Chimney fires, lightning strikes and other traumas to the chimney can cause damage to the liner and, in some cases, to the masonry structure itself.
Wood, coal and, to a lesser degree, oil, fires combust fuel incompletely. The resultant smoke exits the chimney with these unburned particulates in suspension. As the smoke rises, it cools. When it reaches a certain temperature, condensation occurs and the particulates are deposited on the interior of the flue in the form of soot or creosote.
Soot, from oil appliances, and creosote, from wood and coal appliances, are the chief culprits in chimney fires. Under the right conditions, these deposits will ignite, sometimes explosively, and burn at very hot temperatures.
Terra cotta tile liners will withstand large amounts of heat, but not a rapid rise in temperature. If the rise is severe enough, the liner can crack and in some cases, even develop holes. While the chimney fire may be contained in the first instance, this damage needs to be repaired to be certain that fire protection and proper function are maintained.
Lightning strikes can also be the cause of trauma to the chimney. Lightning may enter and exit at the top or travel all the way to the bottom. Again, a damaged tile liner needs to be repaired before the chimney can be considered safe for use.
Earthquakes, downed trees and high wind are also documented causes of trauma to a chimney.
Many chimney professional companies use a Chim-scan video camera to assess possible damage to a liner. Insurance claims usually require such documentation and often the camera picks up situations that the naked eye cannot.
Repair usually involves removal of the damaged chimney liner. A bladder is then lowered down the chimney and inflated to the proper size. A specially formulated cementitious material is pumped around this bladder, filling all the voids in the chimney and sealing all the cracks from the inside. Once the mortar is dry, the bladder is deflated, removed from the chimney and the new liner has been cast-in-place within the structure.
See also the advantages of choosing pumped masonry over stainless steel.