Advice for preventing toilet from backing up into shower trap

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seanp

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So I was feeling good about my DIY plumbing work until I discovered today that the toilet has been backing up into the shower trap. Now I have a real dilemma on my hands. This is part of a remodel of a small bathroom to add a shower to what used to only be a toilet and sink.

Details: 2nd floor/half story; undersized 2x6 floor joists; no way to elevate the floor without a major remodel

In order to be able to add a shower, I had to move the toilet from its original location and go to a wall-mounted model so I could run the waste line through the wall rather than bore through what are already undersized joists. The toilet waste 45s into a wye where the shower waste connects. Due to the undersized framing, I couldn't roll this wye to 45, but it is slightly elevated. With only liquid waste on a full flush, the water level in the shower trap doesn't budge, so I thought I was good to go. However, the discovery of toilet water in the trap today after a week of use suggests that solids and paper on a full flush provide enough of a slowdown that its backing up into the shower line.

I think when the shower is hooked up it will wash the line, but I know there will be problems down the road if I don't fix this. Due to the 2x6 joists, I just can't elevate the 2" shower line and come in from above the 3" waste. Ideally I would go all the way to the outside wall where the cast iron turns vertically and tie in there, but I looked today and it would be extremely difficult to do this -- tight space, lots of HVAC ducts. Would it make a difference if I tied in the 2" line further down the 3" line so it's not right after a 45 turn? Seems like this might ensure that everything is flowing straight before the 2" enters. Are there any other ideas that I'm just not seeing?

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FullySprinklered

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The problem must be downstream. One jurisdiction I've worked in would not allow flushing a toilet past a pipe of a smaller size. You could do a 3" wye and immediately bush it down to 2" for the lav or tub, but I don't see where this has any bearing on your problem. Something is backing things up downstream of the shower, looks like to me. Got kids? Might be a fistful of Legos.
 

seanp

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fullysprinklered -- thanks for the response. At least it sounds like I haven't done anything obviously wrong -- I'm doing my best to do everything by the book with my limited space. Your question about kids got me thinking though. Yes, I do have kids and while I'm pretty sure it's not a fistful of Legos (that would be bad!), I'm starting to wonder about toilet paper volume and the flush mechanism. The toilet is one of those Euro jobs with options for a half flush or a full flush. I told them to use the half flush for "number one" and full flush for "number two". However, since kids use way more toilet paper than is necessary, I'm wondering if a series of half flushes that each had a lot of paper could have created a backup. I'm pretty certain there's no significant blockage downstream, but I could see where the half flush isn't enough to push everything all the way through the cast iron -- maybe toilet paper gets dropped part way on a half flush, then the next one just keeps building up until things start backing up. So it's enough to back up into the shower eventually, but not enough that a full flush doesn't push everything through and start things over. I'm going to try testing that idea by using only the full flush and keep an eye on things. Thanks again for your response -- it will make my week if the solution is this simple.
 

Jadnashua

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FWIW, it is not uncommon for solids to not make it all the way to the sewer with one flush! If the pipes are sloped properly, the liquids will, and the solids will get moved along when the next flush, or other water use. The dual flush toilets are not really designed to move anything other than liquids out through the thing when using the lower volume...IOW, it may not clear any paper that might be used except when the larger volume is activated.
 
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