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I live in Seattle. My 6 years old composite shingles start to grow moss this year.
I read one post on YouTube that you can use a long zinc strip placed on top of your roof to prevent moss. I am wondering how effective it is. Another post mentioned that one strip can stop moss growing for about 15 feed down, so I need a few more strips to make whole roof protected.
Any thoughts and experiences? 
I live across the water from Seattle. I've seen several houses around here with zinc strips. They also have moss on the roof. I think there's no stopping moss in this climate on composite shingles.
Yes, it works - although most recommendations I've seen prefer zinc to copper (both more effective and cheaper). In fact, you see it all the time on older roofs underneath galvanized roof fixtures. For example, my neighbor's house:
Basically, each rain washes metalic zinc down the roof and basically prevents any growth and lasts for decades. Copper does the same thing, although in that case it would be copper oxide washing down the roof. Note that if the house is brick, you should go with the zinc - copper oxides react with periclase in bricks and can cause damage.
If you want some more reading, there's a short piece on the Oregon State website.
I have a similar moss problem on my roof in the Berkshires, MA. Based on some online recommendations I placed a bunch of pennies on the roof - half under several shingles. I believe pennies are roughly 97.5% zinc. This seemed to work for a while but over time the pennies fell off or blew away whenever I cleared the roof of pine needles. I will try the zinc strip under the shingle approach. Thanks.
3M Scotchguard on shingles seems to work. here
