A downstairs tub can be the first place wastewater appears when a blockage lies downstream of several fixtures. Water from a washer, toilet, or sink can collect in the shared pipe and rise through a low opening. The tub can back up even if nobody has used it that day.
Record what happened before the backup
Note which appliance or fixture was draining, whether a toilet bubbled, and whether another low fixture backed up. If the tub drains slowly only after someone showers, tell the plumber that too. That pattern can point toward a blockage closer to the tub. If wastewater is already rising, pause water use instead of running more water to reproduce it.
Give the plumber a starting point
Say which floor the affected fixtures are on and where you believe the cleanout is. Leave the cap closed for the technician. If the problem returns after clearing, ask what section was reached and whether an inspection can check that section. See our drain cleaning service for help tracing the affected line.