Ripped off part of the roof of our garage as the company that built it didn't finish it properly.
We had to (extremely gently) prize off four rows of shingles and then cut out the leading 18" of OSB which was largely rotten after water ogress from the edges, then replace the OSB (and some of the fascia where rotten) and install a drip edge before reinstalling the shingles.
Three sides done now, just one more to go and then we can think about installing guttering, which was also never installed!
Something we knew we would have to do when we bought the place but I didn't realise just how bad a shape the wood was before we got up there. It's only about 5 years old too. :/