Applying caulking or silicon over damaged mortar joints, rather than proper masonry Tuck-pointing, is not only an improper repair method, but it also creates further deterioration by trapping moisture in the masonry behind the mortar. The trapped moisture will cause damage during the winter when it freezes and expands, while also deteriorating the mortar joints and bricks around it. Any caulking or silicon should be removed from the chimney and replaced with proper breathable masonry material to allow moisture trapped within the masonry to evaporate evenly, rather than being trapped in certain areas.