The most expensive plumbing repair often involves fixing a damaged sewer line. Sewer line repairs can be costly due to factors like accessibility, depth, and extent of damage. Common issues like tree root intrusion, pipe corrosion, or collapses necessitate extensive excavation and labor-intensive repairs. Additionally, addressing sewage backups or leaks promptly is crucial to prevent property damage and health hazards. Regular inspection and maintenance help identify potential sewer line problems early, mitigating the risk of costly repairs.