Water Backing Up Into Your Tub When Toilet is Flushed? Your Sewer Line May Be Clogged

So you flushed your toilet and noticed water backing up into your bathtub, huh? That’s gross. But it’s not as uncommon as you might think.

And the usual culprit: a clogged sewer line.

How a clogged sewer line can cause your toilet to backup into your tub

Take a look at this diagram of a (very simplified) branch of a home plumbing system. It shows a typical bathroom with a sink, toilet and bathtub or shower.

When there are no problems, a flushed toilet will go straight down the drain into the sewer.

But if you add partial clog to the sewer drain, the sudden flush of water can’t all make it down the drain. Instead, it backs up and flows out the bathtub drain. If the clog is especially bad, you might also see it come up the sink.


The water then slowly drains out of the tub through the small opening remaining in the pipe. 

What causes sewer line clogs

There’s a whole host of things that could be causing the sewer clog. Here are just a few:

  • Tree roots—Tree roots are great at finding water. So good, in fact, that they can get into your sewer line and block it.
  • Grease—Grease is liquid when you put it down the drain, but it can solidify in your pipes, collecting other waste as it passes by, creating a clog.
  • Feminine products—Flushing tampons down the drain is sure to cause problems. Don’t do it.
  • Toys—Have a little one? They might have flushed something down the toilet without you knowing. It’s been known to happen.

“But my sink works fine. It only happens when I flush the toilet!”

That’s common. Faucets and showers don’t send as much water down the drain, so a small restriction in the sewer line might not cause the water to backup elsewhere. 

A toilet, on the other hand, releases several gallons of water all at once. 

“But this problem just started!”

Sewer problems do tend to build up slowly over time. However, they can also occur suddenly, like in the case of a flushed toy. 

How to find and fix the sewer problem

The sewer line will need to be snaked and/or jetted to clear the blockage. Snakes work for harder blockages (toys and foreign objects) while jetting is best for soft blockages (like grease buildup). 

Get a professional plumber to take care of the problem, otherwise you could end up doing more harm than good. A professional plumber can also do a video inspection to make sure there are no other underlying problems (like tree roots).


Benjamin Franklin Plumbing serves Hastings, Minnesota and the surrounding areas, including  Apple Valley, Burnsville, Cottage Grove, Eagan, Woodbury, Lakeville, Rosemount and Stillwater.


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