I put tin on the roof of our storage garage. The tin manufacturer recommended OSB and roofing felt under the tin. Haven't had a problem in a bit over three years.
As an aside I fully understand the pull of the old traditional camp and the needs of a young family. I am part of a traditional "men only" camp. Problem is I have a wife that likes to hunt and two daughters you are showing interest. A close friend invited me and my family into his camp. His idea being, if the kids want to hunt, why exclude them based on gender. So, now I hunt my new camp first w/e and during archery season. I do go back to the old camp on 3rd w/e about every other year.
