What causes the chimney crown to float?

[John Kogel]

I've seen this and repaired one with mortar once. The masonry flue is supporting the concrete crown.

The chimney is straight and on a concrete footing. Bit of a creek behind the house, but a dry crawl. This is an extreme case, and I would like to understand better what has happened.

Built in '88.

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[mjr6550]

Fired clay products like brick and flue liners expand as they absorb water. Concrete based products like block shrink as they cure. If the crown is bonded to the flue liner movement cannot occur so a mortar joint gives way. I have seen this even on chimneys that were never used.

[Trent Tarter]

I would speculate that water has leaked below crown and freeze-thaw cycles have lifted it up.

[inspector57]

Is it not required to use a flexible sealant between the clay liner and crown?

I assume the crown is not allowed to remain attached to the chimney structure due to being bonded to the liner which has expanded in relation to the chimney structure.

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[Charlie R]

What is required and what the mason actually did are usually two different things. Like Mark wrote, the crown was sealed to the clay flue which lifted up due to expansion while the block of the chimney did not lift. I like how it actually broke the one block loose and lifted that block also. Good pics.