What Are The Black Streaks Running Down My Roof?

Your roof goes through a lot—from rain to wind to snow, your roof handles it all. It’s inevitable that there will be some wear and tear over time, and that you’ll need inspections and cleanings from a roof cleaning company to keep your roof in top shape. Especially when you notice black streaks ruining the look of your roof. 

Like most homeowners, you’re probably wondering what those streaks are so keep reading as we explain the possible causes.

What Are The Black Streaks Running Down My Roof?

Before you panic about the black streaks, do know that they’re quite common if you have an asphalt roof, and especially if you live in a humid area. 

They don’t always appear for a bad reason either and are best diagnosed with a professional inspection. That being said, there are a few things that may contribute to the streaks.


You see black and you probably assume it’s mold, but these streaks are more likely to be from roof algae known as Gloeocapsa Magma. Basically, this algae commonly finds its way onto roofs using the wind, turning shingles black.

Despite the coloring of this algae, it’s not dangerous to your health. The most immediate negative thing about it is the fact that it discolors your tiles and may impact the selling price of your home. Over time it can also wear down your shingles, so you shouldn’t let it stick around.

Just having it up there on the roof can raise the temperature of your home as well, leading you to pay more in electricity bills to keep the home cool. 

Experts do debate whether having this algae can lead to mold growth, but ultimately, it’s smarter to professionally clean your roof every year and a half at most to ensure that you’re not allowing the problem to fester.


Yes, the black streaks can also be dirt that gets carried up to your roof from the wind. This isn’t all that harmful, but it can still impact your home value if you’re trying to sell. Getting your roof a professional clean will get rid of the dirt and black streaks in this case.

Mold or Mildew

Even though the black streaks are more commonly algae, there is a chance that they can still be mold. You won’t know the difference until you call in a professional to check and to get your roof cleaned properly. 

Even if you think it might be just algae up there, you don’t want it to develop into mold, so it’s wise to book in a professional clean anyway.

Defective Shingles

Finally, there’s the possibility of defective shingles, which can make the issues with mold, algae, or dirt worse. This is because mold, algae, or dirt can get stuck in cracks or tears and cause more problems in the long run.

How to Get Rid of Those Black Streaks

No matter the cause of your streaks, you can’t clean them off yourself, at least not effectively. 

Some people attempt to pour chemicals along their shingles to get the roof cleaned. This includes chlorine bleach, oxygenated bleach, sodium hydroxide, and trisodium phosphate. All of these might help clean the roof a little, but these are chemicals after all and can wear down the coloring on your shingles and the metal on your roof.

Simply using these chemicals also won’t help you know the cause of those black streaks. If they’re mold or algae and not just dirt, you likely won’t resolve the issue in full.

Professional Cleaning

To make sure that your roof is genuinely clean and to find out what exactly caused the black streaks, your best bet is to call a professional like us to get rid of them in the long term. We have the tools and the expertise to clean properly, and to stay safe up on the roof too.


Black streaks on your roof aren’t always dangerous. They can be something simple like dirt, but they can also point to something more dangerous like algae or mold too. In the end, it’s best to call a professional to get your roof inspected and cleaned so that a small issue doesn’t turn into something more significant.