Chimney with no footing (C) Daniel Friedman Cracked Concrete Block Chimneys: Inspection & Diagnosis
     

  • CRACKED CHIMNEYS, MASONRY BLOCK - CONTENTS: Detailed Inspection & Photo Guide to diagnosing and acting on cracks in a concrete block chimney. Cracked concrete block chimneys risk unsafe chimney flues, presenting fire and flue gas hazards in buildings. Common Causes of Concrete Block ("cinder block") Chimney Cracking. Evidence of Prior Repairs May Indicate a History of Chimney Cracking & Movement in a Concrete Block Chimney
  • POST a QUESTION or READ FAQs about diagnosing & repairing cracked concrete block or masonry block chimneys
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Concrete block or cinder block chimney cracks & damage: this article describes cracks in masonry block or concrete block chimneys and suggests possible causes and crack severity.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

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Cracked Concrete Block Chimneys

Severe frost cracking at concrete block "cinder block" chimney (C) Daniel FriedmanCracked concrete block chimneys risk unsafe chimney flues, presenting fire and flue gas hazards in buildings


Both outdoors and indoors we may also see chimney cracks which could be due to chimney movement (introduced above) or due to compression loads or other chimney construction problems (photograph below).

Our photo (left) illustrates serious frost cracking damage in a concrete block (or masonry block or "cinder block") chimney serving a home in upstate New York. Notice that there is no rain cap and no chimney cap protecting this chimney against water intrusion - defects likely to lead to frost damage and an unsafe flue, especially in freezing climates.

Cracks in a masonry block chimney always deserve further investigation to identify the probable cause, to decide on the remedy and to evaluate the safety and function of the chimney.

We recommend that you have any cracked or damaged chimney thoroughly inspected by a professional, including an inspection of the integrity of the chimney flue and of any fireplaces or other attached heating systems.

Common Causes of Concrete Block ("cinder block") Chimney Cracking

Cracking and spalling masonry block chimney (C) Daniel Friedman

Our photo (left) illustrates a concrete block chimney suffering from both extensive cracking as well as spalling surfaces (CHIMNEY SPALLING). We think there are leaks into the internal structure of this chimney, probably originating at the chimney cap.

It looks as if someone thought they might repair the chimney by coating its exterior with a masonry sealer paint or a thin stucco covering. But as leaks are originating from the chimney top and passing within the chimney structure, that repair did little good.

Cracks in a masonry chimney, particularly concrete block chimneys (this article) and perhaps brick may be caused by

Readers should also see CHIMNEY INSPECTION from GROUNDChimney Cracks, Separation, Movement.

Cracks in Masonry Chimneys May be Very Dangerous

Cracks in a concrete block chimney (C) Daniel Friedman

Watch out: a chimney fire, even if it does not set the building ablaze, is likely to have caused serious damage to the chimney itself, leaving it unsafe, as may other chimney cracks or movements for any reason. The risks include building fires, potentially fatal flue gas leaks, improper draft, and unsafe heating equipment operation.

Our photo at left shows dangerous cracking indoors in a concrete block chimney used to vent a heating appliance.

You might also notice that the barometric damper is not level - a much simpler problem to correct.

As a chimney leans away from the house we might find several serious problems caused by that movement:

  • Damaged, unsafe chimney liner (flue gas leaks or sparks into the building interior, a fire or potentially fatal carbon monoxide hazard)
  • Damaged, unsafe fireplaces where the chimney has pulled the firebox away from the building or created dangerous openings around the fireplace into the building structure, risking both sparks (and fire) or air leaks (and inability to control the draft).
  • Leaks into the building walls (flue gas leaks or sparks into the building interior, a fire or potentially fatal carbon monoxide hazard)
  • Unsafe fireplaces in the building: hearth cracks, fireplace side cracks, chimney damage all present risks of sparks or smoke entering the building cavities, a fire and flue gas risk. Check for evidence of movement at the fireplace. (Flue gas leaks or sparks into the building interior, a fire or potentially fatal carbon monoxide hazard)

Also see CHIMNEY CRACK DETECTION & DIAGNOSIS

Evidence of Prior Repairs May Indicate a History of Chimney Cracking & Movement in a Concrete Block Chimney

Photograph of a brick chimney separating from the building.

If you see repairs such as shown in the our photo (left) the chimney should be professionally inspected, especially to establish the condition of the flue liner as it may be unsafe.

Our pen points to a gap between masonry block chimney and a building wall, indicating chimney movement and raising question about other less obvious crack and damage to this structure.

It may be possible to repair minor chimney racks with masonry products provided that the underlying cause is also identified and corrected. Also, some concrete blocks may be cracked during their individual drying/curing process, a condition not to be confused with chimney cracking.

The usual repair when a significant chimney crack has occurred, damaging the chimney and the flue is to remove and replace the chimney, though in some cases it may be possible to re-line the chimney and to jack an intact masonry chimney back to level and repair its connections into the building.

Also see CHIMNEY CRACK DETECTION & DIAGNOSIS where we describe cracks in brick chimneys or in other chimney materials, and see CHIMNEY COLLAPSE Risks, Repairs.

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