The picture above shows a common problem in the Northwest; moss growth and piles of pine needles on the roof. Our frequent rains and lack of sunny days lead to a lot of water on our roofs . If the roof is not kept clean of pine needles and other organic debris, water will stick around on top of the roof and eventually be a prime area for moss growth. 

Moss loves these wet areas and likes to grow on top of and in between the roof shingles. The shingles are raised up by the moss which allows water to get underneath which will lead to damage of the roof sheathing. If you trap enough water next to the roof you will eventually get a leak. 

The pictures above are the same area before and after cleaning. This corner of the roof had heavy moss growth from a few years of neglect. The tree branches that hang above this corner aren't helping. They provide shade and an extra source of water from dripping branches. 

It just takes a few minutes of work with a push broom to get all the moss off the shingles. You do have to be a bit careful to avoid taking all the granules off the shingles. Moderate pressure with the broom is all that is needed. Too heavy of a push and you can damage the shingles. 

DON'T USE A PRESSURE WASHER. Seems like it would be a lot easier but if you use a pressure washer you are guaranteed to damage the shingles and shorten the life of your roof. 

You can't see it in the pictures but the last step is to sprinkle a moss preventer on the clean shingles. I like to use the shaker bottle of Moss Out. A 6 pound container will take care of most roofs. This should be repeated every year to maintain a moss free roof.