Stop adding flow
Do not flush toilets, run taps, use a washer, or test other drains while a sewer backup is active. Water sent into the drainage system can return at the lowest opening.
Ask other occupants to stop water use until the condition is assessed.
Treat the water as contaminated
Keep children and pets away. Avoid contact with wastewater and do not operate electrical equipment in or near wet areas.
Cleanup, disinfection, damaged materials, and insurance documentation are separate from clearing or repairing the drain; arrange the right help for each scope.
Record the pattern before it disappears
Photograph the first affected drain, note the weather and time, list every fixture involved, and record whether neighbours or the municipality reported an event.
These observations do not prove responsibility, but they help separate a local blockage, private lateral problem, and system surcharge.