An expansion joint around the top flue tile serves a very practical purpose. As the fire burns in the fireplace, the hot gases of combustion vent up the flue system. These hot gases in turn warm the flue system, causing it to expand. If the crown is constructed such that the cement is installed all the way up to the top flue tile, as the top flue tile expands when it heats, it will crack the crown itself which can cause water problems or other problems to occur.
An expansion joint is created by leaving a gap between the actual cement crown and the top flue tile and filling it with a flexibleChimney crowns, even when constructed professionally, are often installed incorrectly. Everyone has a different philosophy on the proper construction of a chimney crown from the material that should be used to construct the crown to its whether the crown should hang over the edge of the actual brick on the chimney or stay flush. One fact that cannot be disputed, however, is the need for an expansion joint around the top flue tile. A chimney crown lacking an expansion joint around the top flue tile will almost always need to be repaired after some time.
This flexible sealant allows the flue tile to expand without pushing against the cement crown, preventing cracking. The flue is most likely to expand and crack the cement in the winter time when it is very cold because the cold cement is even more prone to cracking from pressure exerted on it by the flue tile.
Crowns constructed without an expansion joint around the top flue tile usually need to be reconstructed entirely. Although filling the cracks in the crown is an option, the problem is likely to occur again because the same processes are occurring that caused the cracking in the first place. If your crown is cracking and you do not see an expansion joint around the top flue tile, consult a professional who will be able to assess the situation. Sometimes the cracking is minimal and the professional is able to seal the cracks using a high grade sealant and waterproof the crown to prevent water damage from deepening the existing cracks, but other times the cracking has become so severe that crown reconstruction becomes necessary….. Now I’m starting to sound like a dentist 🙁
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