Plumbing size issue

I received a call from the buyer agent regarding a home I inspected last November. The new owners apparently flushed two toilets at the same time and they backed up. A plumber said that the pvc fixtures and sizes are incorrect and connecting two bathrooms into the same pipe is incorrect. I did not note these things in my report so I am being asked if I should be liable for repairs. I told the agent that I flush one toilet at a time and did not note a back up. He nicely said not to worry about it and that he would pay the $4,100 to fix and replace the cast iron (though it shows no problems) and would just write it off.

If I should have seen this, I’m willing to bear the cost, but I’m not certain and wonder what you all would do.

Here’s pic and screenshot of other crawl space pics FYI.



I believe 4" can accomodate 2 toilets, unless both are coming down the 3" PVC, but you are NOT a code inspector.

Lets see what @mwilles has to say.

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Looks like toilets on a 3” horizontal I don’t see anything wrong with that. The rubber couplings are incorrect those can only be installed underground.

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What did they flush down the toilets at the same time?

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Apparently there are roommates sharing the home. I guess they happen to use them at the same time.

Plumbers have a way of making work for themselves when homeowners don’t know any better.


Has anyone shot a video camera down the cast iron from home to steet? Maybe that’s where the problem is regarding the backup.


There is a clog in the main pipe somewhere, the inside diameter is reduced. When you flush one toilet there is not enough discharge volume to overwhelm the restriction point, so the backup never occurs. When you flush 2, water fills the pipe and starts to backup.

A 3inch main can handle 4-6 toilets (with nothing else flushing) without a problem when everything is installed correctly and pipes are clear. If you step up to 4inch, you can have a ton of toilets for a residential dwelling :slight_smile:


Codes are different for how many water closets you can capture on a 3” horizontal line. The strictest codes that I know of were in Illinois where you could only capture 2 water closets on a horizontal 3”pipe. After that it had to be 4”. Keep in mind a future bathroom rough in inside of the basement would be considered an additional toilet.

This is about 2 toilets flushed at the same time “causing backup”. Point being, it’s not an issue with number of toilets or size of the main line as the buyer agent is claiming the plumber is telling them. The strictest code you speak of took into account other fixtures, not just water closets. I was strictly talking about a number of toilets a main could handle when connected to it and used at the same time, thus my remark “nothing else flushing”. Hope this explains better :smiley:

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It does Simon but the code I mentioned from Illinois is strictly water closets and no other fixtures, as mentioned. 6 water closets are allowed on a vertical 3” pipe before increasing to 4”.

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2 WC on 3" horizontal line and nothing else, is this true even today? That’s extremely limited. How would 2 full bathrooms connect?

Thank you guys for your input. I suggested the agent get a second opinion from a company that could scope the pipes before he pays a plumber to replace everything.


The code specifically spells out no more than two water closets on a 3” horizontal pipe. If other fixtures are included such as lavatories and showers they are included in the total DFU’s. This is why I always say if there’s an issue with plumbing contact a local plumber.

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You’re referring to a horizontal branch, not a building drain (what I was referring to), which in IL must be 4in and can handle 180DFU @ 1/8" per foot grade. In IL, the 3" branch is not limited to 2 water closets only as far as fixtures. There can be 2 water closets and other none-WC fixtures up to 20DFU on 3" branch, just like I mentioned above (PART 890 ILLINOIS PLUMBING CODE : Sections Listing) or IRC. Sometimes, Martin, you make me think I’ve lost my mind :rofl:

I thought that’s what I said :slightly_smiling_face: I was only talking about water closets and no other fixtures. I did not say the pipe could not contain other fixtures. The code is very specific on water closets only. Another code will reference the total DFU’s on the said pipe. This is another example of contacting a local plumber that is familiar with local codes. I was in that jurisdiction for 20 years.


The Tee fittings might be wrong.
Where’s the circuit vent for the sink?

Martin, I enjoy your expertise here. I recently had the same issue and after going back I discovered no dedicated vent stack. Could that be the issue, or at least a significant factor?

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Daniel it’s possible that the vent may have been tied into a main stack in the attic with only one protrusion through the roof.