When it comes to black streaks on roofs many people shrug and think “it’s just dirt and grime”. The truth is, black streaks are almost always the result of blue-green algae, you can’t really judge black streaks as bad for your roof purely because of their color. Black streaks often quicken the deterioration process of a roof’s shingles, but black streaks aren’t necessarily bad.

What Are Those Black Streaks?

So what is a black streak and why does it happen? Gloeocapsa magma, otherwise known as blue-green algae, gains nutrients from commercial fertilizers that runoff into waterways. Blue-green algae feed off those commercial fertilizers that wash into waterways then find their way onto roofs (typically gutters), where it spreads like black mold. If you do not already have them, you may want to invest in gutter guards. The black mold we see on roofs is actually the black-green algae, gloeocapsa magma, thriving and spreading because it loves commercial fertilizers. Black streaks aren’t necessarily bad for your roof because black-green algae aren’t as damaging as black mold.

The black streaks you see on roofs though are anything but good for those who own them, so if black streak really isn’t all that bad why should you care? Black streaks cause a buildup of algae and moss that can actually detract from the shingles and other building materials and become very difficult to remove (you don’t want to scrape these off). These black streaks can also cause problems by causing water damage – they block drainage – and promote rot on building materials. Black streaks also make your roofs look unsightly and can cause black mold growth on nearby tree trunks, sidewalks, and walls if left untreated

Why Are They Bad?

So black streaks are a problem for those who own the roof but what causes black streaks in the first place? Well, as stated before black streak is caused by gloeocapsa magma, blue-green algae that feed off of commercial fertilizer run-off from fields into waterways. This ice alga doesn’t necessarily cause problems on its own because it’s not black mold – black mold is black rot – so scraping black gloecapsa magma off isn’t going to do anything for you even though it looks disgusting. The reason streak happens though is because of black mold in gutters, black streaks spread black mold which in turn spreads black streaks. Obvious I know but the black streak comes about by feeding off commercial fertilizers that run into waterways allowing it to feed and grow on roofs where it settles with other building materials as black mold would.

Black streaks are caused by black-green algae thriving because of commercial fertilizers. Black streaks can cause rot on roof materials through drainage blockages and black-green algae itself doesn’t cause any problems besides occasional black mold growth when left untreated  This means you should take care of your roof because these black stains affect more than just how your house looks – they also attract harmful bacteria that will damage your home if left untreated.

Black streaking isn’t black mold – black mold can grow right on black streaks – black streaks cause black mold which spreads black streaking. So what do you do if your roof starts black streaking? If you have a public water system nearby, meaning a municipal water system for your town or city, the first thing to do is get that checked out by your local health department just in case there were some toxins from that area that leaked into the water.

The second step would be to clean up your gutters and fix any drainage problems so commercial fertilizers don’t settle near them and help black-green algae thrive. Lastly, hire a company like CKG Contractors to come out and kill off any gloeocapsa magma black-green algae, and black mold on your roof using the proper products and methods. If black streaks don’t bother you then you can take care of them yourself but for those who don’t want black streaks because black streaking leads to black mold (and black mold grows right on black streaks) then get rid of it. This should be done yearly anyway to protect your home from leaks and damage brought about by water that settles around your roofing materials where black mold likes to grow.


So yes, black streaks look bad and they attract bacteria, black streak itself isn’t all that bad because black-green algae don’t cause any problems by itself other than attracting black mold which in turn can ruin your roof but if black streaking isn’t black mold it’s black rotting, black black black black black black black mold. So get rid of those black streaks because they are bad for your roofing materials and protect your home from water damage that can turn into black mold growth that grows right on top of black streaks to damage building materials.

For more information reach out to CKG Contractors at 973.599.0811 or via the form on our home page here for a free home roof inspection to see what we may be able to do to help.