An algae infestation is a common problem to those homeowners who live in a humid area and have asphalt shingle roofing; however, this growth can also grow on tile and on wood shakes. Regardless of the type of material installed on the housetop, algae will jeopardize the integrity of the roofing system.

There are several different opinions regarding the removal of algae. According to some contractors, such type of removal can cause a deterioration of materials resulting in a loss of function. This opinion is not completely accurate because asphalt shingles can be cleaned without damaging the roof.

The Culprit

‘Gloeocapsa magma’ is the technical term for algae that is responsible for black stains and streaks. This growth is a primary problem with asphalt shingles. These plant-like organisms accumulate, grow and feed on moisture and calcium carbonate from fiberglass shingles. Basically, they spread rapidly because the spores are easily blown off by the wind as well as a result of rising humidity.

Furthermore, when the older of these complex-celled organisms decompose, they harbor moisture and allow the bacteria to multiply. Infestation of algae, if left untreated, will create rotting, loss of granules and premature break-down of the roof. Considering that the spores are blown by the wind, it is essential to take preventive measures and perform an annual inspection to avoid the colony from re-growing.

The Solution

The good thing about algae is that it can be cleaned using different methods. The following should be done to address this unwanted formation:

Low-Pressure Washing - The power of high-pressure is good but it has been discouraged because it can damage the integrity of the roofing material. The National Roofing Contractors Association (NRCA) and Asphalt Roofing Manufacturers Association (ARMA) state that high-pressure washing should not be used to remove algae because the granules that protect the shingles from ultraviolet rays will be jeopardized. Consequently, low-pressure washing is an acceptable solution to avoid damaging the shingles.

Chlorine Bleach Solution - The cheapest cleaning method is the application of chlorine bleach solution to the shingles. A homeowner can DIY this plan. Mix about 13 ounces of chlorine bleach to a gallon of water and spray gently using a garden sprayer on the affected areas. After spraying, let it sit for five minutes then rinse it off to avoid further damage. This is only effective if it is applied to a dry roof. The negative effect of using this solution is that it can decrease the elasticity of the shingles resulting in rigidity, brittleness and curling. In addition, a chlorine bleach application is hazardous to the environment, so vegetation on the ground must be covered with plastic.

Oxygen Bleach Solution - Oxygenated bleach powder is non-toxic and the stain removal capabilities can be activated by mixing it with water. The mixture can be soaked longer than chlorine bleach; the solution can sit for about 30 minutes and then be brushed in a downward motion to remove the algae. Always make sure to wear personal protective equipment during the cleaning procedure because any roof is slippery when wet.

Introducing Copper - These complex-celled organisms do not like metal. To prevent it from growing, a homeowner can cut 5 or 6 inch wide strips of copper or zinc and place them under the ridge of the roof. When it rains, copper carbonate will be washed down preventing the algae from growing.

This growth can cause dark streaks on any housetop material. Those who live in areas with high humidity are vulnerable to this manifestation. The best way to effectively remove an algae infestation is by utilizing different cleaning methods, always being cautious when cleaning as it may destroy the integrity of the roofing material. Copper and zinc offer a sensible solution to combating this colonization but will add to expenses. When deciding what to do for algae growth, always remember that it can be cleaned by using different solutions - or call the nearest roofing contractor to do the job. Don’t let algae invade and destroy any roof!