What Causes Cracks and Other Damage to Chimneys?

Closeup of brick chimney.

Cracks in chimneys, missing mortar or separation should all be noted on a home inspector’s report.

Potential home buyers don’t want to see a cracked chimney. In fact, they probably don’t want to see cracks in anything in their home-to-be.

But the reality is, cracks are part of most structures — concrete, brick, stucco, plaster, and more. Some are expected and inevitable. Some are just ugly. And some are dangerous. As a home inspector, it’s important for you to be able to differentiate among the causes and consequences of cracks.

Most visible cracks in chimney bricks come from the freeze/thaw cycle. Bricks and concrete are porous, and as such, will absorb water from rain or thawing snow. Once that water is inside the structure and the temperature drops again, it freezes and expands. Often the brick — or other material — can’t withstand the pressure, and cracks develop.

Soil Preparation Is Key

Like any structure, the base has to be properly prepared to support it adequately. As with a retaining wall, driveway or walkway, the area beneath must be excavated and filled with gravel. If it isn’t, air pockets can develop, destabilizing the structure.

No mason can completely eradicate air pockets, but an effort should be made. If it isn’t, the chimney could eventually shift and suffer cracking.

Even if the cracks are small and only in the mortar, if vines are allowed to grow on the chimney, their roots may penetrate the cracks, making them wider and eventually destroying the chimney. It may sound hard to believe, but vines can grow thick and strong enough to support the weight of a person and even pull down a healthy tree.

Regardless of how a chimney gets cracked, it should be repaired. The longer a crack is left open to the elements, the more moisture can infiltrate it and further undermine the structure.

Fire Can Be Dangerous to Chimneys

Even though chimneys are built for the purpose of safely containing fires, the truth is that the heat can threaten their durability. A chimney that is the frequent site of large, hot fires can suffer cracks because of it. Also, a damaged liner or a buildup of creosote can cause a fire in the fireplace to climb up the chimney and damage it.

Tree roots that grow under the chimney may push up and cause it to shift and crack, and drainage problems may cause the area under the chimney to become unstable, also possibly resulting in cracking.

When inspecting a chimney, Inspectapedia advises you to look for proper footings. If they are missing, make a note of this even if the chimney is not cracked. It might be just a matter of time.

Also, note any suspicious caulking or flashing you find between the chimney and the roof. Of course, there should be flashing here, but if it looks extensive or like it has been repeatedly replaced, it might be because the building and the chimney are separating.

Brick chimney with black soot at the top.

Chimneys are an integral part of many homes, so their safety and integrity must be certified.

Some Chimney Cracks Can Be Repaired

John Warde, home improvement columnist for the New York Times, says superficial cracking around the chimney top can be repaired with caulk, but any cracked or crumbling mortar or loose bricks need to be replaced.

Certain crack patterns are more dangerous than others, Inspectapedia says. Carefully inspect the chimney from the indoors as well as the out, noting any cracks, their width and length. Also, note any leaning of the chimney. Any of these discoveries merits a recommendation that the chimney be evaluated by a professional chimney inspector for safety and integrity.

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