Chimney cracking

[Bruce A. King]

Chimney on rear of 3yr old house with deck wrapped around chimney.

Small cracks above the top of the gas grill...





Larger cracks behind the gas grill....





The crack does not extend lower than about 4 feet above the ground.
No rain cap on chimney....
Freeze expansion?

[Troy Farmer]

Any separation from the house at all?
Small or large gaps?

[Bruce A. King]

No other issues are present, just this seperation that is not near the base/footer.

[Larry D. Kage]

It looks somewhat like some heat cracking I've seen on other chimneys. What does the chimney service?

[Brian A. MacNeish]

Originally Posted by lkage

It looks somewhat like some heat cracking I've seen on other chimneys. What does the chimney service?

By the widening base, it looks like a fireplace.

I have seen chimneys crack like that due to severe creosote based chimney fires but the cracks appear farther up the actual chimney not behind the fireplace (if it is a fireplace).

Wonder if something venting into the chimney was overfired and the temps rose quickly, causing thermal shock? This is the phenomenon that causes cracking in chimney fires. The flue gases were chugging along at 300-500 or so degrees F, then the chimney fire starts and temps rise within 2-3-4 minutes to 1200+ degrees....that's a shock to the chimney as the tiles expand much quicker than the unit masonry does.....pressure and the crack occurs.

[Larry D. Kage]

Originally Posted by Brian A. MacNeish

By the widening base, it looks like a fireplace.

I have seen chimneys crack like that due to severe creosote based chimney fires but the cracks appear farther up the actual chimney not behind the fireplace (if it is a fireplace).

Wonder if something venting into the chimney was overfired and the temps rose quickly, causing thermal shock? This is the phenomenon that causes cracking in chimney fires. The flue gases were chugging along at 300-500 or so degrees F, then the chimney fire starts and temps rise within 2-3-4 minutes to 1200+ degrees....that's a shock to the chimney as the tiles expand much quicker than the unit masonry does.....pressure and the crack occurs.

Yes, my question was more of a wood or gas type question but you included good info in the direction I was headed. Thanks.

[Barry Adair]

Brian provided one very good theory.

Unless other evidence is present, as in the crack express' itself in the firebox (CSIA Level II), just monitor for further movement.

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