The toilet backs up into the shower or the tub.

Stop running water anywhere in the building, including the washing machine and the dishwasher. Water appearing in a fixture nobody used means the blockage is downstream of both of them, in the line leaving the house, and everything you send down any drain now has to come up somewhere.

That somewhere is the lowest opening in the building, which is why it is appearing in a ground floor shower rather than in the toilet you flushed.

Why this particular symptom matters

A blocked toilet is a toilet problem. Waste coming up in a different fixture is a system problem, and the distinction is the most useful thing you can know before you call anyone.

Every fixture in the house drains into one main line. When that line is blocked, the water has nowhere to go but back, and it rises until it finds the lowest opening. In most houses that is a basement floor drain, a ground floor shower, or a tub. The toilet is usually higher than both.

So the fixtures that are affected tell you where the blockage sits:

• One fixture only, everything else drains: a local blockage in that fixture’s branch. Treat it as a blocked toilet.
• Two or more fixtures on the lowest floor, or a floor drain flooding: the main line. This is a drain specialist, not a plunger.
• Every drain in the building, and the neighbors have it too, or it started during heavy rain: likely the municipal side, or a system overwhelmed by storm water. Call the sewer authority. Most run a 24 hour line and most will come out.

The gurgle is a clue, not a nuisance

If flushing the toilet makes the shower gurgle, or running the washing machine makes the toilet bubble, air is being pushed through standing water in a shared line. That is the same blockage announcing itself before it overflows, and it often shows up days or weeks earlier.

A drain that has been slow for a month and now backs up was never a sudden event, which matters later: gradual is the word most policies use to decline.

What to do in the next hour

1. Tell everyone in the building to stop using water. This is the whole instruction. It is also the one people forget within ten minutes, so say it twice.
2. Do not plunge the toilet. The blockage is not there. Plunging pushes against a solid column of water in the main line, and the pressure comes back up somewhere else.
3. Do not use chemical drain cleaner. It will not clear a main line, and it leaves a caustic pool for whoever opens the pipe next.
4. Lift what you can off the floor in the affected room and in adjoining rooms.
5. Find the cleanout if you can, so you can tell the plumber where it is. It is a capped pipe, usually four inches across, either outside near the foundation on the sewer side of the house, in a basement, or in a crawl space. Opening it yourself will release whatever is backed up behind it, so this is a locate rather than an open.
6. Photograph everything before anyone starts.

What actually blocks a main line

Worth knowing, because it changes whether this comes back next month.

• Tree roots. The most common cause in any house with mature trees and older pipe. How roots get in and what the fixes are.
• Wipes. Including every wipe sold as flushable. They do not break up, and they knit together around anything else in the pipe.
• Grease, poured down a kitchen sink over years, which cools and lines the pipe like cholesterol.
• A collapsed or offset section, common in clay pipe and in anything old enough to have been laid before plastic.
• A belly, meaning a sag in the line where solids settle because the fall is wrong.
• Storm water overwhelming a combined system, which is not your blockage at all.

A camera inspection after the line is cleared is what tells you which of those it was. Without it you are paying to clear the same blockage on a schedule.

Who fixes it, and who pays

The line from your house to the sewer main is usually the homeowner’s, and it is usually the part that fails. The main under the street is usually the city’s. Where the boundary sits, and what happens when the blockage is on their side, is worth knowing before you agree to any excavation.

If waste got into the house, the cleanup and the plumbing repair are two separate bills, and your policy probably treats them differently: the backup itself usually needs an endorsement.

After it is cleared

The blockage going away does not clean the house. What came up is category three water, and the material it touched is decided by what has to come out rather than what can be dried.

Run water gradually at first, one fixture at a time, and watch the shower drain while you do it. A line that clears and then slows again within days was partially cleared, not fixed.

The short version

Waste in a fixture you did not use means the main line, not the toilet. Stop using water everywhere. Do not plunge and do not pour chemicals. Locate the cleanout for the plumber. Ask for a camera inspection afterwards, because otherwise you will be doing this again.