As you know, l j, there can be a gazillion places that a roof can leak. One place to start is just by going up in the attic and looking for a pattern of water traces on rafters. If you don't see anything when it is dry, try it when it is raining or have a friend go up on the roof with a hose while you watch in the attic. One thing that happened to me was that the clay liner tiles got eaten out near the top of the chimney. Condensation of furnace gases caused this and it ate right thru the mortar from the inside. Hope this helps.