possibly condensatino developing on the underside of the roof decking. you did say the wet wood you saw was the underside of the roof and the roof rafters correct?? if so,,,then sounds like either you have a leak in the roof or condensation may be developing due to the difference in temperature between both sides of the roof...i.e.cold on one side ,,and warmer on the inside plus lots of humidity in the attic for some reason. Do you have ridge vents or some other type of vent?? it needs to be corrected before it becomes a major problem. I would hae it check by a reputable roofing contractor. in fact get several looks/estimates and compare their estimate/opinions on the cause. good luck LJ