Thank you for your explaination. I have checked the thermal insulation in attic are dry and has no water mark. The roof boards are dry. But, because the slope of the roof, the bottom of the roof is hard to reach and also filled with thermal insulation material. I did not check it and I will try now based on your suggestion.Peter, what Neal was referring to was the insulation in the attic, if it is wet or if it has been wet it will have a crusty texture to it. Check on the horizontal, both side of the chimney. You will have to crawl over to the edge to check because of the slope of the roof. Also, can you see any water staining on the bottom of the roof decking where the chimney is located on the outside wall?