If you have a large amount of moss build-up, you will first have to scrub it off with a brush; we recommend a long handled brush to make this task as safe as possible.
One of the most common solutions to remove algae or moss is to lightly spray wash the roof with a 50/50 mix of water and bleach to remove the algae or moss (no pressure washers, they will strip off the granules and may cause even more damage).
- Be sure to wet any foliage near the home first. Bleach is no good for plants as you can imagine, and getting them wet first with plain water will help to protect them.
- Just make sure to rinse everything with clean water when you're all done.
To keep the algae from coming back, you can insert strips of zinc or copper under the row of shingles closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. Then, whenever it rains, the chemicals in the metal will wash down the roof and kill any moss or algae trying to form on your shingles.
Hope this helps and Good luck!
For more information:
Read "How can I control moss growing on my roof?" a Q&A answered by Erik Henry.