Moss can severely damage, even destroy a roof by breaking down the fibers in the shingles, forcing them apart and allowing leaks to occur. Wet and cold conditions are conducive to the growth of moss. that is why many home owners find moss growing on their roof in the spring after a long cold winter.
Moss retains water on the surface of an asphalt roof. It keeps the roof wet for a longer period of time. A wet roof encourages the growth of mildew and mold. The mildew and mold develop underneath the shingles, that damage is likely to extend inward. A musty smell eventually develops in the house when mildew and mold are extensive. also, the constantly wet asphalt roof, where the moss is, will likely rot because it simply can not dry out. The moss basically acts like a sponge, and never really dries up. this is very detrimental to any roof.
Quite often as the moss grows, it will weaken the sealing on the shingles. The moss pushes up the asphalt shingles, a condition which causes dislodging and misalignment. Since moss fragments asphalt shingles, the roof is unlikely to last long. If you have a lot of moss on your roof, when it rains, the water will tend to run off sideways, causing leaking underneath the shingles.
Moss often grows more rapidly or abundantly where there is a lot of shade. This would probably be either on the North face of your home, or possibly a roof area that might be blocked from the home by a second level to your home. This is why I often find moss issues on garage roof that has a roof shaded by the structure of the main home. Moss is a very common problem I find on customers roofs. This moss is most effectively eradicated by using a moss killing solution that will also loosen it from the shingles. This process takes about 20 minutes once the solution is applied. The next step is to use a hot water (200 degrees) hard rinse to remove the moss from the roofs surface. After this step is completed, your roof should be completely functional again, and look 100% better than it did just hours before! Remember, a strong and secure roof adds value to any home. It is one of the strongest selling points of a house. Unchecked growth of moss will negatively affect the value of your home.