But what they don't tell you is about every 10 to 15 years you need to clean and retreat the roof. I have a close friend that built his house in 1973 using first class, top of the line cedar shakes. In 1988 he cleaned and retreated his roof. Last year he did the same thing again. No small task, and not cheap either. Aparently the shakes "dry out" and need to be treated to maintain the oils, etc that prevent them from rotting and to maintain their water resistance.
You might want to do a google search on "cedar shake roof" to find tips on repairs.