I have a 1989 two-story saltbox house with a walkout. On the top level are two full bathrooms including the bathtub. The main level has a half bath, a laundry room with a laundry tub and the kitchen sink. The basement has a half bath (with a cheater valve since it was not tied into the vent pipe), installed when we finished the basement 20 years ago.
A few years after installation we noticed that sometimes the basement toilet will fill up and slightly overflow with sudsy bath water when the top level bathtub is draining. No other water is ever running at the same time, just the tub draining.
The water from the tub goes down two stories and runs just a few feet from the toilet. If causes the problem, why doesn't it cause the problem consistently?
We have never had any problem with any general backups, slow or plugged drain or other drain issue and have therefore never called the roto-rooter. The drain line exits under the basement and out to the city sewer line. all drain pipes are PVC.
I'm in Wisconsin, but weather is not an issue. The drain line has never frozen and this has been a very mild winter so far.
Any ideas??



Edited 1 times.

1. Drain lines do NOT freeze, unless they are clogged and full of water, or you have a leaking faucet
2. Whether it happens will depend on how much soap is in the water.
3. The "cheater valve" can contribute because it forces any positive pressure to come up in the tub.

Think about it logically, when the tub water can't get downstream as fast as it's draining, it follows the path of least resistance which is the nearby toilet. So you have a partial clog somewhere downstream of where the tub line ties into the main. Why it doesn't happen all the time means the clog is intermittent.

Had a similar situation years ago where a root had entered the main line in the yard. Wasn't enough to block drainage by itself, but if someone flushed a lot of TP, the wad of TP would catch on the root and create a restriction. After a short time the TP would dissolve and pass the root and all would work fine again.

If it was me I would have a guy come and camera the drain line. Even if it's working fine at the time he may identify a spot where a partial clog could occur under the right circumstances. If you are PVC all the way to the sewer it's probably not a root, but could be something else.

I would think if it was a vent or design issue in the plumbing the problem would be consistent.

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Not a plumber by trade but a fierce DIYer

suds pressure zone:

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Hj id like to learn more about your comment that it could be soap suds, could you please explain how that happens? I have something similar happening with my washer and a floor drain which might be related! Thanks.

See Berns reference to "suds pressure zone". When water, and soap, free fall they get velocity, but when they hit the turn at the bottom, it creates a pressure change. when that happens the soap turns to suds, which increases the volume, forcing the suds out of any nearby drain. It is not common to residences, due to their limited height, but it is not impossible.

so try low suds soap, shampoo and laundry detergent first?

Wow that's interesting! I wouldn't think that suds, being mostly air, would be able to exert enough force to push past the trap on the toilet and overflow the bowl.

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Not a plumber by trade but a fierce DIYer

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Quote
buildermike
The water from the tub goes down two stories and runs just a few feet from the toilet.

So the bowl was installed too close to where the tub drains into the main? It says in the code residential dwellings and less than 2 stories exempt from 8' code requirement.

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Not a plumber by trade but a fierce DIYer