What the Ohio River valley does to a Cincinnati chimney
Cincinnati hands a chimney a hard set of conditions that compound on one another. The city sits in and above a river valley, and the damp air that pools along the Ohio and its creek beds rides up through neighborhoods on the slopes, keeping masonry wetter for longer than a drier inland climate would. Brick and mortar are porous by nature, and a stack that stays damp is a stack that absorbs water deep into its joints. Add to that the burning habits of a real Cincinnati winter, where a wood fire smolders low on a cold, gray afternoon, and you get the perfect recipe for creosote, the tarry residue that coats a flue and is itself the fuel of a chimney fire.
Then the cold arrives and the worst single force in the local climate goes to work. Water that has soaked into the brick, the crown, or the mortar freezes overnight, expands, and pries the masonry apart a fraction of a millimeter at a time, then thaws and lets the next rain soak in deeper. Across a Cincinnati winter that cycle runs over and over, and it is why the crowns crack, the joints open, and the faces of older hillside chimneys begin to spall and shed their brick. A small gap that let a little water in last November can be a structural problem by March. This is exactly why we press Cincinnati homeowners to have the chimney looked at before the burning season, while there is still time to seal and repair the vulnerable spots before water and freeze ever reach them.