Fungus on a Roof
Stains and growth on a roof are often due to fungus or algae discoloration. I use the term fungus as a generic reference to all types of growth, including mildew, algae, moss and lichens.
Fungus grows on a roof when there is a food supply (dirt), moisture, and moderate temperatures. A lack of direct sunlight, mostly damp conditions, and debris on the roof also promote growth.
Once fungus develops, the areas under and around it stay damp, which promotes further growth.
Flashing can stop fungus
Most fungus can be eliminated by installing metal flashing on the roof. Oxidation, a form of metal corrosion, occurs as water runs over the flashing. As an example, leaching of the metal strips prevents fungus from developing around and below this roof dormer.
On this hip roof , light stripes appear below the roof vents and the metal base of the antenna. Again, metal oxidation runs down the roof and prevents growth. The dark stains around the lighter streaks are probably due to black algae or dead algae. This algae often moves from roof to roof in a neighborhood when the wind blows.
Zinc strips also stop growth
Zinc strips placed high on the roof can help prevent fungus on shingles below. The zinc reacts with water and oxygen to form zinc oxide that prevents fungus growth. Strips made of copper or galvanized metal would also work.
Shingle manufacturers now offer asphalt shingles that have granules with a metal component to inhibit growth.
What you should do
Make notes and inform your customer of visible growth on the roof. Cases of dark stains with no visible “clumps” of growth usually won’t affect the life of the shingle, but your customers may not be happy with the roof’s appearance once they close on the home. Clumps of growth or debris need further evaluation, as this condition can damage asphalt, wood and other types of shingles.
Routine maintenance should include removal of debris on a roof. You could recommend cleaning and maintenance by a professional. I would never suggest that a homeowner try to clean a roof, though. I see far too many do-it-yourself articles on the internet about cleaning a roof with water, bleach and detergent. All we need is a customer climbing on a pitched wood or asphalt shingle roof covered with slippery cleaning solution. Brushing or pressure-washing a roof can also risk safety and roof damage. Leave all types of roof treatment to professionals.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com (htoyh.com), he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.
To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters and local groups. Tom can also provide his knowledge for your educational event; contact him at Tom@htoyh.com.