Black Streaks on Your Roof?

Black Streaks on Your Roof?

August 26, 2014 1:34 pm

Is your roof starting to look a bit drab and unsightly because of black streaks?  Well, there is a solution and it doesn’t require a whole new roof.

The black streaks are actually the result of algae growth and will typically be found on north facing roof areas or sections that do not receive a lot of sun.  The algae spores thrive on organic materials, such as limestone granules, found in the material that makes up your roof shingles.

Generally speaking, the overall consensus is the black streaks do not diminish the integrity and functionality of the roof covering material.  This is unlike moss growth where, over time, the continued and unchecked growth of moss can affect the performance of your roof shingles.

There are a variety of both homemade solutions and commercially available products that the serious DIYer can use to battle the problem.  Alternatively, you can engage the services of a professional to do the job for you.

In any case, you will need to either walk the roof or be able to reach the area with a sprayer from a ladder.  Therefore, it is imperative you are comfortable with working from a ladder or accessing your roof in a safe manner.

Many of the homemade solutions use bleach and can be toxic to plantings around the perimeter of your home.  Some of the commercially available products claim to be environmentally friendly and less toxic.

To Start

  • Always use safety goggles and gloves.
  • If you are uncomfortable working from a ladder then do not even attempt this task
  • Follow all manufacturer’s recommendations and instructions
  • Wear old clothes
  • Follow all safety precautions for working at elevated heights and/or ladders.
  • It’s best to clean your roof on a cloudy day to prevent the cleaner from evaporating too quickly.
  • If you know the manufacturer of your shingles, contact them or check their website for specific recommendations on cleaning.
  • When using a product made for cleaning roofs or oxygen bleach, follow the directions on the container.
  • DO NOT power wash the roof area, you will likely damage or shorten the life of the shingles.

The Recipe

Recipes for cleaning solutions containing bleach vary, but the basic formula for roofing applications is:

  • 1 quart bleach (6% sodium hypochlorite)
  • 1 gallon water
  • 1/4 cup TSP (trisodium phosphate) or other heavy duty cleaner (DO NOT use a cleaner that contains ammonia or is not recommended for mixing with bleach, as it can result in poisonous chlorine gas)

The Process

Pour the cleaning solution into a common garden variety pump sprayer and saturate the affected areas with the solution.  Wait about 15-25 minutes to allow the solution to work, then rinse with water.  You should also consider rinsing any plantings that may be affected by the solution prior to and following the roof application/rinse.

There are also commercially available products, such as Spray and Forget, which appear to be easier to apply.  However, these products may take a little longer to work and remove the stains.


To prevent stains in the future, you can install a copper of galvanized steel strip of metal just below the peak of the roof, with approximately 2 to 4 inches of the material exposed.  The copper and zinc are toxic to the algae and will kill the spores thereby preventing continued growth.  Each time it rains, the water flows over these metal strips and releases tiny particles of metal over the roof area, hence preventing algae, moss or mildew growth.

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