Thanks for your feedback on composite roofing. You've clearly had a lot of experience.
I actually like our wooden roof a lot and am not terribly concerned about the aesthetics. We live on an Island. It's par for the course.
I am concerned about extending the life of our roof (now almost 20 years old). This is why I was asking about ways to remove the lichen without damaging the shingles.
On another site, a product called Shingle Shield was recommended by one homeowner, who had used it on an asphalt tile roof. The Cleaner is marketed by Chicago Metalic, the maker of a Zinc-based Roof Protector Strip, which is designed for use on asphalt roofs. Here is the website: http://www.chicagometallic.com/product_frameset.asp?product=®ion=1&company=29&division=30&viewby=none&prodid=37&menuitem=52&rowcount=1
I spoke with the head of the Division, a very helpful guy, who was reassuring about its use on a wood shingle roof, despite the age of ours. This product, is sodium hydroxide-based and, therefore, needs to be handled with caution, even after it is diluted 10:1, as instructed. I'm actually more concerned about the effect it could have on painted surfaces and the side of our house, which does not need cleaning. If I decide to use it, I will not do it myself, but instead find someone who has sufficient experience.