Chimney crowns. The crown is the slab that seals the top of a masonry chimney around the flue. When it cracks, water runs straight into the chimney's core. The CSIA's guidance is that a crown should project beyond the chimney by two inches on all sides, so water drips clear of the brick instead of running down it. Small cracks can be sealed; a crown that is breaking apart is rebuilt.
Flashing and leaks. Flashing is the metal seal between the roof and the chimney. When it fails, rainwater runs down the chimney into the house, where the CSIA notes it can damage ceilings and walls and rot rafters and joists. A stain on the ceiling near the chimney is the usual first sign.
Mortar joints and brick. Mortar wears out before brick does. Tuckpointing means cutting out the failed mortar and packing in new, which restores the joint and keeps water out. Bricks whose faces have flaked off are replaced.
Waterproofing. In the CSIA's words, "common brick is like a sponge, absorbing water and wicking moisture to the chimney interior." A water repellent made for masonry slows that down. It goes on after the cracks and joints are repaired, not instead of repairing them.
Fireboxes and dampers. Inside, we repair cracked firebrick and failed firebox mortar, and free, repair or replace dampers that are rusted or stuck. Where the old damper is beyond saving, a top-mounted damper that seals at the top of the flue is often the better fix.
Flue liners. Cracked or missing flue tiles are found by inspection, not from the living room. When a liner is damaged, the report explains the options for repairing or relining it. Caps and chase covers have their own page: chimney cap installation. For gas units, see gas fireplace service.