What are the Black Streaks on my Roof Shingles?

What are the Black Streaks on my Roof Shingles?

If you have an asphalt/fiberglass shingle roof (most common roof covering in this area), chances are you will get black streaks on it over time.

Why is that and what can you do?

The why has been well researched.  Though often attributed to an accumulation of dirt, defective shingles, mold, or mildew; the most common culprit is actually a blue-green algae known as Gloeocapsa Magma that is spread by airborne spores. It is most prevalent on the north and west sides of the home in neighborhoods with lots of mature trees.

In the 1980’s, 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 and these algae are tough! Over time, it has continued to migrate north and now flourishes in less humid environments.

Roof algae contains a dark pigment that serves as a protective coating, which shields it from the sun’s ultraviolet rays. This is what gives it the unattractive blackish color. As the algae grow, 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, however, this is generally a minor concern. The main issue is appearance, especially if you are getting an appraisal or selling your home. Many will interpret it to mean the roof needs to be replaced, and in most cases that is not true.

What to Do?

In some cases, nothing. While the experts are still evaluating if it causes early shingle wear, there is no definitive proof. That said, based on aesthetics and potential accelerated wear potential, it is a pro-active maintenance step to address it. And it may mean a quicker, more profitable sale or appraisal.

What are the Black Streaks on my Roof Shingles?

In our opinion things NOT TO DO are using bleaches and power washers because environmental concerns and shingle damage are the main concerns.

There are several options:

  • Hire a company that specializes in this work and can perform a structural inspection and apply remedies – but review what they are using, insurance for the company, warranty, etc. As always, be careful who you hire.
  • For Do It Yourselfers, remember this is a roof and proper fall protection is a must. There are several products, such as Wet and Forget ( that will slowly remove the discoloration. Periodic reapplication will likely be needed every few years.
  • Use a metal inhibitor. You can see how this works in the first picture where there is no algae below the chimney. Metals like galvanized steel, copper and zinc are natural biocides that inhibit algae growth. Companies like Zinc Shield provide products in this regard –

Nowadays, roofing industry trends include asphalt shingles that are manufactured with specially designed algae-resistant granules, noted by the “AR” or “algae-resistant” specification – an important innovation, as they help inhibit the blue-green algae growth that can cause harmless, but unsightly, black streaks and stains. Also, special UV stabilizers have been developed and added to granules to make the color last longer and enhance longevity.

In summary, if you have an old roof (10 years or more) chances are there will be stains – they are unsightly, but normally not damaging. If you are not selling, we would recommend doing nothing and waiting until the shingles are replaced or do proactive maintenance with a safe, non-damaging cleaning method. If you are selling and have a newer roof, treatment may make sense.

Sometimes structural and other problems arise that require the expert inspection and evaluation of an engineer. Every roof is different, and if you have questions, please contact Yingst Engineers & Associates, Inc. and we can work with you to find the best solution.