Waste water backing up into basement shower

Hi everyone I am new here so let me know if there is a different forum to post this in!

I have been having multiple plumbing issues for a few weeks now. It all started when I was giving my son a bath in our upstairs bathtub. When I drained the tub (and flushed the upstairs toilet) water starting coming up from under the upstairs toilet and also pouring out the ceiling into our basement bathroom (which is directly under our upstairs bathroom). I had a plumbing company come out, he said that he thought the upstairs toilet was clogged so he took it off, snaked it, thought he unclogged it but after a few uses it backed up again.

The next day he came out, cut the basement bathroom ceiling out, cut the pipe from under the toilet, and snaked down it. He couldn't find the clog so he cut the drywall out down to the floor (which is the basement slab), ran a snake and then a camera down and said that he couldn't see because it was under water but he felt like he was pulling up mud and so he thought that there was a break in the cast iron sewer line that runs under the slab.

They came out a few days later, pulled up the flooring, jack hammered the slab, and replaced about a 5 foot section of cast iron pipe at the point where all of the outgoing lines to the house connected at a Y and a T. Then he back-filled the hole with the gravel and called it done. We still have an open hole in our basement floor that we planned to repair this weekend.

That was all 2 days ago. Then last night I come home and in my basement shower there is a ring of toilet paper and poop bits around the drain, but no water, so I guess it backed up and then drained back down again. No one has used the basement toilet so it had to have been from the upstairs toilet. As my husband and I were discussing what to do our 11 year old son tells us that yesterday morning (same day we found the paper in the shower) he used the bathroom and used "a lot" of toilet paper. When he tried to flush it clogged and so he plunged it and got it to go down.

Could a brief upstairs toilet paper clog cause a backup into the basement shower? Should I call the plumber back? Do these sound unrelated? They recommended replacing the cast iron with PVC all of the way to the street as a preventative measure but I just can't afford that right now, especially after all of the money I've already paid them. I feel almost 100% certain that if I call them back out they are going to tell me that is the problem (despite running a camera that way and saying they didn't see anything that needed immediate attention).

Thanks in advance everyone.. sorry to be so long-winded but I wasn't sure how much information you needed!

I would be hesitant to call that plumber back.

Did you see the section of plumbing that was removed? Was it defective?

The upstairs clog and plunge should not back up water by itself. You could consider a bidet seat that will cut down on toilet paper use.

Was the line to the street checked? Do you have a cleanout that could be used to rod the sewer to the sewer main at the street? If so, I would either carefully open that to see if there is water standing there after bigger water use. If there is have a rodding company rod that line to the street.

Are there more cleanouts? I would inspect them. Is there a cleanout in or near the new under-floor section? If not, maybe consider having one added before the concrete work would be in order. I am not a plumber.

It may be that the plumbing is not designed that well. It could be your recent plumber was not that effective.

I would set a water alarm by the basement shower drain. I like the Glentronics Basement Watchdog Water Sensor and Alarm. It is pretty inexpensive and louder than some.

I did see the section that was removed. They called me down to look before they removed it. It was a corroded part in the cast iron pipe, on the belly of it. It was still attached and covered in sludge but when they flushed the basement toilet a few feet away the hole filled with water so I assume it was broken.

They told me that they were going to check the line to the street but I am not sure if they actually did :/ When I next checked they were back-filling the hole. One of the guys said that he recommended replacing the cast iron to the street as a precautionary measure, but that they had fixed the immediate problem.

I do have a sewer cleanout that is in my garage. When you open it there is about an inch of water in it. The plumber who fixed the other issues told me of this before he ever started any repairs and said that about 50% of the houses in our area that he saw had a "belly" and that they usually didn't cause a huge problem. He also said he thought that our sewer line had "fallen back some" but that it was slight and also not a big problem. I assume he meant that it was no longer at the ideal angle for gravity to pull the water to the sewer main at the street.

That is the only clean out that i know of. it is about 10 or 12 feet from the area that they replaced. There isn't one outside.

Thanks for your help so far I have called the plumbing company back to ask if they checked the line to the street but haven't heard back. I will post their reply when I do.