Can an open 1st floor toilet drain overflow if the 2nd floor toilet is flushed?

[hurleypatrick9]

I would like to remove our 1st floor toilet so I can redo the bathroom floors, vanity, walls, etc. A light remodel. I did this to the 2nd floor bathroom and it went surprisingly well.

My question is related to potential back-ups or overflows once I have the removed the toilet and 'opened' the drain flange/pipe.

When I did the job upstairs, I just packed the drain pipe full with a rag to prevent sewer gases from escaping and continued to use the shower, sink, etc. as needed. That worked well for the 2-3 days it took to finish.

My concern is this: once I remove the 1st floor toilet, what is stopping the water draining from the 2nd floor toilet or shower from backing up and flowing out of the 'open' 1st floor toilet drain? I understand there should be a Y junction for the 1st floor drain to connect to the waste pipe and this should 'theoretically' prevent the 2nd floor wastewater from flowing out the open 1st floor drain, but is that enough to rely on?

I can't see the waste pipe since its behind the 1st floor bathroom wall and then shrouded by a soffit in the basement. I don't suspect any blockages in our waste drain, but the thought of this going wrong has been stalling me from getting started.

Thanks for any help!

 

[Geofd]

Yes it could overflow,there is a fitting that's sold at home Depot ,Lowes or a umbing supply house, i call it a spin tight, it's a plastic+rubber
Fitting that tightens in with a wing nut