Repair Crack in a New Masonry Chimney

By James Dulley

August 1, 2019 4 min read

Dear James: I added a family room with a fireplace to my home last year. The solid brick chimney has developed several long cracks through the bricks and the mortar. Is this something I should be concerned about? — Johnny G.

Dear Johnny: Yes, you should be concerned about those long cracks in a recently built chimney. A masonry chimney, brick or block, needs to be very strong and stable, and the cracks you described may seriously weaken it. Chimneys should extend well above the roof line, and you can imagine the damage one could do if it were to crumble and fall onto your roof.

Some small cracks in the mortar between the bricks is not unusual and is generally nothing to worry about. These small mortar cracks can be repaired with flexible mortar compounds, which look similar to real mortar but are more durable. Using a liquid sealer on the entire chimney exterior can also help.

These small cracks allow water to enter the chimney structure, particularly during a driving rain. This water may drip out inside the chimney or migrate downward and out weep holes at the bottom. In cold climates, this water may freeze. Water expands when it freezes, and that can cause additional cracking.

In your situation, with long cracks running through the mortar and a series of bricks, you will not be able to repair it yourself. Definitely have a qualified chimney sweep or a structural engineer inspect it and advise you on how to proceed. If the cracks are serious, the professional may suggest tearing it down and rebuilding it properly.

Long cracks have several possible causes, but it's usually not the bricks themselves. It's highly unlikely that several bricks with internal flaws would all be lined up on the chimney. Most of the possible causes are related to the design or to the installation procedure.

Starting at the bottom, the footers may not have been strong enough. A full masonry chimney can weigh tens of thousands of pounds over a relatively small area. The chimney footer should be at least 1 foot thick and extend out at least 1 foot longer than the base of the chimney. It should also have reinforcing steel rods (rebar) extending up from it.

Poor-quality or weak mortar may have been crushed under the weight of the full masonry chimney. If that's the case, the bricks may have been stressed enough to crack.

If the wide side of the chimney is facing the prevailing winds, the force of the winds may have caused the chimney to bend, creating the cracks. Mortar feels hard to the touch after it begins to set, but it can take days or weeks to reach its full strength. Inserting steel reinforcing rods in the chimney can keep it more rigid while the mortar is developing strength.

If you have to rebuild the chimney, consider installing an approved metal chimney liner without solid masonry walls. This will reduce the overall weight of the structure and minimize future settling problems. Make sure the masonry is reinforced with steel rods and spray on a sealer. Use a special chimney cap sealing compound to keep moisture from entering from the top.

Send your questions to Here's How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit www.dulley.com. To find out more about James Dulley and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

Photo credit: Free-Photos at Pixabay

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