Don, I have no direct experience but do know that birch has a winter bark and a summer bark. I would imagine the winter bark would be the one to use, canoe builders used it. I have seen a Yurt roof covered with birch bark, it curled up and was replaced with cedar shingles with in a year. And I wonder what the insurance companies would think? Historically it has been used as a weather layer under trim and splices on clap boarding, et cetera.

Tim