What old North Philly brick does to a chimney over time
A North Philadelphia chimney lives a rough life. Many of these stacks have stood for the better part of a century, mortared up when the rowhome was built and asked to vent whatever heating system the family installed next, from coal to oil to gas, each burning a little differently and leaving its own residue inside the flue. Decades of Philadelphia weather work on the outside at the same time. The freeze and thaw of a Delaware Valley winter gets into every hairline crack in the brick and the mortar joints, prying them a little wider each cold snap, and the soft old brick common on these blocks soaks up water and spalls, shedding its face in flakes until the joint behind it opens up.
The trouble compounds because so much of the chimney is hidden. Water that gets past a cracked crown or a missing cap runs down inside the masonry, rusting the damper, rotting the mortar between the flue tiles, and on an attached rowhome it can track sideways into the party wall and surface as a stain in the bedroom next door. By the time a homeowner notices the white efflorescence on the brick or the patch of damp on the chimney breast, the water has usually been working unseen for a season or two. That is why we press so hard on inspecting before the cold sets in, while there is still time to cap, seal, and point the brick before another freeze pulls it apart.