Can Algae Ruin a Roof?

Location

• Algae grow on all types of roof surfaces, including wood, metal, asphalt, and clay or concrete tiles. Although algae can be a problem anywhere, homeowners who live in humid areas that receive a lot of rainfall have more problems with algae and moss growing on their roofs. North-facing areas have more growth because they don’t receive as much sun and those parts of the roof stay wet longer. In addition, roof areas located underneath large trees have more problems due to shade.

Gloeocapsa Magma

• The most common algae that attack roofs are a black algae called Gloeocapsa magma. This alga’s dark pigment protects it from the sun’s ultraviolet rays. It’s harder to remove than green algae because of its ability to secrete a material that allows it to grip tightly to fiberglass shingles. Gloeocapsa magma algae produce their food from photosynthesis. In addition, they absorb the nitrates they need from nitrogen in the air and take in other nutrients from the dirt and dust around them. Algae spores are spread from roof to roof by the wind.

Shingle Construction

• Shingles made 20 years ago or more repelled algal growth because the oil in the bottom layers of asphalt leached to the surface of the shingles. The fiberglass in modern shingles doesn’t let oils pass through to the surface. In addition, the top layer of asphalt is very thin, and the oils it contains are quickly lost to evaporation and weathering, leaving the shingle susceptible to algae.

Bleach

• The most common method for killing algae is with bleach. Mix 1 gallon of liquid bleach with a gallon of water then apply the solution to your roof with a garden sprayer. You’ll need between 1 and 1-1/2 gallons of solution per 100 square feet of roof area. Let the solution sit for about 15 minutes then rinse it off with a garden hose. Note that bleach may not be the best choice for asphalt roofs because the high sodium content in the bleach can damage the asphalt. Protect nearby plants by spraying them with water before treating your roof with bleach.

Power Washing

• Some types of algae cannot be removed with bleach. Using a high-pressure washer is an effective way to remove them. Power washers remove the top layer of wood fibers from the shingles, and they can damage your shingles if you use a high setting. According to Brian Buchanen, of the Texas Forest Service, a pressure of 1,000 to 1,5000 psi is usually sufficient to remove algae.

Sealers

• Sealing products are available to use on clay and concrete tile roofs that have algae growth. However, sealants only provide a temporary solution and may need to be applied again within six to 18 months. The chemicals used in sealants can also cause the roof to age prematurely.

Metal Inhibitors

• Algae shy away from metal. For this reason, installing copper or zinc strips on your roof can help prevent algae, especially when it rains by bathing your roof in metals. While these strips are effective in controlling algae and only release minimal amounts of metal at any one time, they can cause a pollution problem over time. According to the Cornell University Cooperative Extension, copper strips work better than zinc for a longer period of time.