Homeowners who are just now getting a first glimpse of their rooftops this winter may notice some dark streaks or discoloration on their shingles. This could be algae growth or moss may have settled in and begun to grow on the roof. While both algae and moss are known to thrive in moist environments, they are not confined to any one region of the country.
In understanding why algae and moss grow on rooftops, it is important first to learn what each is, how the two differ and what causes them to spread. Although some similarities exist, algae and moss are not the same and require slightly different methods to prevent or to remove.
Often mistaken as mold or mildew, algae can form where moisture tends to dwell on roof surfaces. The most conducive conditions for algae growth are coastal or humid climates. While there are many types of algae, the most common type, gloeocapsa magma or blue green algae, protects itself from damaging ultraviolet rays by producing a dark-pigmented sheath. The sheath is what causes the discoloration. Algae discoloration typically begins as small dark spots that can transform into vertical streaks on the roof surface. By the time it's visible, algae likely has been present for several months or longer.
Algae spores can be carried by the wind or animals. If it's on one house in the neighborhood, it will probably appear on others. While there is no scientific evidence that algae is damaging to asphalt shingles, it certainly can affect the aesthetics of a roof. And in the case of a highly-reflective or "cool" roof, algae can impact the effectiveness of the reflective performance of the shingles.
Moss is a non-vascular plant that obtains water through its leaves, unlike most plants that gather water through their root systems. Moss must have a constant moist environment to survive. In North America, it tends to grow on north facing roof planes that receive less direct sunlight and stay damp longer than south facing planes. Overhanging tree branches provide additional shade and drop debris on the roof that further holds in moisture and acts as a food source for moss.
Unlike algae, moss can be harmful to asphalt shingle performance. Moss growth can cause the leading edges of the shingles to lift or curl, which can increase the risk of shingle blow-off during wind events. Moss build-up can also act as a damming material and cause lateral water movement, resulting in moisture damage to the roof deck or leaks in the system.
Today, there is a relatively easy process homeowners can take to remove algae from their rooftops, but it should be noted that it will return. Before starting any job, the Asphalt Roofing Manufacturers Association (ARMA) recommends that safety precautions be adhered to and a professional roofing contractor be contacted to perform a full roofing assessment.
The most effective cleaning method is with a 50:50 mix of laundry-strength liquid chlorine bleach and water. Apply the mixture with a low-pressure sprayer and allow the solution to settle on the roof surface for 15 to 20 minutes, before rinsing thoroughly with low-pressure water. Extended dwell times may be necessary, but avoid letting the solution dry completely as this may prevent complete rinsing. Take proper precautions to protect landscaping and surrounding areas from the chlorine bleach solution. Always use appropriate personal protective gear when working with chlorine bleach. The algae stains will disappear and wash away with subsequent rains, but could recur in the future.
Moss will loosen over time and may be removed with a leaf blower; however, be careful not to break the seal of the shingle. This could affect its performance. Keep in mind, it may take more than one bleach treatment to kill all of the moss. Never use a pressure washer to clean an asphalt shingle roof as this could cause granule loss and very likely premature failure of the roof system. It is advisable to be aware that improperly cleaning your roof could result in voiding the manufacturer's product warranty.
Some ways homeowners can help prevent moss and algae from growing include:
- Trimming tree branches back to allow more sunlight to reach the roof and minimize debris accumulation will provide an environment less inviting for such growth
- Debris should be cleared regularly with a leaf blower or similar, non-abrasive method as part of a regular maintenance program
- Air flow should be directed down the slope to avoid driving debris under the edges of the shingles, or breaking the sealant bond
- Keep gutters clean to promote proper water drainage. Do not allow gutters from an upper roof to drain directly on a lower roof; extend the downspout from the upper roof into the lower gutter
- Purchasing an algae resistant shingle; these shingles offer a warranted period of algae growth protection. Check shingle warranty for details and requirements.