget work on a log sided house in northern Minnesota? We have dark spots mainly on the north side.

    1. Wet And Forget

      Hi Joe,
      Wet & Forget will remove the dark mold and mildew spots from the north side of your log sided house. For best results, apply to the dry logs and leave it. Over time, the weather will hit the surface and rinse and scrub the dead growth for you.

  5. Dennis

    Our local library has black streaks on the limestone wall parts.

    1. Wet And Forget

      Absolutely Dennis,
      You’ll see black streaks, green algae stains, and black mold stains definitely growing on limestone surfaces and surfaces that don’t get cleaned very often.

  6. Georgiana Kotarski

    Can I use it on galvalume metal roof? Would a very low setting on a pressure washer work or would the ratio be unattainable? Thanks!

    1. Wet and Forget

      Hi Georgiana,
      Wet & Forget is safe to use on galvalume metal roofs. We do not recommend using Wet & Forget with a pressure washer because it will dilute our product too much. We recommend using a garden pump sprayer or a backpack sprayer to apply our product.

  7. David Coulsey

    Is wet and forget safe to use o n a roof with copper gutters?

    1. Wet and Forget

      Hi David,

      Wet & Forget is safe to use on a roof with copper gutters. Wet & Forget is a gentle product that works with the outside elements (rain/wind) to remove the moss, mold, mildew, and algae stains.

  8. Charissa Woods

    Wet & forget outdoor is the best product ive ever used!!! Its a lil pricey but well worth its weight in gold!! My pool deck was old and Had lots of green algae and mildew all over it that i needed to remove before Restaining and sealing. I bought over 12 different products averaging about $150 before trying this and nothing worked to remove 1 single spot!!! Wet & forget worked on the whole deck over night!! I was amazed because i thought it would take a week or so before it would start working!!! This is by far the most impressive and magical cleaner ever!! If it was a lil cheaper id buy at least 10 more jugs of it to clean the outside bricks on my home, the roof, my carport, patio and 2 storage bldgs!!!

    1. Wet and Forget

      Thanks for your feedback, Charissa! Wet & Forget has a longer residual effect than many other cleaners on the market as it thoroughly cleans exterior surfaces. 1 gallon of Wet & Forget concentrate will make 6 gallons mixed so it can cover 750-2250 square feet. It also doesn’t expire so if you only use part of bottle, you can use the rest at a later date without a problem!