Algae Discolorations of Roofs

32 • Interface January 2003
For many years, roof discoloration caused by algae has
been observed throughout the United States and Canada.
Although this is commonly referred to as “fungus,” in actuality,
it is “algae” growth. The discoloration usually has a brown
to black appearance and may be mistaken for soot, dirt, moss, or
tree droppings. The long and short of it is that this discoloration
is an aesthetic irritant to many people because of its unsightly
appearance.
The Asphalt Roofing Manufacturers Association’s (ARMA)
member companies, which manufacture residential asphalt shingles,
continue to receive frequent inquiries from homeowners,
contractors, remodelers, and others with questions and concerns
about algae discoloration of roofs. In response to these inquiries,
the Residential Roofing Committee, comprised of ARMA residential
roofing member company representatives, prioritized
development and publication of the information contained in
this article. The recommendations contained herein are based on
and excerpted from the ARMA Technical Bulletin, “Algae
Discoloration of Roofs,” readily available by request to the
ARMA.
The primary species of algae being observed is Gloeocapsa
Magma. This type of algae is contained in and transported
through the air and tends to collect and grow on roofing surfaces.
Natural pigments produced by these algae may cause a
white or light-colored roof to gradually turn dark brown or
black. The algae discolorations should not be confused with
moss or tree droppings, which typically produce only localized
discolorations.
Although roof discoloration is widespread in coastal areas, it
is not confined to these areas. It is commonly found in the Gulf
states of the U.S., along the Northwest and Eastern seaboards,
and in the Midwest.
Algae growth occurs to varying degrees in all regions of the
country, especially those subjected to warm, humid conditions. It
should be noted that almost all types of roofing systems are susceptible
to algae discoloration. It is, of course, most readily visible
upon white or pastel roofs, while it is not so visible upon
darker shades of roofing.
A preventive measure to algae roof discoloration is the use of
commercially available algae resistant asphalt shingles. Copper,
which is well known for preventing algae grown in marine applications,
is being incorporated into the granules of asphalt shingles.
Appropriate levels of copper also effectively block the
growth of algae on roofs. Most asphalt roofing manufacturers
provide shingles containing copper as an option to their residential
product offering.
Algae discolorations are difficult to remove from roofing surfaces
but may be lightened by applying a solution of chlorine
bleach, trisodium phosphate, and water. Solutions for these
ingredients may vary between shingle manufacturers and depend
on the amount of discoloration. Solutions range from one cup
TSP, one gallon bleach, and five gallons of water, to one cup
TSP and 2.5 gallons each of bleach and water.
To apply these solutions is a relatively easy procedure. First,
gently disperse the solution on the roof surface. Use normal precautions
for handling bleach. Be sure to apply it carefully to
avoid damage to other parts of the building and its surrounding
landscape. Avoid scrubbing the surface, as this friction may
loosen and remove granules, shortening the life of the shingle. If
possible, always work from a ladder and/or walkboards to avoid
direct contact with the roof surface. Observe all possible safety
precautions when working on or near the roof. Finally, rinse the
solution from the roof by gently spraying the surface with water.
Be warned that this solution application and rinse process will
make the roof slippery and potentially hazardous to walk on during
treatment.
The effectiveness of such cleaning techniques is only temporary,
and discoloration will likely recur. As mentioned earlier,
several types of algae resistant roofing products have been developed
and are now commercially available. These asphalt roofing
products are specifically designed to inhibit algae growth for
extended periods of time.
An important note of caution is that high pressure washing
systems for algae removal should not be used.
These recommendations were prepared by and have the
approval of the ARMA for informational purposes only. They are
not intended to revoke or change the requirements or specifications
of the individual roofing material manufacturers or local,
state, and federal building officials. Any questions or specifications
of a manufacturer should be directed to the roofing manufacturer
concerned.
For additional information, contact the roofing materials
manufacturer or ARMA at 301-348-2002 or visit ARMA’s website
at www.asphaltroofing.org. 
By ARMA