I have a 6′ claw foot tub on the second floor of my house. The drain flows through a P-trap into a 2″ line that connects to a 3″ stack that drops straight to the basement. In the basement, the stack is 4″. All of this is PVC.

As the waste line goes into the floor, it is converted to 4″ cast iron and goes all the way to the septic tank from there.

There is a shower and a toilet just upsteam of the main stack in the basement. Recently, when the wife drains the tub, water and bubbles push up out of toilet, but not out of the shower. When the tub is draining, I can hear bubbling in the toilet.

A quick search of the internet suggested that:

1. The water volume was too great for the waste line to carry.
2. The waste line is clogged.
3. The tub isn’t properly vented.

My response to the first is that simple physics implies it would take a lot of pressure for a two inch line to fill a four inch line enough to back it up.

My response to number two is why isn’t water backing up into other fixtures like say the shower pan or sink right next to the tube? All other fixtures in the house appear to be working normally.

My response to number three is why isn’t the tub trap being sucked dry? The tub vent appears to be working properly.

So, before I run out to get some stuff and snake the waste line from the stack to the septic tank, I thought I’d see what some others with more experience than I do would have to say.

Suggestions? Thanks.

JL.