Exposure on all five surfaces. Four faces and a top, with nothing sheltering any of them. Wind driven rain off the lake, snow sitting on the crown, direct sun on the south face and freezing air on the north. Every other wall on the building gets protection from somewhere. The chimney gets none.

Freeze thaw, amplified. Mortar absorbs water, it freezes, it expands, it pushes. Chicago runs through that cycle dozens of times a winter. On a chimney it happens faster and more severely, because the stack has no thermal mass of a heated building behind it. It freezes harder and stays frozen longer than the walls below, so the same winter does considerably more damage up there.

Heat cycling from inside. When the fireplace or furnace is running, the inside of the stack heats while the outside stays at Chicago winter temperatures. That differential expands the interior masonry against a cold exterior, stressing the structure from within while the weather works on it from outside.

Snow sits on the crown. Unlike a sloped roof, the top of a chimney is a flat surface where snow and ice accumulate and then melt slowly into whatever cracks exist. A hairline crack in a crown becomes a genuine water pathway over one Chicago winter.

Most of them are a century old. Chicago’s bungalows, two flats and greystones were built with masonry chimneys serving coal and later gas appliances. Many still have their original clay tile liners, sized for equipment that has not been in the house for fifty years, and many have never been repointed since they were built.

The house moves and the chimney does not. Chicago’s clay soil moves foundations seasonally. A chimney is a heavy independent masonry column, often on its own footing, and it does not always move with the rest of the building. That is what opens the gap you sometimes see between a chimney and the wall beside it.