Frequently Asked Questions
For customers who wish to know more about roof stains and what causes them, we've posted the following links
that we found to be very helpful. It's important to have reliable information because there is a lot of
misinformation about roof stains and how to treat them.
Read what 3M has to say about the stains on your roof, and how to prevent them in the
Read what Tim Carter from Ask The Builder has to say about your roof stains.
What 3M Has To Say:
If you are considering Algae resistant shingles in the future, 3M has some suggestions as well as some
information on your existing roof stains.
Click here to read the original article
Following are questions and answers that provide detailed, helpful information about algae staining on
asphalt shingle roofs.
What causes black streaks on asphalt roof shingles?
Unsightly black stains on shingles are often mistaken for fungus, mildew, dirt or oil, but are actually
caused by a hardy type of algae. This common problem has increased during the past 20 years, indiscriminately
settling on rooftops from coast to coast. Dark algae streaks are visible when algae cover the normally
vibrant colors of white, cobalt blue or hunter green roofing granules.
How do algae grow?
The primary type of algae found on rooftops - Gloeocapsa magma - thrives in humid environments, often
appearing first on the north slope of a roof where extra shade and moisture support its growth. Algae can
derive mineral nutrients from fillers such as calcium carbonate in asphalt shingles. Increased use of
moisture attracting calcium carbonate fillers cause asphalt shingles to retain moisture longer, thus
promoting algae growth.
Why are algae black?
The organism Gloeocapsa produces a dark-pigmented sheath to protect itself from ultra-violet rays. By the
time the black streaks are noticeable, the algae usually have been growing for months or years - the result
of generations of dead algae cells built up over time.
Are algae harmful to shingles?
There are no proven damaging effects of algae accumulation on shingles. However, the dark algae streaks are
the number one appearance-related problem reported to roofing contractors. Consumers commonly mistake the
dark stains for faulty shingles, leading them to complain to the contractor or manufacturer about a shingle's
How do algae stains spread?
When environmental conditions are right for algae development, the problem rapidly expands, covering an
entire roof. Once algae are present in an area, airborne spores are carried from rooftop to rooftop,
infesting whole neighborhoods. All roof types, including wood shakes, concrete and clay tile, and asphalt
shingles are susceptible to algae infestation.
What methods are available to clean an algae-infested roof if a homeowner isn't ready to replace the
Cleaning is the most commonly used homeowner defense against algae build-up on asphalt roofing shingles, but
at best, it's only a "quick fix." Typically, roofs must be cleaned frequently to minimize obvious staining.
Not only are repeated cleanings costly ($400 - $700), but some cleaning methods can damage a roof and shorten
the life of the shingles. Excessive scrubbing or power washing loosens the granules - and in essence, the
color. It also deteriorates the asphalt once the protective granules are lost due to sunlight, causing
further degradation to the shingle. Potent bleach solutions also can damage shrubs and bushes, and can be
caustic to the homeowner or roof cleaner.
What is the most effective asphalt shingle product available to inhibit algae growth and prevent the
Look for the Scotchgard™ Algae Resistant Roofing System logo on quality shingle brands. The
Scotchgard™ Algae Resistant Roofing System means protection. Many shingles feature algae resistance.
But your assurance of the optimal long-term protection from granule discoloration due to algae can only be
achieved when the precise level of granules with Scotchgard™ protection is uniformly distributed across
the surface of the shingle in combination with standard colored granules.
How can a homeowner be sure of the performance of an Algae Resistant (AR) product?
Consult with a quality professional roofing contractor about asphalt shingles with built-in Scotchgard™
protection. The Scotchgard™ Algae Resistant Roofing System logo on quality shingle brands is your
assurance of the optimal long-term protection from algae staining.
Do asphalt shingles with built-in Scotchgard™ protection cost more than regular asphalt
Generally yes, but many experts and homeowners agree that it's much less expensive to prevent algae growth
from the outset. The price difference is minimal when one faces the accumulated cost of frequent roof
cleanings. The extra cost associated with built-in Scotchgard™ protection refers to the materials only,
because the labor cost to install these shingles is the same as it is for regular shingles.
What Tim Carter from Ask The Builder has to say:
This article is from Ask The Builder.
Are you suffering from the stained roof syndrome ? It's that dark staining that looks like someone spilled
coffee up on your roof. This problem used to be associated with mildew in years past. Mildew is a by-product
of fungi. However, it has been determined that another organism has invaded millions of newer asphalt shingle
roofs around the USA. This organism is an algae, Gloeocapsa magma. It has experienced an explosive
growth curve during the past 15 to 20 years quite by accident.
A large number of homeowners who are experiencing these roof stain problems are doing so for the first time.
Even though they live in areas where the climate has not changed, the problem appears out of the blue.
In years past, the algae simply didn't have a food source. This changed with the introduction of fiberglass
shingles during the past 20 years. The fiberglass shingles made today contain massive amounts of food that
the algae loves to eat -that food being limestone.
How in the world does limestone get into fiberglass shingles you might ask? It's simple. The limestone is
added to the asphalt mixture as a filler or thickener. Years ago, this was not necessary, as an organic felt
mat was used as the base for the shingles. This mat was saturated with asphalt and then coated with granules.
Because of the absorptive qualities of the mat, it was able to soak up enough asphalt to give the finished
shingle sufficient weight. The materials used for the mat were derived from the cotton rag scrap market.
Well, about 20 years ago this rag market dried up because of the widespread introduction of synthetic
textiles such as nylon and polyester. The shingle manufacturers scrambled and decided to use fiberglass as a
However, a problem quickly arose. The thin fiberglass mat didn't absorb asphalt like the old organic mat.
Something had to be added to the asphalt to increase the weight of the shingle. BINGO! They decided to use
limestone which is plentiful and cheap. Well, you know the rest of the story.
Flashings Solve the Problem
As people complained to the shingle manufacturers about this problem, they began to notice that on certain
portions of some roofs the algae wouldn't grow. Commonly these areas were just downslope of a chimney or a
plumbing vent stack. It didn't take too long for them to figure out that just above these areas were zinc or
copper metal roof flashings.
Apparently, each time it rains, small amounts of copper and zinc wash down the roof in theses areas. The
minute amounts of these metals seem to be enough to poison the algae.
Well, the shingle manufacturers called up the Industrial Mineral Products Division of the 3M Corporation. You
see, these are the guys that supply the shingle manufacturers with all those nice colored granules.
The 3M people figured out a way to coat certain granules with copper. These granules are then covered with
the same color as the rest of the granules. Anyway, the copper actually leaks through the coloring and makes
the shingle unsavory for the algae.
These shingles carry a lifetime warranty from many of the shingle manufacturers, so that you don’t have
to worry as to how long the copper will last.
Newer, Existing Roofs
Millions of people have roofs that are stained. But, they are not yet ready to replace all their shingles. So
what should they (you) do? It's easy! You just need to clean your roof and introduce some copper as high as
possible on your roof.
Cleaning a shingle roof that has this algae infestation is not that hard. It is DANGEROUS work, but not hard.
The trick is to apply standard treated wood deck cleaners to your roof and follow the instructions.
However, you shouldn't use just any deck cleaner. You must use a deck cleaning product that contains a safe
cleaning agent such as oxygen bleach. Sodium percarbonate does the same job as chlorine bleach.
However, chlorine bleach is harmful to you, your roof, your gutters and downspouts, and any plants it might
come in contact with. Want more information as to how to get oxygen
bleach? Just click the link!
If you decide to clean your own roof, BE CAREFUL. Wet roofs are slippery. These deck cleaners are slippery.
You can easily fall off your roof and either die or become permanently crippled. I'm serious. So, if you
still want to do this, and not hire a professional, be sure to wear an OSHA-approved safety harness. Also, do
not direct the hose spray UP the roof! Water can get under shingles and leak into your house. Always spray
down the roof.
26 Jan 2010