Wood shakes are natural split wood. They naturally crack, split, break and expand and contract even after a couple of years from installation. They also need maintenance or an applied stain/protectant every 6-8 years. Unfortunately, most homeowners don't provide this maintenance. Second, further maintenance issues include: insects, wasps, bees, squirrels, raccoons, etc. infest the wood product and provide further issues with the roof. Lastly, the wood shake quality has decreased substantially since the early 1990's. Old growth forests are a thing of the past, therefore wood shake roofs no longer last 40 years and you may see 15-20 years maximum out of them.