Chimney mortar crumbling or deteriorating — San Francisco, California

The mortar joints between chimney bricks endure the worst conditions in your home — direct sun, driving rain, freeze-thaw cycles, and constant exposure to combustion gases from the inside. Over 20–40 years the mortar erodes, cracks, and eventually crumbles away, leaving gaps between bricks. This process (called spalling or joint deterioration) lets water penetrate the chimney structure, which accelerates the damage: water freezes inside the gaps, expanding them further, and can travel down into the flashing, attic, or even interior walls. Left unrepaired, a deteriorating chimney becomes a structural risk — loose bricks can fall, and water infiltration causes mold and rot. The fix is called tuckpointing (or repointing): a mason grinds out the old mortar to a uniform depth and packs fresh mortar into the joints. Cost is typically $500–$2,500 for a standard residential chimney, depending on height, access, and how many joints need work. A chimney cap ($200–$600 installed) prevents most direct rain and animal entry.