During one of our morning web research sessions, Bill read the advice of several tiny housers suggesting it was worth springing for something a little more substantial than roof felt. We returned our $19 tar paper rolls and splurged on $60 rolls of WeatherWatch, used primarily for high-leak areas (eaves, rakes, valleys) on big house roofs.
Now the roof just needs its sheets of corrugated metal. But that will be another day....