What Causes Black Streaks and Black Stains on My Roof?

Jun 9, 2023 | Soft Washing

If you live in the Southeastern U.S., you have probably seen the unsightly black streaks that plague roofs in the area. These long black stains put a damper on your coveted curb appeal. However, you may not know that they can also signal a larger issue.

The same factors that cause black streaks on your roof can lead to significant roof damage. If you notice these stains on your roof, you need to understand what they are, the best ways to prevent them, and how to treat them when they appear.

What are the black stains on your roof?

You may think dark stains on your roof are dirt, mold, or mildew. But they are actually algae! More specifically, these streaks are usually caused by gloeocapsa magma a bacteria more commonly referred to as blue-green algae. This type of algae turns black when it forms a hard, dark outer coating to shield it from UV rays.

Are black streaks harmful to your roof?

The gloeocapsa magma algae itself isn’t harmful to humans but it can certainly hurt your roof. Over time, the algae that create the black streaks on your roof will erode the UV protection granules in shingles, causing them to become dry and brittle.

If left untreated, the algae can also lead to lichen growth. Lichen contains roots that are much harder to remove and do more damage to your roof. Damaged shingles can allow water to penetrate your roof’s defenses, leading to rot, costly leaks, and even the necessity of replacing your roof.

How do black streaks get on your roof?

Unfortunately for other roofs in your neighborhood, algae spores are airborne. That means that when one house on the block gets black streaks, the rest usually follow.

Once the airborne spores attach to a roof, a gloeocapsa magma infestation begins. Over time, small clusters of algae grow and join to form the noticeable black streaks. By the time you notice stains on your roof, the algae have likely been there for several months – meaning you need to act fast.

Most asphalt shingles are now full of filler materials – like fiberglass and crushed limestone – which feed algae. When combined with commercial fertilizer that contaminates our water supply and good ol’ fashioned southern humidity, you have a recipe for organic algae growth.

You may notice that the majority of black stains appear on the north facing portion of your roof. This is no coincidence and it’s due to the sun’s direction here in the northern hemisphere. This dynamic means the north facing portion of your roof usually receives less sunshine than the southern facing side, so it maintains a higher amount of moisture that accelerates algae growth.

How to Prevent Black Streaks on Your Roof

There are plenty of short-term and long-term options to prevent algae growth. A simple short-term solution is to keep overhanging trees trimmed. This prevents debris build up and allows more sunlight to hit the roof. Also, keep gutters and downspouts clean and free of clogs to promote proper drainage. Further, never allow gutters or downspouts from an upper roof to drain directly on a lower roof. Instead, always extend the downspout from the upper roof into the lower gutter. Additionally, make sure the downspouts are routed away from the foundation of your home.

A longer-term solution is to install copper or zinc strips close to the peak of the roof. As it rains, compounds in the metal will wash down the roof and inhibit algae growth. If you are planning to replace your roof or have it re-shingled, there are new roofing products that have copper mixed in with the shingles to prevent algae build up. Consumers have had mixed results with these products, so make sure to do your research before installing these specialized roofing materials.

How do you remove black streaks from your roof?

Roof algae and its characteristic black streaks can be easily remedied with a simple roof cleaning by a trained professional. You have several options for roof washing which we will discuss below.

DIY Roof Cleaning

If you are a DIYer, there are several cleaning solutions that have some effectiveness against algae. If you choose to apply one of these solutions, be careful! First and foremost, with untrained people the risk of damage is high and this can be a very costly mistake. Further, falls from roofs can be devastating. This is why you should leave this work to the pros. However, if you are comfortable taking this risk, be sure to wear eye protection, gloves, protective clothing, and slip resistant shoes.

Make sure you follow the suggested mix of chemical and water for your roof type, then use a pump sprayer from a home and garden store to coat the roof with the desired solution. Spray the roof in an even coat and be sure not to step on the area while wet as it will be extremely slippery. Once the solution has been given time to dwell – usually 15 – 20 minutes – rinse the roof with a garden hose with no pressure.

You should never clean or rinse your roof with a pressure washer. Power or pressure washing shingles can void manufacturer warranties. In addition, it will damage the protective coating and can even remove the shingles themselves.

Hire a Professional to Soft Wash the Black Stains from Your Roof

If you want to stay safe on the ground and don’t want to deal with chemicals potentially harming your landscaping, work with a trained roof cleaning company to take care of those problematic black streaks and stains.

Be sure to choose a roof cleaning company that uses industry standard equipment and chemical formulas recommended by the National Roofing Contractors Association [NRCA]. Also, choose a company that can apply the roof treatment using a technique called Soft Washing. Unlike power or pressure washing, soft washing allows the cleaning company to lay down a layer of treatment with very little pressure. Often, they can apply the treatment from the ground without having to climb on your roof.

Soft washing also protects your landscaping. Before the treatment, an experienced roof cleaning company will prep your home for treatment and wet the perimeter to protect plants.

Contact The Roof Cleaning Experts at Terra Pro-Wash to Remove Black Stains from Your Roof

Pesky black streaks can be unsightly and diminish curb appeal. However, with proper maintenance and routine cleanings from a professional exterior cleaning company, you can put these worries behind you.

At Terra Pro-Wash, we clean all types of roofs including asphalt shingles with eco-friendly chemicals delivered through industry leading soft washing techniques. In addition, our roof cleaning technicians are trained to protect your roof and landscaping. Remember, Happy Roof, Happy Life!

If you have any questions about the black streaks on your roof in the Metro Atlanta area, give us a call today or request a quote online. Our estimates are always free, so there’s no reason to hesitate!