I got up into the attic and there was water all along a truss starting at the chimney. I removed the cellulose insulation to allow the sheetrock to dry, and I put pans under where the water was dripping.
When we put up Christmas lights last week, I noticed a fair amount of moss on the roof, especially around the wood chimney. I took wire brush to it, and it looks like that was not the best idea - the moss was keeping the water out. When we bought the house, I knew that I would need to replace the wood on the chimney at some point. There is some dry rot, but luckily it has not moved down to the decking or trusses. I was planning on repairing this when I did the rest of the siding, but now my timetable is moved up.
I did a temporary repair, and here is a brief summary of how I did it...
To start, the design of the thing is poor. It has the wood coming down way too low. Also the flashing is overlapped with the trim wood, and it's wicking water up.
Best practice in wet climates it to put a "cricket" above the chimney. It's like a little dormer roof that sheds water off to the sides - marked in red:
I didn't have time to add this right now, so I decided to add a large piece of flashing that has extra width to help divert the water.
First, I added a piece of trim to make an even face for nailing the flashing:
I added the flashing as shown, which goes up underneath the course of shingles. It is nailed to the trim.
Then I put roofing patch all over the top of the trim piece and onto the flashing. I pitched it so it would shed water. I also slid a piece of tin up under where the two shingles above come together to direct water on to this flashing, not under it.
Here is the side of the chimney - its pretty bad looking. The flashing is wrong (continuous piece), plus there is a second layer of shingles, making a little gutter along the wood. This could not have been more poorly done. I think its where the water is coming in. If you add a layer of roof, you have to add a layer of flashing, or it will direct the water underneath.
I added some simple step flashing along the side. I cut these 8" long, and slid them up under each course of shingles. This is a only sure way to direct water away from the wood.
Normally, I would add the top flashing, cut into the wood. In this case I screwed the flashing to the wood and covered it with roof patch.
I want to stress that this is a temporary fix. It is obviously not acceptable to flash and seal over dry rotted wood and leave it that way. This is only going to stay in place until the I have time (and the weather permits) to tear the chimney down and rebuild it.
Once I confirm that the water is controlled, I will see if I can bleach, prime and paint over the water spot. If not, I'll have to replace that small area of ceiling (hope not).
Here is good article on step flashing in Fine Homebuilding.