Wood Shake Roofs

Can someone answer how long a average wood shake roof should last in the northwestern portion of the US? Should they be regularly treated and if so with what?

Thanks

What is the average life span of a cedar roof?
The life expectancy of a roof will vary depending on many factors. Some of these factors include the following: age, environment of installation area, quality of wood, quality of installation, care and maintenance performed, foot traffic, and amount of overhanging trees. The CSSB administers a limited product warranty on behalf of its manufacturing members, and the length of the limited warranty is for 20 or 25 years, depending upon the product type. If more specific information about your roof is required, the CSSB suggests that you hire a professional Approve Installer or obtain a roof inspection to provide you with an experienced, local assessment.

The longevity of a roof depends on many things: the grade and quality of the product used, the quality of the application, climate, salt-water proximity, exposure, and maintenance. The average life span is approximately 30 - 40 years. We recommend investigating the life span of existing cedar roofs in your area.

http://www.cedarbureau.org/faq/product.htm