We have an old granary we made into a summer home. We have not insulted it. One side of the roof (NW side)was repaired . The original roof was torn off but the wide planking was left on and they added plywood over it to nail the shingles to. The problem is we are getting white mildew or something on the inside of the planking in some spot. There is black algae starting on the outside of the shingles. What could cause the white mildew, it is very sad to see the beautiful wide planks turning. Do we need a new roof.