I have 2 separate fixtures (bathtub & shower) in the main bathroom which both drain into the same 1 1/2 inch ABS pipe through a Y fitting and then about 12" later drain into a 2" vented drain. When a full tub of water is drained I get a 'burble' of water coming back up through the shower. Both the tub and shower have their own separate P traps. Is it okay to have 2 fixtures draining into a single line prior to draining into the main 2' line? Is it possible I'm having a venting problem?

First, all showers should be piped in 2" drain piping (that is code everywhere that I know of), and the tub is OK on an 1.5" drain. Now, in terms of your question, I think you nailed it...sounds like a venting issue to me. Here, in most states, it is OK to have two fixtures drain into one line as long as one of the two fixtures has a vent. Since it sounds like you have two unvented fixtures draining into a "2" vented drain", it certainly sounds like a problem with venting, for sure!

At a minimum, you could install an AAV (mechanical vent), or if codes in your area don't permit these then cutting into the drains and installing vent(s) is the only way to fix this issue!

Back to you...

Mark

First, all showers should be piped in 2" drain piping (that is code everywhere that I know of), and the tub is OK on an 1.5" drain. Now, in terms of your question, I think you nailed it...sounds like a venting issue to me. Here, in most states, it is OK to have two fixtures drain into one line as long as one of the two fixtures has a vent. Since it sounds like you have two unvented fixtures draining into a "2" vented drain", it certainly sounds like a problem with venting, for sure!

At a minimum, you could install an AAV (mechanical vent), or if codes in your area don't permit these then cutting into the drains and installing vent(s) is the only way to fix this issue!

Back to you...

Mark

Thanks Mark. I may not have explained my problem correctly or I'm missing something from your answer. I have attached a drawing to explain it better. Hopefully this helps...

The drawing you posted backs up everything I said, however, I may have missed the idea that all this plumbing as presented worked just fine up until awhile ago, right? If that is the case, then the common drain to the tub and shower is clogged and the drain needs to be snaked to clear the PARTIAL blockage...;)

If this is a question about how the drains are piped, then I was correct in stating that you have two unvented fixtures combined into one drain and that is what is causing the "burble".

Let me know more, OK?