Several affected fixtures are an important clue
One slow sink may have a local trap problem. When several fixtures back up or the lowest drain receives water after an upstairs fixture runs, the shared drainage path needs assessment. This does not by itself prove a structural failure.
Pay attention to repeated backups
A blockage that returns after clearing deserves further investigation. Record the timing, affected fixtures, and previous work. Pipe condition, obstructions, and the location of the problem determine what repair, if any, is appropriate.
Stop adding water during a backup
Avoid laundry, showers, flushing, and dishwashing when sewage is entering the property. Keep away from contaminated water and electrical hazards. Call for plumbing assessment and speak to your insurer about safe restoration.
Clarify the line location and responsibility
Ask whether the suspected problem lies in the private property connection or municipal infrastructure. Toronto 311 can provide guidance for municipal concerns. Do not authorize a full replacement based on symptoms alone; ask for the diagnosis and alternatives.
Discuss your specific plumbing concern
General guidance cannot diagnose your property. Learn about the relevant plumbing service, or send a service request with your location and symptoms.