Why Do I Have Black Stains on My Roof?


Roof with black moldThose unsightly dark streaks that look as if someone dripped black watercolor paint on your roof aren't grime, mold or mildew. They're aggressive, blue-green algae growth called Gloeocapsa Magma.


The spots appear black because the algae forms a protective shield against the sun’s damaging UV rays that result in the darker pigmented shades. They are the inevitable result of living in the Southeast, a region of the country that sees excessive discoloration on roofs due to high humidity.


If you have algae growth on your roof, it’s likely that your neighbors do as well. That’s because the algae spores are airborne, carried from one rooftop to another via wind and animals. Unless and until the algae is treated, the intrusive organisms will continue to thrive and take up residency throughout the neighborhood.


Asphalt shingles are a food source.

Although all roofs are vulnerable, asphalt shingles—made up of mainly asphalt, fiberglass and crushed limestone—are even more conducive to algae growth because the material, particularly the limestone, serves as a food source for the aquatic organisms. Couple the algae-friendly shingled surface with the warm, humid climate and you’ve got the ideal environment for algae proliferation.


Living in the Palmetto State comes with mostly year-round sunshine and warm temperatures, but the higher humidity creates this breeding ground for algae. Growth will typically begin on the north side of your home’s roof since that area is shaded throughout most the day. Moisture lingers longer in shaded areas, promoting algae growth. Since accumulated moisture sets the stage for algae growth, overhanging trees that block sunlight or gutters that drain onto the roof should be trimmed or cut down to hamper its development.


Have the stains professionally removed.

The black stains aren’t harmful or dangerous, but if left untreated, the organisms can eventually erode the protective UV granules of the roof shingles’ and weaken their effectiveness to protect your home.


A thorough cleaning is necessary to remove the black stains, but the method is best left to a professional, who can treat your roof safely and without damaging nearby landscaping and vegetation. You can tackle the job yourself, but it requires tools, chemicals and an abundance of caution. Using a power washer to remove the stains is not advisable. Power washing can affect roof warranties as well as damage the shingles’ protective coating.


To avoid climbing around on a slippery, algae-covered roof top, contact us at Joye Roofing. We can assess any roof damage, make repairs and create a plan to restore your roof.


Although it’s not possible to keep algae at bay forever once it’s removed, there are things you can do to maintain your roof and deter future algae development.


Be proactive!

Make it harder for algae to take up residency. Remove tree limbs, leaves and other debris on your rooftop. Make sure gutters and drain spouts are clear, and avoid allowing gutters to drain onto other parts of the roof, where water can pool.


Consider installing metal strips.

You may have noticed the absence of black stains below chimneys or around skylights. Metal is a natural inhibitor for algae growth. Consider having copper or zinc strips installed at the roof ridge, which would allow rain water to trickle down over the metal strips, depositing traces of metal over the surface to hamper algae growth.


Opt for metal roofing.

If your roof needs replacing, consider choosing metal roofing, which not only adds curb appeal, but is lightweight, energy-efficient, and resistant to fire, insects, mildew, rot and algae growth.


Don’t let those unattractive black stains on your roof diminish the look of your home, your prized masterpiece. Have your roof inspected, and if necessary, re-roofed so it looks more like a blank canvas rather than a botched watercolor painting.

Schedule your roof inspection


Derek Joye has been roofing homes and businesses in the Midlands for over 10 years.