Why Jersey City chimneys age the way they do
The chimneys in this city were almost all built tall, exposed, and masonry-bound, and the local weather works on every one of those traits. A chimney stack stands above the roofline with brick exposed on all four sides, soaking up rain off the Hudson, then freezing solid on a January night and thawing the next afternoon. Water that has crept into a mortar joint or a hairline crack expands as it freezes and pries the masonry apart a little more with each cycle, which is why the brick above the roof is almost always in worse shape than the brick you can see from the sidewalk. By the time a homeowner notices crumbling mortar or a leaning crown, the freeze-thaw grind has usually been at work for several winters.
Then there is what is happening inside the flue, which is invisible from the street and where the real hazards live. Jersey City has a lot of fireplaces that still get used in the colder months, and a wood fire deposits creosote on the flue walls that builds into a fire risk if it is never swept. Just as common, and more dangerous because it is silent, are the gas furnaces and water heaters venting through old masonry flues that were lined for coal or oil. When a modern appliance vents into an oversized, cooled, or cracked clay liner, condensation and combustion byproducts attack the flue and can push carbon monoxide back toward the living space. Both problems are routine to catch on an inspection and both are easy to ignore until they are not.