A sewer backup occurs when sewage flows backward through pipes and emerges in basements, typically through floor drains or toilets. Backups can result from blockages in municipal mains or private laterals, overwhelmed systems during heavy rainfall (especially in older combined sewer systems), tree roots infiltrating pipes, or improper disposal of grease and debris. Backups cause property damage and health hazards. Municipalities may be liable if backups result from inadequate maintenance or design, though homeowner insurance often covers backup damage with specific riders. Preventive measures include backwater valves, proper maintenance, and infrastructure upgrades to reduce backup frequency.