What the Delaware Valley does to a Northeast Philly chimney
A brick chimney in Northeast Philadelphia takes a beating no part of the house below it ever sees. It stands fully exposed on every side, above the roofline, soaking up the soaking spring rains, baking through a humid July, and then absorbing the freeze and thaw of a Philadelphia winter that swings back and forth across the freezing mark again and again. Brick and mortar are porous. They drink in water during a wet stretch, and when that water freezes inside the masonry it expands and pries the brick apart from within. Repeat that a few dozen times a winter for a few decades and you get the spalling faces, crumbling joints, and split crowns we are called out to fix all over the Northeast.
Inside the flue a different kind of damage builds up. Every fire, whether wood in an old rowhome firebox or gas through a converted appliance, leaves something behind, and on a wood-burning chimney that something is creosote, a tar-like residue that coats the flue and will burn ferociously if it ignites. The clay tile liners common in the Northeast's older stacks crack under the heat and movement of years of use, and a cracked liner lets heat reach framing it should never touch and lets combustion gases seep where people breathe. The slow weathering of the exterior and the quiet buildup inside are two halves of the same story, and a real inspection reads both.