Algae and moss growth on roofs is an unsightly nuisance. It is important to learn what moss and algae are, how they grow, and how they spread. You want your roof to look nice so it makes a good impression of your house. There are many other reasons why algae and moss build-ups are not a good reason to leave un-noticed. Two examples of what will be talked about in this would be roof damage or shingle blow-off.             

                To start off, you should be familiar in what moss and algae are. The black discoloration and streaks that are visible on many roofs across the US and Canada are actually algae. Algae tends to form where moisture dwells on roof surfaces. The most common type of algae is gloecapsa magma which is also known as blue green algae. It protects from damaging ultraviolet rays from the sun by producing a dark pigmented sheath on the roof plane. The algae had likely been present on the roof for several months or even longer by time it’s visible.

                Moss is a non-vascular plant. It obtains its water through its leaves unlike most plants that gather their water from their root system. Therefore, moss must have a moist environment to survive. Moss tends to grow on north facing roof planes that stay damp longer than south facing planes and that receive less direct sunlight. Overhanging trees act like food sources for moss and they provide additional shade and drop debris onto the roof that further holds in moisture.

                Algae and moss spores are spread by animals or carried by wind. They can quickly spread from rooftops to another. That is why it is common to see algae growth on rooftops throughout affected houses in a neighborhood. There is no scientific evidence that algae is damaging to asphalt shingles but it does affect the aesthetics of a roof. On highly reflective roofs which are also known as cool roofs, algae will have a negative impact on the long-term effectiveness of the roof systems. Moss can cause the leading edges of shingles to lift or curl, which increases the risk of shingle blow-offs on roofs during wind events. In severe cases, moss build-up could cause leaks or could cause lateral water movement resulting in moisture damage to the roof deck. This is why it is important to address moss and algae problems as soon as possible.

                Here are some steps that can be taken to discourage moss and algae growth:

-Trimming tree branches to allow more sunlight to reach the roof and minimize the debris accumulation. It will provide an environment less inviting for such growth.

-If debris begin to accumulate on the roof, it should be cleared regularly with a leaf blower or a similar non-abrasive method as part of a regular maintenance program.

-Air flow should be directed down the slope of the roof to avoid driving debris under the edges of the shingles.

-Keep gutters clean to help promote proper water drainage. Don’t allow gutters from an upper roof to drain directly on a lower roof. Instead, extend the downspout from the upper roof into the lower gutter.

                There are other preventative measures such as adding zinc or copper strips to a new roof to prevent both algae and moss (it is not recommended to add them to an existing roof because this will require the strips to be put on with exposed nails and this could lead to leaks over time or breaking the sealant bond which may eventually result in wind damage). You could also apply asphalt shingles that incorporate algae resistant copper granules. Currently, there are asphalt shingles available that use algae resistance technology.