The chimney on my house is basically a flue that has been framed in, sheathed and sided (aluminum), all of which sits on some joists that have been cantilevered out about 2 feet. In the process of replacing a piece of trim (the corner boards, frieze boards and skirt boards are all 1x cedar), I discovered extensive rot in the framing at the bottom. It looks like the problem is a gap where the corner boards meet almost at the top of the chimney (above the roof line), which I never saw because when I paint that particular section I have to get up on the roof and hang my arm around the corner to reach it. The water was basically collecting in the bottom and sitting there.
I took off the siding and sheathing and have removed almost all of the rotten material, which included some floor sheating, some of one joist and most of another, and some studs. I am planning to double up the damaged joists, replace the rim joist, plate and studs, then sheath and re-side.
Other than fixing the original source of the problem, can anyone think of anything else I should do?