I've moved back to my parents' 60-ish year old farm house that has had pretty much no maintenance or repairs done on it. Several years ago, my Dad installed a metal roof over ashphalt shingles. After he put the new roof down, he decided to add some vents (there weren't any there to start with). He chopped a hole in the roof, and screwed the vent over top, and dressed liberally with tar. As it leaked, he added more tar. He gave any other transition areas (the electrical mast, the plumbing vent...) the same treatment.
Before I begin any plaster repair and painting, I want to get this fixed properly. But after a lot of research, I'm still stymied as to the best way to put a vent in a metal roof, never mind a retroactive repair like this one.
Can anyone out there offer me some advice on how best to deal with this?