Fix the roof above, along with the flashing around the chimney, is how you do it correctly. You have water intrusion on the roof surface, is why you are getting rot.
__________________ Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?