Algae Growth on Roofs by DeanSampson



Algae Growth on Roofs1
Virginia Peart2

     When light colored roofs turn green, brown or                              fiberglass roofing. Very high pressure sprays can do
black during warm, humid weather, people seek help.                             the same thing. Additionally, chlorine bleach can
What is this stuff? How can they get rid of it? How                             damage plantings, vinyl windows and other
can they keep it away?                                                          construction materials if used in a solution too strong.
                                                                                The force of a spray from a garden hose may not be
     First, what is this discoloration that appears on                          strong enough to remove an accumulation of dirt on a
houses, especially roofs? Algae spores will grow                                roof. You may need to check the warranty that came
wherever they find minerals, moisture, warmth and                               with your roof before cleaning. Some roofing
light. It is a plant and it likes the summer climate in                         warranties become invalid if you walk on them.
Southern Florida and other southern coastal areas. It                           All-in-all the very difficult job of cleaning algae from
does not grow directly on any specific roofing material                         a roof may be better turned over to a professional.
such as asphalt, concrete or clay tiles, fiberglass or
aluminum, but as soon as light accumulation of dust                                  In choosing a professional cleaning service you
and other organic materials settle on a roof and enough                         might want to ask what precautions are taken to
heat, moisture and a little light are available, algae type                     protect plantings and house materials. A good
spores, ever present in Florida, make the roof their                            procedure would be to wet down the roof and
home and start to grow.                                                         plantings with plain water before using a cleaning
                                                                                solution. Next use a cleaning solution with a safe
Can You Fight Back to Remove Algae                                              concentration of chlorine and moderate pressure. (One
         Growth on Roofs?                                                       part chlorine to 10 parts water. Spray pressure as high
                                                                                as 1,000 psi may be too strong.) Finally, rinse the
     Yes, you can. Chlorine bleach kills and the force                          roof, plantings with clear water. Does the service
of a scrub brush or a strong spray will remove the soil                         check the condition of the roof before and after
and stop the growth — at least for a while. But read on.                        cleaning? Talk to others in your neighborhood who
There are precautions. Working on a roof can be                                 have used a cleaning service and talk to several
dangerous. It is not good for your roof to walk on it.                          services before you choose one.
Using a brush can remove the fine granules that reflect
the sun and protect the surface of asphalt and

1. This document is FCS 3242, one of a series of the Department of Family, Youth and Community Sciences, Florida Cooperative Extension Service, Institute
   of Food and Agricultural Sciences, University of Florida. Publication date: October 2001. First published: August 1992. Revised: June 1995. Reviewed:
   October 2001. Please visit the EDIS web site at
2. Written by Virginia Peart, former associate professor, Housing and reviewed by Nayda I. Torres, professor, Family and Consumer Economics, Department
   of Family, Youth and Community Sciences, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville,

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin.
For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension
Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.
Algae Growth on Roofs                                                                  2

    For cleaning small area that you can easily reach     Is Your House a Sick House
here are some suggestions:

  • Wear rubber soled shoes that are both skid
    resistant and secure.

  • Rinse roof and plantings with clear water before
    and after using a cleaning solution.

  • Use a mild chlorine solution (1 part chlorine to
    10 parts water). Remember that chlorine is
    caustic. Protect yourself when using chlorine by
    wearing rubber gloves and avoiding spatters.

  • Use a safe ladder placed so it is secure at both
    the top and bottom.

  • Heavy accumulation of dirt and algae may
    require scrubbing with a brush but this may
    damage asphalt roofing.

  Can You Keep Algae Problems on
    Your Roof From Returning?
     Unfortunately, whenever the conditions are right
(minerals and organic material for algae, moisture,
warmth, and a little light), algae type growth will
return and treatment must be repeated. Yearly
treatment may suffice in most areas. New roofing
materials resistant to algae growth are now available.
They will retard algae discoloration for longer periods
of time.

    Additional Mildew Publications
      How to Prevent and Remove Mildew

      Say Goodbye to Mildew and Save Energy

      Home Study Course: Say Goodbye to Mildew
      and Save Energy

      Welcome? Mildew Months

      The Return of Mildew

      Chemical Mildew Inhibitors

      Mildew on House Paints

      Algae Growth on Roofs

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