How to Get Rid of Black Streaks on Your Roof

Closeup of a roof with visible discoloration on the shingles possibly caused by the growth of algae or lichen — Credit:
Mcbooz / CC BY-SA 3.0

If you've started noticing black streaks spreading across your rooftop, you're not alone. After a spell of wet, humid weather, many homeowners spot dark stains creeping across their shingles. And while it may look like dirt or mold, the culprit is often a type of blue-green algae that thrives in humid environments.

While the streaks may seem minor, the algae can spread quickly and, when left unchecked, shorten the lifespan of your roof. Plus, insurance companies are taking note of the damage, with some sending notices to homeowners to remove the growth on their roofs or risk having their insurance canceled.

The good news? The discoloration isn't permanent. Ahead, our experts answer your biggest questions about what causes black roof streaks, how to remove them safely, when to hire a professional, and the best ways to keep them from coming back.

The Most Common Cause of Roof Streaks

Those dark streaks running down a roof are rarely dirt, mold, or mildew. In most cases, they're caused by Gloeocapsa magma. "It's a type of cyanobacteria that grows in moist, shaded areas and feeds on the limestone in shingles," says Danny Kostro, owner of NJ Softwash. "Left untreated, the growth can shorten the lifespan of roofing materials and leave your home looking neglected."

Homeowners typically notice the stains becoming more prominent after long winters, rainy springs, or extended periods of humidity, says Kostro. Moisture creates ideal growing conditions, allowing algae colonies to expand and darken over time. "Homes surrounded by mature trees may experience faster algae growth due to reduced airflow and increased shade," adds Ellen Valinotti, co-owner of YardWrap.

While this particular alga is known to thrive on asphalt shingles, it can also survive on other porous surfaces where heat, moisture, and shade are present, including wood shake, clay, and slate. "It's most commonly found on the north side of a roof, which gets less sun and stays damp longer after rain," says Angel Roman, owner of the owner of Wet Bandits Soft Wash.

Why It's a Problem

Black algae streaks start as an aesthetic issue, reducing a home's curb appeal. However, over time, algal growth can contribute to structural problems in roofing. “The organisms can trap moisture on the roof surface, creating conditions that encourage deterioration,” says Roman.

While algae itself doesn't typically destroy shingles, prolonged moisture retention can accelerate wear, especially on older roofs or roofs with existing vulnerabilities. "The longer algae growth is allowed to spread, the more difficult and expensive it becomes to address," notes Roman.

How to Clean Roof Algae

The preferred method for removing roof algae is a process known as soft washing. Unlike pressure washing, which can damage shingles, soft washing uses low-pressure water and specialized cleaning solutions to kill algae at its source. After the solution has had time to work, stains may lighten immediately or gradually fade over the following weeks.

"The most common and effective way to remove roof algae is with a solution of bleach, water, and a surfactant, which helps the mixture adhere to the roof surface," says Glen Pettigano, founder of Bergen Soft Wash. "For professional soft washing, we typically use commercial-grade bleach that's about 12 percent sodium hypochlorite. A common mixture is about 1 gallon of bleach to 5 gallons of water, although we adjust the strength depending on the severity of the staining." Household bleach typically contains about 6 percent sodium hypochlorite, so the ratio needs to be adjusted accordingly.

If you're considering tackling the job yourself, you'll need a pump or electric sprayer, an extension ladder, a safety harness, gloves, eye protection, and a garden hose. Once you have these essentials, follow these steps to remove algae from your roof.

Protect nearby plants and landscaping: Minimize the risk of chemical damage by covering plants or thoroughly wetting them.
Clean your roof: Before spraying, clear away any leaves and branches, and ensure your gutters are empty, advises Pettigano.
Spray your roof: For a DIY solution, combine bleach and water with a few drops of dish soap to help the mixture adhere to the roof (the exact ratio varies depending on the severity of the staining). Apply the solution using a pump sprayer or electric sprayer. Alternatively, homeowners can purchase a roof-cleaning product. Some home improvement stores carry homeowner-friendly hose-end sprayers that attach directly to a garden hose for easy application. Regardless of which product you use, test it in a small area before treating the entire roof.
Let it sit: Allow the solution to sit according to the product instructions. If you're using a DIY bleach solution, let it sit for about 15 to 20 minutes.
Rinse (if permitted): If you are using a DIY bleach solution, rinse the roof thoroughly with a garden hose after the solution has had time to work. For other products, follow the manufacturer's instructions for rinsing, as some roof-cleaning products require rinsing, while others are designed to be left on the roof and rinsed away naturally by rainfall.

Roof work carries significant fall risks—homeowners should only attempt DIY roof cleaning when conditions are dry. "Wear non-slip shoes, eye protection, and gloves, and always use a stable ladder," says Kostro. "Never work alone on a steep roof."

Avoid windy days and never walk on fragile, damaged, or excessively steep roofs. "If reaching all areas of the roof requires climbing onto a steep slope, hiring a professional is often the safer option," says Valinotti.

How to Prevent Black Roof Streaks from Returning

While algae is common in areas with humidity and regular rainfall, there are ways to prevent it from returning.

Routine maintenance: Roof inspections and periodic cleaning (typically every five to 10 years) can help prevent small outbreaks from becoming major staining problems. Because algae thrives in damp, shaded conditions, improving airflow and increasing sunlight exposure is key. "Trimming trees can increase sunlight exposure and help roofs dry more quickly after rain, making conditions less favorable for algae growth," says Kostro.
Using algae-resistant roofing materials: Many modern roofing products feature algae-resistant technology, often in the form of copper-infused or mineral-coated granules that inhibit algae growth. “These shingles can significantly reduce staining and help roofs maintain a cleaner appearance for longer periods. While they are not entirely algae-proof, they generally outperform traditional shingles in humid climates where black streaking is common,” says James Circle, president of A1 Professional Exteriors. Other roofing materials, including metal, slate, and certain tile, also tend to be more resistant to algae growth than conventional asphalt shingles.
Installing zinc or copper strips: Homeowners with existing roofs can also help prevent future growth by installing zinc or copper strips near the roof ridge. "When rainwater flows over these metals, it carries trace amounts of copper and zinc down the roof surface, creating an environment that helps inhibit algae growth," says Pettigano. "Homeowners should note that metallic runoff may be harmful to nearby plants and animals."

Common Mistakes to Avoid

One of the biggest mistakes homeowners make is reaching for a pressure washer. "Power washing can remove the protective granules from shingles, damage roofing materials, and force water underneath shingles, potentially leading to leaks," says Kostro. "These granules shield asphalt shingles from UV damage and weather exposure. Once those granules are removed, shingles become more vulnerable to premature aging, cracking, and water intrusion."

Other common mistakes include using bleach solutions that are too concentrated, scrubbing shingles aggressively, ignoring the manufacturer's cleaning recommendations, applying products without protecting nearby plants, and attempting to clean a roof in wet or slippery conditions. Many homeowners also expect instant results, forgetting that soft-wash treatments often continue working for several weeks after application.

Signs It's Time to Call a Professional

Cleaning your own roof may be reasonable for single-story homes with gentle slopes and minor staining. However, Circle says professional help is usually needed if the following circumstances occur:

The roof is steep or hard to access
Algae growth is extensive
Moss is present in addition to algae
There is significant granule loss
Shingles are missing or cracked
There are soft or spongy areas
Roof sections are sagging
Water stains are present inside the home

"If these issues are present, cleaning alone will not solve the underlying problem. In some cases, roof repair or replacement may be necessary," he says. Professionals have specialized equipment, safety training, and cleaning solutions that can remove growth more effectively while minimizing risk.