Why Are There Black Streaks on My Roof?

The next time you examine your house exterior, you might notice several unsightly dark streaks cascading down your roof. There’s a good chance you’ll see similar markings on other roofs around town, too. Indeed, these vertical blemishes are common, and most people don’t quite know what they even are -- they might look like dirt, soot, or mold, but they’re actually something else entirely. Let’s go over what causes these black streaks on your roof and what you can do to remove and prevent them.

What to Know About Black Streaks on Roof

The Main Cause of Black Streaks on Roofs

The primary culprit for those dark rooftop eyesores is actually algae -- usually gloeocapsa magma (also known as blue-green algae), to be precise. This aquatic lifeform might go undetected early on, beginning as tiny spots speckling your roof. Over the course of several months, however, this blue-green algae can grow, visibly covering sections of your roof in the form of those notable black streaks. If you’re wondering why this substance appears black rather than blue-green, it’s due to its protective coating that keeps the sun’s harmful UV rays at bay.

Why Does Asphalt Attract Algae, Anyway?

Gloeocapsa magma and other types of algae don’t really discriminate when it comes to which surfaces they decide to live on. That said, some surfaces are more hospitable for this type of algae than others, and asphalt tends to be an easy target. So, while these black streaks can affect just about any type of roof, they’re most prevalent on asphalt roofs. But why is this, exactly?

For starters, asphalt is a porous material, meaning water, dirt, and other small particles can permeate its surface, including algae spores. Algae also thrives in humid environments, which covers about half of the U.S. To make matters worse, gloeocapsa magma has grown hardier over the years, expanding to less humid territories it would have avoided in the past. Algae spores spread through the air via wind, animals, and other carriers, so it doesn’t take long for one roof to essentially infect another.

Today, this type of algae can be found on asphalt shingles across approximately 80% of the country, and some regions are more at risk than others. Tennessee, for instance, is completely entrenched in the “high danger zone” for algae-related roof discoloration, so it’s no wonder that homes across Nashville, Franklin, and Brentwood, TN regularly run into this issue. The fact that many modern asphalt shingle manufacturers now incorporate fillers like limestone and fiberglass (which cultivate algae growth) doesn’t help matters, either.

Potential Problems Related to Roof Streaking

So, the algae that causes black streaks on roofs is common and unsightly -- but is it a real problem beyond these cosmetic concerns? Fortunately, the algae itself won’t do any real damage to your roof, especially if it’s dealt with in a timely manner. However, allowing this algae to linger for an extended period of time can have negative effects on your roof and the rest of your home. As time goes on, a significant presence of algae can remove the important UV-blocking granules that coat your asphalt shingles, which leaves your roof more vulnerable to sun damage.

Moreover, airborne fungal spores can link up with algae on your roof and transform into lichen, which can gradually eat away at your roof. Lichen is also more difficult to get rid of than algae, as it can develop sturdy roots as it grows. If enough time passes without intervention, this algae, lichen, and other environmental debris can begin to affect your exterior house paint and more.

Eliminating Those Black Streaks on Your Roof

As common as these black streaks may be, they’re relatively easy to remove with the proper cleaning tools and methods. If you want to take a crack at this project yourself, make sure you steer clear from pressure washing. While pressure washing offers an efficient way to blast away debris on exterior surfaces, it can also do serious damage if you’re standing too close to said surface or aiming at the improper angle. To remove black streaks without damaging or removing your shingles, opt for a garden pump sprayer instead. As for your cleaning solution, your best bet is a combination of trisodium phosphate (TSP) (1 cup), bleach (1 gallon), and water (5 gallons). Always check with your shingle manufacturer for optimal cleaning recommendations before settling on a solution, of course.

Before you hop on the roof and start spraying, however, it’s important to make various preparations. After all, you don’t want to harm any nearby surfaces or vegetation in the process. Make sure you clear away and/or cover these nearby surfaces. Additionally, plan to clean your roof on a day with no chance of rain, decent cloud cover (minimal sun), moderate temperatures, and low winds. To protect yourself, gear up with eyewear, slip-resistant shoes, rubber gloves, and other protective clothing.

Once these preparations have been made, spray your cleaning solution along your roof with long strokes from the top of the roof to the bottom. Avoid standing on wet areas as you clean to keep from slipping. After you’ve fully cleaned your roof, let the solution sit for 15-20 minutes. Then, rinse your roof with warm water through a standard hose.

If all of the above seems too complicated or involved, it’s a good idea to hire roof cleaning professionals. You’ll find that a good number of pressure washing services and full-service contractors can also properly clean your roof and remove black streaks even though pressure washing isn’t called for in this context.

How to Prevent Future Black Streaks from Showing Up

After you’ve successfully cleared away all those ugly black marks from your roof, you’ll never want to see them again. And while there’s no way to completely prevent algae and other contaminants from landing on your roof, there are plenty of ways to mitigate environmental roof damage, including these black streaks.

Maintenance is the name of the game, both for your roof and the rest of your exterior features. The cleaner you keep your roof, the less susceptible it will be to various contaminants. It’s also important to periodically pressure wash your gutters, as clogged gutters can allow moisture to build up outside your home. Landscape maintenance is a part of this equation, too -- trimming your trees to let more sunlight onto your roof is a smart way to keep your roof dry and keep algae at bay (keep in mind that too much sun can harm your roof, too).

Beyond these basic maintenance tasks, you can further prevent black streaks on your roof by installing a zinc or copper strip underneath its shingles (those nearest the roof’s peak). These strips aid in water drainage, sending moisture down and away from your roof when it rains. You can further waterproof (and algae-proof) your roof by applying specialized coatings. Professional painters will perform the necessary surface preparation for waterproofing your roof depending on the type of coating best suited for it.

Rid Your Roof of Unwanted Streaks

Now, if you ever see black streaks on your roof, you’ll know exactly what’s causing it and what to do about it. And if you don’t have the confidence or equipment to handle this issue on your own, the experts at Nash Painting are happy to help.

To learn more about us, our services, and our values, call us at 615-829-6858 today!