Question: > I am requesting as much information as possible on the reasons why clay flue liners crack. I'm a fire investigator for Donan Engineering and occasionlly I have to evaluate the possibility of a creosote fire in a fire place. Answer: Clay flue liners can crack for many reasons. Many of the chimney sweeps that I meet tell me that MOST of the flues they clean are cracked. I'd suggest the #1 reason is the inability of them to allow for expansion and contraction. Some of this is the flue liner itself, another part is the installtion of the liners. Another reason is the settling of the chimney structure. When it comes down to it, todays standard flue liners are actually a poor material for a chimney...but the use of them has continued for more than a century.