In houses built prior to the 1950s, masons often did not use terra cotta flue liners. Chimneys that are brick, stone, or concrete block only, without terra cotta flue tiles, are unsafe for use. Gaps form where mortar has fallen out, and these gaps may allow excessive heat to come into contact with combustibles, or noxious flue gases to enter the home. Chimney fires can easily start house fires, and produce carbon monoxide and other deadly gases. Our blog post on HeatShield Flue Repair goes into more detail on this subject.