What river damp and freeze-thaw do to a Southwest Philly stack
A masonry chimney in Southwest Philadelphia takes a beating that homeowners rarely connect to the brick. The proximity to the Schuylkill and the low, wet ground near Eastwick keep humidity high, and brick and mortar are sponges. They pull in moisture all year, and then a Philadelphia winter weaponizes it. Every time the temperature drops below freezing, the water trapped in the masonry expands, prying at the mortar joints and at the face of the brick itself, and every thaw lets more water in to do it again. Over enough winters that cycle spalls the brick, opens the crown, and works the flashing loose, and on the exposed upper few feet of a rowhome stack, which catch wind and weather with nothing around them for shelter, it moves fast.
Inside the flue the trouble is different but just as real. Wood smoke leaves creosote, a tarry, flammable deposit that builds on the flue walls every time you burn, and a flue that is not swept on a sensible schedule can accumulate enough of it to feed a chimney fire. On the older clay-tile flues common in these rowhomes, the freeze-and-thaw movement that cracks the exterior masonry also cracks the tiles, and a cracked tile lets heat and combustion gas reach the wood framing or the neighbor's wall on the far side of a party chimney. This is why we inspect before we ever talk about what a chimney needs. The visible soot is the easy part. The cracked tile and the loose mortar are what an honest inspection is for.