Not only do I call it out during home inspections, I see everywhere while walking or driving around town–the slow, silent, and steady expansion of moss patches on roofs.
The problem
Moss build up damages asphalt shingles and shortens the life of your roof.
Zinc Strips and Moss Killers
One popular way to prevent moss is to use zinc or copper strips. As the metal slowly leeches from the strips, it forms compounds toxic to the moss. However, as with most compounds that are toxic to one living organism, these compounds also negatively impact other organisms as they make their way through soil and water systems. For example, zinc compounds are known to harm aquatic wildlife. The same goes even more so for commercial compounds such as the ones listed here.
Pressure Washing
Just don’t do it. The granules on the shingles protect the asphalt part of the shingle from UV damage. Pressure washing removes the granules. This can damage your roof, possibly more than the moss!
The Green Solution
You can help prevent moss growth by trimming trees to give your roof as much sun as possible. Take a look some roofs while you drive around town. The effect of shade on moss growth is dramatic. North-facing sections are especially prone to moss. If you can reduce shade, you can expect significantly less moss growth and longer roof life.
The green solution to killing moss and preventing it is common, super inexpensive, and already in your pantry… baking soda! Moss requires a slightly acidic habitat. Baking soda is alkaline, so it kills the moss but is gentle on the environment. You can even apply it as a preventative.
To maximize the effect, apply baking soda in warmer weather with less rainfall. It can take a few weeks for the moss to die completely and turn brown. If the wind and rain do not entirely remove the dead moss, gently help the process along with a very soft broom or brush. Then clean your gutters! Apply more baking soda as a preventative measure.
The Northwest Center for Alternatives to Pesticides has loads of great information about alternatives, including moss control:
http://www.pesticide.org/prevent_and_treat_moss_on_roofs
This flowchart can also help guide you: