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1963 home. CMU chimney had a straight, vertical, hairline crack from the top to about 2/3 the way down. No offset, etc. No crack on the other side or at the mortar crown. Footing looked solid. Serves two woodburning fireplaces.

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Two questions...

1: How did this happen? (Excessive heat from a large fire?)

2: How much of an issue? The chimney appeared to be solid otherwise.

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No crack on the other side or at the mortar crown.
Was the crack inside the TC flue?

Not that I could see. In other words, not in the top few feet.

Perhaps a chimney fire? I would recommend a chimney sweep do a level II inspection.

That's as good a guess as an excessively large fire. I would think that something other than our occasional quakes made it crack in a straight line like that. And, yes, inspection called for.

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As a mason, we used to be warned not to completely mud in terra-cotta liners because they expand and contract and can crack. Maybe the liners cracked the block chimney, by pushing outward on the block. (notice that the crack: 1. is very straight and centered on the flue. 2. stops when below the firebox or thimble area. 3. isn't larger at the top, as if the chimney were spreading apart due to settlement). Concrete block are such big units that they make for a rather fragile system, in some ways, when compared to brick - they handle dead weight great, but other stresses, not so good. Just a guess.

Also, during my disaster restoration days, I saw lightning crack the heck out of brick chimneys. I suppose a lightning strike could crack a block chimney pretty easily.

But, as straight as the crack is, my money is on the flues expanding outward against the block and cracking the outer surface.

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As a mason, we used to be warned not to completely mud in terra-cotta liners because they expand and contract and can crack. Maybe the liners cracked the block chimney, by pushing outward on the block.

Rhymes with the recent thread on cracks in brick veneer on structural block buildings. Worthy of a permanent etch in my memory. I'd never learn this anywhere else.

So, brick and terra cotta have a similiar thermal coefficient of expansion which is different from that of concrete block? Would that be a fair statement?

Marc

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