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My basement bathroom has recently been remodeled, everything was working fine for a few weeks, but now all of a sudden I am hearing bubbles in the toilet when I shower, at which point the shower drain begins to back up. The shower drain runs downstream to the bathroom sink (which has a vent stack) then to the toilet. I have not had any problems with the sink drain slowing or backing up.. just the shower and toilet. Within an hour or so of the shower draining out I can use the toilet again without any problems, but when the shower is used again the same problem persists.. What is causing this?

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Are you sure that the shower goes to the sink, then toilet? If the sink is not affected, my guess would be that it sounds more like the sink is closet to your main pipe, while your shower and toilet are towards the end of a line which was not been properly vented. The vent allows the water to flow, but improper venting will create a vacuum, causing restricted flow and backups. In this case, the toilet is attempting to provide airflow, causing the bubbling sound. If it was recent, I would ask your remodeler to fix the problem. –  Jacob S Jul 1 '13 at 19:38
    
Do you have a septic system, or public sewer? If you have a septic system, has there been a lot of rain lately? –  Tester101 Jul 1 '13 at 22:57

2 Answers 2

Sounds like the main drain for the bathroom may be restricted.

As you shower the water doesn't drain away fast enough, and the plumbing begins to fill with water. As the pipes fill, air may be forced out through the toilet trap causing "bubbles". Since the tub is the lowest drain, the water backs up there first. If you continued to shower, you'd eventually see the toilet overflow as well (if the tub edge is higher than the toilet rim). If the tub was deep enough, you'd eventually see the sink overflow too.

If you look at this crude representation of your bathroom, you'll be able to see what I mean.

Bathroom

Now, if we add in a clog we can start to see what happens when you shower.

Bathroom with clog

As you shower the water cannot pass through the clog fast enough, and the pipes begin to fill.

Bathroom starting to back up

At this point, any air trapped in the line between the drain and the toilet trap will be forced out and bubble up trough the trap.

Air bubbling through the trap

As you continue to shower, the tub is the first to back up.

Bathtub backing up

Eventually, if you shower long enough the toilet could also overflow.

Toilet overflow

Depending on the height of the sink, quite a bit of the bathroom would have to fill before it overflowed.

enter image description here

The first thing I'd try is to clear the drain using a Plumber's snake. A hand auger would likely work, and can be purchased at a local hardware store fairly cheaply. Drill powered augers are also available for a bit higher price, but require a bit more of a gentle touch (and obviously a drill).

enter image description hereenter image description here


Another option is that the vent for the bathroom is clogged, or not sized appropriately. If vent stacks are not covered properly, leaves and other debris can become lodged in the pipe and prevent air flow. A clogged vent will cause the fixtures it's venting to drain slowly, and the results can be similar to a clog as described above.

As with the clog, the solution to clear the blocked vent is a Plumber's snake.

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On review -- I agree with this as being a plausible explanation (was originally my first thought, but I didn't account for the vent equilizing the system). When the backup occurs, the simple test would be to run the sink and check whether the water level in the shower goes up. –  Jacob S Jul 2 '13 at 12:47
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I love & hate snakes. When using a snake, be sure to take small steps. Run the snake in a short way, pull it out, clean it, then repeat a little further in. If you try to go the whole way at once, it will get stuck on a bunch of gunk and be very hard to remove & clean. It helps tremendously if you have access to a drain cleanout, so you don't have to push through the P-trap. Adding a clean out is a great project for the ambitious DIY-er who isn't afraid of the muck. –  Jay Bazuzi Jul 3 '13 at 19:21

I do not agree with Tester's assessment but I do agree with his pictures. If the shower is not vented the air in the pipe will go out the easiest place which is the toilet - and this is a plumbing design flaw. If you had a restriction then you would probably have water backup in shower.

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The OP said that the shower is backing up "at which point the shower drain begins to back up". –  Tester101 Jul 1 '13 at 22:53
    
a vent is not necessary for draining the water. It is only there for two reasons. 1, to allow for the escape of sewer gases. 2, to prevent siphoning out of the traps when water rushes past the opening. –  user18713 Dec 18 '13 at 22:32

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