Yes moss is a serious problem for a composition roof. The effects of moss growth on your roof can be devastating even after a relatively short period of time. Moss can be an attractive and welcome growth in some areas of the home or garden, depending on a persons taste. I happen to feel that moss provides a sort of rustic ambience in the right place and setting. Sometimes when a homeowner wants moss cleaned off of a brick wall in a garden for instance it can seem like a real shame. However deciding to allow moss to grow on your composition (asphalt) roof unhindered, whether you enjoy the way it looks or not, is a bad idea and can be an expensive mistake.

Moss will actually attack and ruin a composition roof (this aspect is discussed in more depth below). Also moss can dam up water which will then run sideways under your shingles causing a leak. Another problem observed is that sometimes moss grows underneath the bottom edge of the shingles and can push it up breaking the seal and making your roof more prone to wind damage. Moss also soaks up water and keeps your roof wet. When most things stay wet for a long time they break down, rot, grow mold and mildew and like wise your roof suffers the same fate when it is not able to dry out.

Moss Damaged Shingles

This roof valley was covered in moss and as you can see the impact on the shingles was extremely adverse. The root like structures (rhizomes) which the moss sends out to anchor itself and to supply nutrients for itself dislodge the protective granules from the surface of the shingles leaving your roof exposed to the elements as well as exposed to further undefended attack by more moss. I have observed roofs that were leaking for no other reason than the simple fact that moss had eaten so far into the shingle that water was able to seep through in the damaged areas. In more extreme cases the moss had actually eaten clear through the shingles. Moss erodes shingles quickly and especially in the critical areas of the roof such as valleys (shown here)  that is very bad news. Removing moss quickly before it overtakes your roof is vital to its longevity. Better yet though is preventing moss from growing on your roof at all. See this discussed here: Prevention is the Key