I have purchased a small house in Seattle area that has a detached studio building with a cedar shake roof. I just cleaned and pressure washed the roof. It looks new now and I'd like to keep it that way. How do I seal/preserve the shakes so I don't have to do this very often? Thanks!

Cedar is like redwood a naturally rot resistant wood. It weathers to a natural gray appearance. Most people just leave it like that. If you want a new wood appearance you could try a stain now that your wood is cleaned up but you would have to reapply that periodically just like you have to paint your house. Between water and sun a roof really takes a beating.

Can I use something like Thompson's Water Seal to preserve and seal the wood? What is the best way to apply?

Yes that will work. You can put it in a deck sprayer and spray it. Try to avoid spray drift onto plants around your building, and protect any windows from streaks.Did I mention move your car? Cause I have done that one.

Thanks for the tips! There's no wind today, so I'm going to tackle the job. I appreciate the advice! Have a good day.