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The bath and sink are connected to the same pipe as shown on schematic below - pipe has a gradient downwards towards the main pipe (yes, I have found some people fit it with an upward gradient!).

When I empty a bucket into the sink or empty a full sink, the bath starts gurgling or burping, sending water through the waste outlet into the bath. Note there are no blockages anywhere in the system.

  1. Is there anything that can be done to prevent this without connecting all bath components directly to the main pipe?

  2. Can multiple bathroom sinks, baths, showers etc. share a common drain pipe and if so, are there any rules?

UPDATED SCHEMATIC after posting to show toilet - this works fine and does not cause any problems to the sink or bath when flushing. updated

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    A vent is plugged or non-existent.
    – Tyson
    Commented Jul 9, 2018 at 11:56
  • So go up on roof and run water from a garden hose down vents which might be plugged? Maybe put a generous squirt of dish washing detergent in before the water. At first be sparing with amount of water put into vent until one is sure this would not run out inside the house. If vent would fill up, then let it sit and see if detergent and water would dissolve clog.
    – Jim Stewart
    Commented Jul 9, 2018 at 12:08
  • @JimStewart adding liquid plumber or something like that to the drain vent pipe might be helpful if it is allowed in his locality.
    – Ken
    Commented Jul 9, 2018 at 12:13
  • Now thinking about this more. Before going up on the roof try clearing from below by filling the lavatory and the tub with hot water from tap with detergent. Open drains and use a plunger on the tub drain. Push softly and pull quickly. This might clear a plugged vent.
    – Jim Stewart
    Commented Jul 9, 2018 at 12:33
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    Doesn’t the sink drain line need a branch vent? When the sink drains it’s pulling on the water in the trap of the bath, isn’t it?
    – paul
    Commented Jul 9, 2018 at 20:02

5 Answers 5

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T Piece

Is the bath trap "gurgling" because of bubbles coming out of the trap, or because of air being sucked into the trap?

This is important, because I suspect the T Piece could be the issue. Since it's not a gentle bend pointing toward the waste pipe, when the water hits the T, it could go upstream, compressing the air up to the trap, and bubbling out.

Alternatively, if the force of the water is sucking the air from upstream, the trap will lose water and air will get sucked in from the bath.

See this page about vertical waste drops. It says a Sanitary Tee is incorrect for vertical downpipes:

When connecting any vertical waste drop to a horizontal run the proper way to do it is to either use a Wye & 1/8 bend (45 deg), or a Sanitary combo. A sanitary combo is a combination Wye & 1/8 bend already made in one fitting.

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    this is very useful and a potential solution along with the air vent answer too
    – onepound
    Commented Jul 10, 2018 at 9:36
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    I've chosen your answer because yes there is a t -piece and it should be a wye. Though other answers are interesting yours is the actual problem no blockage.
    – onepound
    Commented Dec 23, 2019 at 14:42
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1:Can multiple bathroom sinks, baths, showers ect share a common drain pipe ? Yes they can and do all the time.

If so are there any rules? of course there are the International Plumbers Code being one of them - and depending on what other rules your locality has adopted.

Now on to your issue :

  1. Did this problem just start (on previously working system)?
  2. Is any of it new construction ?

Potential Causes:

  • Your drain from sink to drainage pipe is partially blocked!

  • Your vent is is either blocked or partially blocked!

Solutions:

  • Run a plumbing snake down the pipe towards the sewage drain.
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  • 1. Did this problem just start (on previously working system)? answer: It has done this from new.
    – onepound
    Commented Jul 9, 2018 at 16:00
  • 2. Is any of it new construction ? no it was built with house years ago 1980's
    – onepound
    Commented Jul 9, 2018 at 16:01
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    Your drain from sink to drainage pipe is partially blocked! I'll look at this to make sure.
    – onepound
    Commented Jul 9, 2018 at 16:01
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    @onepound If from new: A rag or construction debris could be stuck in that vent pipe, they might put something in during construction or some site visitors might do something like that. The drain pipe might show a down angle but is it really .. That would be something to check - but I venture a good guess you have a blockage in the vent pipe or the drain.
    – Ken
    Commented Jul 10, 2018 at 4:21
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If your diagram is correct, the problem may be that you don't have a vent for the sink at all. There should be a loop that goes up from the sink, then tees into the vent pipe.

Vents allow air to follow the waste water; otherwise, the water flowing down the drain creates a vacuum, pulling the water from the trap along with it. In your case, water starts flowing down the pipe in the sink. As it falls, it 'pushes' air out in front of it, up the vent pipe. Once all the water has drained, it sucks air through the sink's trap - you probably don't notice the gurgling from that, since it's following the already gurgling water. Once the 'end' of the water hits the tee, it applies suction to both the sink trap and the tub trap; whichever is shallower will gurgle (in this case, your tub). The toilet doesn't cause problems, because one it acts as its own vent, and two it has the stack vent to draw from.

If water down the sink is causing the tub to gurgle, it's likely that water down the tub will cause the sink to gurgle. Dump water into the tub, and as soon as it drains, plug the tub. If there is no vent, you will hear the sink gurgle as it equalizes the pressure by letting air through.

The difficult fix is to open your wall and install a vent for the sink - and maybe for the tub too, depending on distance.

The easy fix is to get an under-sink vent; I haven't used them, nor do I know if your building code would allow it, but as long as it's legal, it would be the fastest and simplest option: you won't have to open the walls!

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  • Sink vent a very useful answer and a potential solution along with the wye answer too though this might work out the easier of the two to implement as the T-piece is concealed in a box that may not have enough room to accommodate the Wye.
    – onepound
    Commented Jul 10, 2018 at 9:38
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The behavior you described might mean your sewer line is be clogged, roots and flushing large items can cause this...

If draino doesn't fix it, might be time to call a professional, they're not too bad on cost, just don't get any of the extra services lol.

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  • I'm thinking it might be more to do with bath trap being lower than the shared waste pipe.
    – onepound
    Commented Jul 9, 2018 at 18:20
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Have a similar problem with poor plumbing in the bath. When pipes are clean, things are fine, but when they become partially clogged, then water starts flowing out to the bath floor when washer machine drains water.

The only solution for me has been steel plumbing snake. You need two people and it should be easy unless you have too many turns in the system. I do have but it has been tolerable so far.

No amount of pipe cleaning chemicals was able to help so I tend to spare the money and avoid destroying environment by using them.

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