Repair cracks and maintain your chimney
Question: My wood-burning fireplace has cracks in the brick and mortar outdoors. Is this something that needs immediate attention?
Answer: Generally some minor cracks in a chimney mortar between bricks are common, but the long cracks you described might indicate a serious problem. The fact that the cracks continue through the mortar and the bricks is what you should be concerned about.
It would be wise to have your chimney professionally inspected. Your first step might be to call a chimney sweep to have the chimney cleaned. Although they are not structural engineers, an experienced chimney sweep has seen most of the problems that can occur with a chimney. He/she may recommend you contact a licensed structural engineer to evaluate it.
Bricks and mortar create an extremely strong struct ure because t hey have tremendous compressive strength of thousands of pounds per square inch. To the contrary, they are relatively weak when they are stretched in tension. If the chimney begins to tilt at all from being unevenly supported, this puts some of the bricks and mortar into the weaker state of tension instead of compression as it was designed to be.
There are several possible causes of the chimney cracking problems you are experiencing. A chimney, due to its extreme weight, should have a large concrete footer beneath it to support it. Code usually requires a chimney footer to be at least one foot thick and extend out a foot or more further out than the chimney. Check your building plans for the size of the footer.
The footer should have been reinforced with steel rods (called rebar) when it was poured. Even if it is large enough, if the concrete was not adequately reinforced or there was a problem with the strength of the concrete mixture, the footer may be disintegrating from the weight of the chimney.
Settling of the ground beneath the chimney footer is another possibility. If your house was built on fill, then the footer, even though it was properly sized, may be rocking or sinking deeper. If it moves, even a small amount, the stresses in the heavy chim- ney can be tremendous.
Another less likely possibility is faulty bricks. If several weak bricks were placed on top of one another, unusual stresses could have caused them to break. Once several of them break, it may create additional stresses on the other bricks. The structural engineer will be able to test this for you.
Whether you are able to repair the chimney or you have to totally rebuild it, consider installing a chimney cap and a metal flue liner. The cap will keep debris, rain and animals from getting into the top of the chimney. The liner will minimize the possibility of future chimney fires from creosote buildup. The liner will also make it easier for a chimney sweep to keep it clean.