The two forces that age an Upper Darby chimney
If you want to understand why chimneys in this part of Delaware County need a regular hand, you only have to track two things, the water hitting the masonry from outside and the residue building up inside. Take the water first. The brick, the mortar, and the concrete crown at the very top all drink in moisture, and our winters cannot make up their mind about the freezing line, crossing it and recrossing it dozens of times between December and March. Every time that soaked-in water turns to ice it swells, levering the masonry apart a hair at a time. A crown that looked perfectly whole in October comes out of the winter with a web of fine cracks, the topmost mortar joints have started to wash out, and the brick faces near the cap have begun to flake. That is why so many older chimneys here lose their crowns and their top courses long before anything lower down shows its age.
Now the inside. Every wood fire leaves a deposit on the flue wall, and on the open-hearth fireplaces still common in the older twins and rows around here a couple of seasons can lay down a layer thick enough to ignite on its own. When that happens it is not a polite event. A chimney fire burns ferociously hot, hot enough to fracture the very clay tiles that are supposed to keep the next ordinary fire contained, and once those tiles are split, the next normal fire can send heat and carbon monoxide straight into the cavity of the wall. Water prying at the brick from the outside and creosote stacking up on the inside, working at the same chimney from opposite directions, is the whole reason a flue here earns a yearly look instead of being ignored until smoke comes back into the room.