What a Delaware County winter does to the brick above your roofline
The toughest thing a Lansdowne chimney endures is not the heat of the fires it carries, it is the water and the cold gnawing at the masonry standing exposed above the roof. Our winters do not settle into a steady deep freeze, they seesaw across the freezing mark over and over from December into March, and each time the rain or melt that has soaked into the brick and the crown turns to ice, it swells and levers the masonry apart by a hair. Stack enough of those nights together and a crown that looked sound in October is webbed with fine cracks by April, the upper mortar joints have hollowed out, and the brick faces have begun to flake and shed. This back-and-forth is the reason so many older chimneys around here surrender their crowns and their top courses long before anything lower in the structure shows its age.
Inside the flue a second, slower hazard accumulates. Every fire leaves a film of creosote on the liner walls, and on the wood-burning fireplaces still common in the borough's Victorians and twins, a winter or two of fires can lay down a layer thick enough to ignite on its own. A flue fire is sudden, loud, and hot enough to fracture the very clay tiles meant to bottle it up, and once those tiles are split, the next ordinary fire can shove heat and carbon monoxide straight into the wall cavity. Hard freeze-thaw working on the brick outside and creosote building on the liner inside are the twin reasons a chimney in this climate needs a regular look rather than a glance once a decade.