My basement drain started backing up and after giving it a good look things look odd and I'm wondering how exactly things are set up. Below is a link to a diagram I tried to make and I'll try to explain it. The only thing that causes the drain to backup is running 1 sink. After removing the drain cover and cleaning it out a bit things weren't as I expected. There is the horizontal pipe coming from the sink which water comes from. But then there is an elbow joint with no openings which is the first thing you see, then in the back corner (which took a bit to find) there is a vertical pipe coming straight up which seems to be what should be draining the basin but is clogged. This isn't a normal setup, right? Would that vertical pipe have a p trap? When removing the drain cover and another rubber seal the smell coming from the drain is awful.
The house was built in the 1890s and has had I think 2 major additions.