What Can You Do About Black Streaks On Your Roof?

October 6, 2023

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black stain on roof

Black streaks on rooftops are an unsightly problem that can damage shingles and shorten the life of your roof if left unchecked. Left to spread, this roof algae can lead to leaks, roof rot, and costly repairs. 

So, what can I do about black on my roof? To remove unsightly black algae stains from your roof, use a strong bleach or detergent solution, a stiff scrub brush, and a pressure washer set to under 500psi, then improve roof ventilation and drainage to prevent the stains from recurring.

Thankfully, black roof stains don’t have to ruin the look and longevity of your home. With some simple cleaning methods and preventative measures, you can outsmart the black stains and keep your roof looking like new.

What Causes Black Streaks on Roofs?

Black streaks on roofs are caused by a type of algae known as gloeocapsa magma. This algae grows in moist environments and feasts on the limestone filler used in some asphalt shingles. Professional roof pressure washing can help. The algae aren't harmful, but it is unsightly and can damage your roof over time. Here's why gloeocapsa magma grows on roofs:


Gloeocapsa magma thrives in damp environments. When moisture accumulates on your roof from rain, condensation, or excess shade, it creates the perfect conditions for algae to grow. Proper roof ventilation can help moisture evaporate before algae have a chance to form.

Limestone Filler

Some asphalt shingles contain limestone filler. This mineral provides an abundant food source for gloeocapsa algae spores carried by wind and rain. Over time, this leads to black streaks on limestone-based shingles.


Warmer climates provide ideal conditions for algae growth. Cool northern regions may see black streaks on poorly ventilated sections of the roof or shaded areas that don't dry quickly after rainfall.

What can I do about black on my roof

Dangers of Black Streaks on Roofs

Left unchecked, black algae streaks can cause a number of problems:

  • Roof damage - Algae retains moisture against shingles, causing deterioration of the asphalt over time.
  • Reduced curb appeal - Black streaking simply looks bad on an otherwise attractive roof. This impacts resale value.
  • Shortened roof life - By damaging shingles and accelerated wear, algae growth shortens the effective life of your roof.
  • Leaks - Severely damaged areas can begin to leak into the attic and ceiling.
  • Mold risk - Wet algae can support mold growth in damp attics or eaves.

Clearly, black algae streaks should be addressed as soon as they appear. The longer you wait, the more ingrained the algae becomes.

roof rot

What Can I Do About Black On My Roof?

There are several effective methods for removing algae from roof shingles. Here are a few DIY options:

Mix a Roof Cleaning Solution

An effective algae-removal mix can be created using:

  • 1 part bleach
  • 3 parts water
  • 1/8 cup detergent

Mix well and apply liberally to stained areas with a sprayer. Let it sit for 10-15 minutes before rinsing thoroughly. Avoid plant life.

Use a Pressure Washer

Adjust a pressure washer to no more than 500 psi to avoid damaging shingles. Spray algae stains using a 25° nozzle. This should lift most algae without impairment of granular surfaces.

Scrub with a Long-handled Brush

Use a stiff bristle brush with at least an 18-foot handle to allow roof access. Dip brush in cleaning solution. Scrub vigorously, but avoid abrasion of shingle surfaces. Rinse thoroughly.

Employ Roof Cleaning Professionals

For large or severe black streaks, hire a professional roof cleaning company. They have industrial equipment, high-power sprayers, and strong algaecide chemicals that can thoroughly clean a roof without damage. This option may also include algae-resistant treatments.

cleaning house with pressure washer

How to Prevent Black Algae Streaks

Once your roof is pristine again, take steps to prevent gloeocapsa algae from returning:

  • Improve ventilation - Ensure air circulates under the roof, and moist air can escape through vents.
  • Direct downspouts away - Keep roof drip edges clear and redirect rain runoff from pooling on or near the house.
  • Apply algaecide treatments - Roof coatings containing copper, zinc, or silver compounds deter algae growth.
  • Schedule semi-annual inspections - Detect algae early before staining becomes severe.
  • Plan professional cleaning every few years - Periodic cleaning prevents heavy buildup of difficult algae stains.

What can I do about black on my roof

Protect Your Roof Investment

Your roof represents a major investment that impacts the beauty and value of your home. Keep it looking great and extend its life expectancy by acting quickly when black algae stains appear. A little prevention also goes a long way towards stopping algae growth before it takes hold. With proper care, your roof will provide many years of uninterrupted curb appeal and weather protection.

About Tampa Bay Roof Cleaning

Tampa Bay Roof Cleaning specializes in removing ugly black streaks and stains caused by algae growth on residential and commercial roofs throughout the region. Their team uses gentle, eco-friendly cleaners and low-pressure rinsing to thoroughly clean rooftops without damaging shingles. The company also applies protective treatments to keep algae from recurring and ruining the look of your roof. Please call (813) 295-7443 for a FREE estimate.


Are black streaks on roof harmful?

Although black algae streaks on roofs are not toxic or directly harmful to health, the stains should not be ignored because they can slowly damage shingles, lead to leaks or rot, and reduce the lifespan of the roof if left untreated. Unchecked algae growth that causes visible black streaks on a roof indicates excess moisture, poor ventilation, and potential structural issues that will only worsen over time.

What kills black mold on roofs?

To kill black mold on roofs, make a solution of 1 part bleach to 3 parts water and spray liberally over moldy areas, allowing it to soak in for 10-15 minutes before rinsing - this will effectively kill and remove mold spores and discoloration. Another option is to use a chlorine-based mold remover made specifically for roofing materials that kill mold at the roots without damaging shingles or eroding their coatings.

How do I know if my roof is rotting?

Telltale signs your roof is rotting include soft, spongy sections of roof decking, new leaks or stains on interior ceilings and walls, visible mold or moisture beneath shingles, cracked or curling shingles, and sagging roof ridges or edges. If you notice any of these issues upon inspection, it likely indicates moisture damage and decay requiring prompt roof repair or replacement. You can also check for roof rot by probing any suspicious areas with a screwdriver, knife, or awl and looking for sections that seem abnormally soft and penetrable.

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