What an older West Philadelphia chimney is really up against
A brick chimney in this part of the city fights the same battle every year, and it is a quiet one. Water is the enemy. Rain and snow soak into the exposed brick and mortar of a stack that stands well above the roofline with no shelter at all, and when a Philadelphia winter freezes that absorbed moisture, the water expands and pries the masonry apart from the inside. Repeat that freeze-and-thaw cycle through enough seasons and the mortar joints crumble, the brick faces flake off in sheets, and the crown at the top develops the cracks that let still more water in. The damage builds slowly and out of sight, which is exactly why so many homeowners are surprised by how far it has gone once we get up there and show them.
The second pressure is what happens inside the flue. Burning wood leaves creosote, a tarry, flammable residue that coats the flue walls and, left to build, becomes the fuel for a chimney fire. Burning gas runs cleaner but produces acidic moisture that eats at an old clay liner or unlined brick over time, which is its own slow form of decay. On the tall, narrow flues common in these West and Southwest Philadelphia houses, both problems are easy to miss from the ground and easy to find with a proper sweep and a camera. Catching either one early is the difference between routine maintenance and an expensive, dangerous failure.