We live in a house, we have 2 toilets 1 upstairs and 1 downstairs. The downstairs one started bubbling vigorously and the water level in the bowl started rising. Now flushing just increases the water level and the level goes down very slowly but does not reach the normal level.

The upstairs toilet is absolutely fine. Tried plunging it but doesn't seems to be working, any idea what could be wrong and how to fix it?

More details on this issue:

No basement, or a downstairs shower. The downstairs toilet is the lowest thing connected to a drain. The bubbling was spontaneous and the water level rose itself prior to the flushing.

When the upstairs toilet is flushed the bottom toilet's water level goes down then back up again. Is this something I should be scared of? Also not sure if it is helpful but there has been a small storm where it has been continuously raining since yesterday.

An update on the issue, as the user MonkeyZeus suggested the issue seems to have been fixed on its own. Checked the water level in the toilet and it was way below the normal level, so I half flushed it to check and the water has returned to the normal level and is working fine. So most likely the issue was with the main sewer line which has been fixed on its own.

  • Are you on a septic tank? Better call a professional and stop using water unless you want excrement on your floors.
    – MonkeyZeus
    Jun 18, 2021 at 14:54
  • Do you have a basement, or a downstairs shower, or is the downstairs toilet the lowest thing in your house connected to a drain? Was the bubbling spontaneous or when you flushed? Did the water level rise by itself, not after a flush? After the water level reaches its (new higher) normal level, if you run the bathroom taps at full force does the toilet level rise a little?
    – jay613
    Jun 18, 2021 at 14:59
  • @MonkeyZeus that's incorrect and misleading. Jun 18, 2021 at 15:03
  • If you're on public sewer then check if your neighbor is having the same issue. If they are then it's an issue with the main line at the street. If it's just you then you will likely need a professional's help. Either way, stop using water; both upstairs and downstairs.
    – MonkeyZeus
    Jun 18, 2021 at 15:03
  • 1
    @MonkeyZeus since you're being credited in the "update to the OP as answer", you may want to write up all your comments in an answer so the OP can accept it.
    – FreeMan
    Jun 18, 2021 at 17:49

4 Answers 4


When you flush upstairs, does the downstairs unit bubble or react? If not, then the clog is in the waste pipe close to the downstairs unit. If the downstairs toilet water level rises when the upstairs unit is flushed, or if a shower, sink, etc. upstairs is left running, then you have a clog farther down the waste-pipe line.
I wouldn't panic about septic tank problems just yet if there are no other signs (other backups or slow drains, and no soft spots in the ground near the tank or field).


Some guess work here but on the information we have so far, if it were me I would act and I'll describe how.

I'm guessing from everything you have a blockage in the drain out of your downstairs bathroom but before it joins the main line out of your house. Water from the blockage is rising up the vent that services this bathroom, that may be causing some of the bubbling. I don't suspect vent blockage is the primary problem, I think it's being blocked by the backup of water.

I think if the blockage was in the main line out of your house, or in the city sewer, you'd be having more serious issues ... all over the bathroom floor.

What I'd do, depending on how much time I had available:

  • Stop using the downstairs bathroom.
  • Keep an eye on how the downstairs toilet behaves when you flush or shower upstairs. If the water level rises a lot ... stop using all water in the house.
  • Call a plumber to remove the toilet and run a large drain snake through its drain
  • If you can't get a plumber, pour a whole bottle of main line drain cleaner down the toilet. But then wait at least a day and a few more flushes before snaking, and inform any plumber you've done this. You don't want that stuff splashing out at you.

The symptoms, especially the bubbling, suggest a problem with the venting of the drain system. Drains include vents to let gases ("air") in and out of the system. When a bunch of water runs down a drain it tends to pressurize air in the pipe on the downhill side of the water's source while at the same time creating a vacuum in the pipe on the uphill side of the source. Vents allow air in and out to neutralize these pressures.

Up on the roof of the house there should be several pipe stubs standing a foot or more above the roof surface. They're usually plastic, black or white in color, and 2-3 inches outside diameter. Have a look into each one, maybe with the help of a flashlight, to check for obstructions like an insect nest or other debris.

If the ambient outdoor noise isn't too loud (house not near a highway etc) it may be possible to put an ear near a vent pipe and listen for water flowing in the drains. While one person listens at a vent pipe have another person flush a toilet or run water at a sink below that vent. If sounds of running water can be heard through most of the vents, but there's one in particular where there's no sound audible, I'd try running a long drain snake (maybe 25 feet?) into that vent.

  • Would a venting issue explain Edit #2's "When the upstairs toilet is flushed the bottom toilet's water level goes down then back up again." statement?
    – MonkeyZeus
    Jun 18, 2021 at 15:54
  • @MonkeyZeus Yes, that could be a case of the "sometimes the P-traps get sucked dry" phenomenon, but with only enough suction to draw the water level down temporarily.
    – Greg Hill
    Jun 18, 2021 at 17:31
  • As @MonkeyZeus states, look for a blocked stack vent. Birds can build nests that fall in, rodents can crawl in and die, or other blockage could cause all those symptoms. Jun 18, 2021 at 22:13

So many different comments. First of all we need to know whether the OP is on sewer or septic. It makes a big difference.

I had a rental that was on a Glendon System (pressurized pumped system) The controller alarmed indicating there was a fault. The renters silenced the alarm and didn't tell me. It's a two story house (like the OP's house), they continued to use water, toilets, baths, etc. Because the pump system wasn't working they eventually filled up the septic tanks, then untreated sewage back flowed into the lower floor bathtub, sink, toilet and overflowed on the floors, carpets, trim, sheetrock. Had to call Service Master to decontaminate and make major repairs.

There can be lots of causes of plumbing issues like the OP asked about as others have contributed. I'm only sharing this experience to add to the discussion as a possible cause.

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