A missing or damaged cap
A chimney cap covers the top of the flue. Without one, rain and snow can fall straight down the chimney. A cap also helps keep out debris.
A cracked crown
A crack in the crown lets water into the masonry below it, where it can travel a long way before it shows up indoors.
Worn flashing
Flashing seals the joint where the chimney meets the roof. When it lifts, rusts, or pulls away, water can run into the house. Stains on the ceiling beside a chimney often point here.
Porous bricks and mortar
Brick and mortar can absorb water, especially once joints start to wear. Damp walls and musty smells around the fireplace can be signs of this.
Why it only leaks sometimes
Some leaks appear only in heavy rain, in wind from one direction, or when snow melts. Good leak detection looks at the whole chimney, not just the wet spot. Fixing a leak early helps protect both the masonry and the inside of your home.