When the roof on the exterior of your home is dirty, or the paint is peeling, it makes your entire house look dingy.

As homeowners, we know we’re expected to keep our homes looking nice — no one wants to come home to a mess, and no one wants to own the house that the neighbors grumble about whenever they drive past. That includes your roofing.

Not only can a dirty shingles ruin your home’s curb appeal, but it can actually cause damage. 

More than Dirt
Your house is exposed to the elements all day (this includes what is at the very top), every day. Dirt and debris will collect on it. Natural rainfall is typically enough to wash off the appearance of most dirt. However, the dark stains and green areas that appear on the shingles are caused by other natural elements: 

Algae
Fungus/Lichen
Moss
These substances are much trickier than simple dirt and require some explanation.

Algae
Angie’s List explains that the black stains are called gloeocapsa magma or GM. GM algae is a bacterium that feeds off of both asphalt and limestone roofing in moist, cool and shaded areas of the roof. When there is minimal light, GM forms a blue-green color. The more shaded areas of the roof are more likely to have these algae, which can harbor various types of molds. This explains why some composite shingles are stained only on one side.

A report by the Asphalt Roofing Manufacturing Association (ARMA) notes that these algae spores are transported by wind or animals. They can quickly spread from rooftop to rooftop in neighborhoods of single family homes, townhomes and apartment complexes.

Lichens
Lichens are basically the combination of algae and fungus. ARMA describes lichens as fungus that grow symbiotically with algae. These plant-like organisms form a branching growth of “root-like tentacles that penetrate deep into the shingle’s organic, oil base for nourishment.” Lichens are tough to remove from the structure and can reappear from humidity and rain.

Unlike the algae and lichens, moss is an actual plant. Moss has a shallow root system and requires a lot of moisture to survive. Areas with less sunlight remain damp longer. Overhanging tree branches not only provide more shade, but they also drop debris that serves as a food source for moss. Like algae, moss spores are often transferred by wind or animals and can easily spread throughout neighborhoods.

How Can Stains Damage a Roof?

The stains and growth patches caused by algae, fungus and moss can damage your roof through shingle deterioration and wood rot. Both lead to expensive repairs and can shorten the life of your roof. A roof cleaning can help minimize or eliminate this damage.

Shingle Damage

As mentioned above, GM algae is a bacterium that feeds off of both asphalt and limestone. That means that these organisms are literally eating away at your shingles. Furthermore, according to ARMA, roofs with algae stains absorb heat. This results in an extremely high heat in the attic, which can reduce the lifespan of the shingles by essentially baking them from the inside out.

Lichen colonies can be quite damaging to asphalt. The penetrating tentacles create pock marks where they grow and eventually ruin the shingles.

Moss is even more detrimental to shingles. Moss can cause the edges of the shingles to lift or curl—often the first sign that a roof replacement is needed. And because moss holds in moisture, it can freeze in the winter, causing even more shingle damage.