Sewage backup: stop using water and call a plumber: slow sink drain

Why this happens

Seattle Public Utilities describes a sewer backup as sewage or dirty water coming up a sink, toilet or drain, usually because something is blocking the sewer pipe. This Old House says several drains backing up, or sewage coming up at the lowest points of the home, are signs of a main line backup. It also says that continuing to use water can push more wastewater into the blocked pipes and make the backup worse, which is why the first step is to stop using water.

What to do right now

Stop using water everywhere in the house. This Old House says to avoid flushing toilets, running faucets, and using the dishwasher or washing machine.
Keep children and pets out of the affected rooms. This Old House notes that sewage contains harmful bacteria and contaminants.
If sewage has reached outlets, appliances or wiring, stay out of that area. This Old House says to shut off power to the affected circuit only if it can be done safely, and to contact an electrician or emergency services if you cannot safely reach the breaker panel. ESFI says not to touch a circuit breaker with wet hands or while standing on a wet surface. Seattle Public Utilities says not to use electrical equipment while standing in water.
Call a licensed plumber or a drain cleaning service right away.
Leave clearing the sewer line to the plumber. Seattle Public Utilities tells its customers to call their gas utility before a side sewer blockage is cleared, to avoid damaging a gas line. Ask your plumber whether that applies where you live.
Check how your sewer utility wants to hear about a backup, because cities differ. Seattle Public Utilities asks you to call once your plumber thinks the blockage is in the public sewer. The City of Minneapolis asks residents with sewage backing up into the basement to contact its sewer operations hotline, and sends a crew to check. Both say they clear blockages in the city-owned sewer. Minneapolis adds that a blockage in your own line may need a professional drain cleaning service.
For any cleanup, Seattle Public Utilities says to wear long sleeves, long pants, rubber boots, gloves and eye protection, and never to mix cleaners: bleach and ammonia together produce dangerous fumes.

When to stop. Do not plunge, snake or pour anything into a drain that is backing up sewage. Seattle Public Utilities says not to try to stop the flow of sewage coming up through a drain. Do not use water again until the line is cleared.

What to tell the plumber

Where the wastewater came up, and which fixtures are affected.
When it started, and what was running at the time.
That you have stopped using water in the house.
Where the sewer cleanout is, if you know.
Whether your neighbors have the same problem.

Build the full list: what to tell the plumber.

Start this troubleshooter over

References (every fact above comes from one of these pages)

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