Chimney with no footing (C) Daniel Friedman Cracked Concrete Block Chimneys
Inspection, Diagnosis , Repair

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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


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:


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.

Repairs to Cracked Concete Block Chimneys

Extensive concrete block cracks, badly damaged masonry chimney flues, curved, leaning, broken masonry chimneys may need to be re-built to be safe and functional. But minor chimney damage, and chimney damage that is confined to just above the roof line can usually be repaired or re-built.

Watch out: be sure that the roof surface is protected from damage during rooftop chimney repairs, otherwise your chimney may be fixed but you could end up needing a new roof. Also see CHIMNEY REPAIR METHODS.

Question: I was told that the building code in Massachusetts only allows block chimneys to be reparied with bricks

2017/04/26 Eddie Arroco said:

I need my fireplace chimney concrete blocks repaired or replaced above the roofline which are cracked and breaking up. I called in a contractor but I was told that the building code in Massachusetts only allow him to replace the blocks with bricks.

I'm afraid the bricks wihich is of different color will not look good at all. Is there any truth to this code since it is only a repair of existing structure? What do you suggest I should do? I would really appreciate your advise. Thank you very much.

Reply: Really? Ask again. Here are the Massachusetts Chimney Codes based on model building codes

I am really surprised at the claim that concrete block chimneys are not permitted in Massachusetts.

And I'm doubtful. It might be that the mason wants to work with and use the materials with which she is most-comfortable.

But on occasion a builder speaks as if God told him the Truth when in fact he has never read even the instructions on the box of the device he's installing.

Take a look at these two Massachusetts chimney code documents and you will see that the words "brick" or "concrete block" do not appear anywhere in the code.


A modern concrete block chimney uses a hard-fired clay chimney liner inside the blocks and a listed or UL-listed approved chimney cap. You should not have to replace blocks with bricks.

Now, call a chimney repair contractor or two in your area and let me know what they tell you.

While you're at it, have the entire flue interior inspected for safety too.


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