When a house is located in a moist and humid environment, the roofing system is often susceptible to strange types of growth. There is even a higher risk of this occurring if the house is built so that it does not receive enough sunlight due to surrounding tall buildings or tall trees as well as afternoon shade that blocks the sun from the roof. The development of these "growths" not only causes discoloration of the material; it also eventually destroys the shingles, resulting in water penetration and other kinds of major damage. This is actually a very common problem when algae, moss and lichens "grow" on the roofing structure.

Because of this, it is crucial to understand: their differences; their impact on the materials; their habitat preferences; and how to properly control them to prevent destruction of the underlying structure.

Differences Between Algae, Moss, and Lichen

Casual observers usually cannot tell the difference between moss and lichens that thrive on a roof top because they are difficult to differentiate. On the other hand, algae are very easy to detect because of its stain-colored appearance. Whatever the case may be, it is still imperative to understand the individual differences.

• Algae – This organism appears in various colors. On buildings, algae appear as black or green streaks and easily adapt to extreme weather conditions, spreading rapidly through airborne spores. Dew is the primary source of fluid for algae; however, it can flourish for a long period of time without any water at all. To prevent the growth of this organism, copper and zinc is widely used.

• Lichens – This entity commonly grows on asphalt or wood shingles. Described as green, spongy-type clumps, they have a good grip on the roofing material and grow in a random manner, usually flourishing in places where there is high humidity and extensive shade. The colors vary from silver to gray; gray to green; and yellow to orange, which forms into crust-like or leafy textures. Since lichens have a strong hold, if a roof is infested it will likely destroy the protective coating of asphalt shingles.

• Moss – This organism appears in color as white-yellowish or orange spots and thrives in places with high humidity and frequent rainfall. Moss may appear to be harmless; however, it should not be allowed to flourish since it can cause extensive damage to the roofing material. It harbors and retains water, which encourages penetration to the protective layer of the roof, resulting in structural leakage.

Preferred Habitat

These three types of organisms that could grow on a roof traditionally occur during cool weather conditions or during rainy and snowy seasons as well as in shady and moist areas. Since these living organisms produce no hazards to the health of humans, they are frequently actually allowed to flourish. It is a completely different case if they are living on the surface of a roof as they could cause severe damage to the protective layers. It is common for them to develop where there is less sunlight and could eventually cause a problem on the roofing materials and discomfort to the residents of the house.

Growth Control

There are several ways to regulate the growth of algae, moss, and lichens. Two of the most common methods are chemical or non-chemical control that is accomplished by cleaning the surface of the roof.

• Chemical – Chemical products, such as benzalkonium chloride, acetic acid, hydrochloric acid, oxygen, and chlorine bleach are used to clean and remove the infestation.

• Non-chemical – This type of cleaning is done by using a pressure washer, sharp knife, and/or a paving brush. It should be done regularly to prevent the recurrence of the infestation of the algae, moss and lichens. Pruning of any tall trees whose branches have grown over the structure is also an ideal way to expose the surface to direct sunlight.

The three most commonly identified germinating organisms on the roof are algae, moss and lichens. They have different colors, shapes, and characteristics in terms of growth and should definitely not be ignored because of their great impact on the structure. Adequate control can be obtained by cleaning and removal of the proliferation with either chemical or non-chemical solutions. So what is growing on a roof may not be good for that structure – do not ignore it!