The top flue liner joint is where we usually find most of the gaps in liners during our inspections. This is explained in much better detail in the chimney crown section, but many chimneys we encounter are improperly constructed, and the crown is bonded directly to the top flue tile. Since the top tile is held in place, and the rest of the chimney is able to move during expansion and contraction, we often see issues where the top flue tile is offset from the rest of the chimney liner. This allows heat and potentially deadly gasses to escape and possibly come back into the home or catch nearby framing members on fire.
Another issue we often see with the top flue liner joint is that it simply won’t be bonded at all. What happens here is the mason will build the chimney too short, and instead of cutting a portion of a flue tile to make a properly sized top flue tile, the mason will just insert bits of masonry debris below the top flue tile to make it extend above the crown. In either of these situations, along with many others, we may encounter, we always recommend sealing the joint with refractory mortar (basically just mortar capable of withstanding very high heat), replacing the entire top flue tile, or removing brick holding up the top tile, resetting the old top tile, and adding a portion of a new tile to properly extend the liner.