Chimney leaks are usually only spotted after the damage has already been done, and show up as water in the fireplace or leaks in the roof. Three common leaks result from cracks in mortar joints, seepage into the flashing, and a damaged chimney cap. They are all best dealt with by trained chimney professionals as repairs incorrectly performed lead to further damage.
Not only is brick itself porous unless it has been waterproofed with a vapor-permeable sealer, but the mortar joints are susceptible to water damage. Big holes can form in mortar simply from prolonged exposure to water and require at least a patch to prevent further deterioration. This is a common way for water to get into the home and is easily remedied by the application of fresh mortar.
Another common point of entry for water into the chimney is at the flashing. Flashing is sheet metal that increases the resistance of the chimney to penetration by water. Designed to let water pour away from the chimney, it can instead run behind the flashing when a gap develops between it and the mortar. It is only a matter of time before water starts dripping into the fireplace.
A third common cause of chimney leaks is a defective or missing chimney cap. Both high winds and small animals can dislodge a chimney cap, so it may be missing altogether, which is easily confirmed by a quick visual inspection. It may, however, be corroded by rust, as it is most likely made from galvanized steel.
Continually exposed to rain and snow, chimney caps are the front line of chimney defense and, over time, inevitably deteriorate. Holes form through which water enters, dripping into the fireplace and running down the flue. The resulting accumulation of moisture in the chimney leads to far more serious problems and greater expense than the cost of replacing the chimney cap.