We're in the process of finishing our basement. I'm an experienced DIYer, but I don't understand why the plumbing for the current bathrooms is laid out the way it is. My plumbing experience has been replacing sinks, converting copper to CPVC and the like. I've never dealt with venting and sewer before. I'd like some advice on how our house is currently plumbed and how to change it to accommodate our new layout.
The bathrooms in the house are on top of each other. The lower bathroom area looks like:
I have two pipes coming out of the floor:
The smaller has an OD of 2.25" and is plastic. The larger has an OD of 3.25 and is cast iron. The upstairs bathroom has the toilet and sink connected to the larger pipe. This pipe goes up through the roof for the vent. This much seems pretty straight forward to me.
How the tub upstairs is connected has me confused. I've attached two shots, front and back of the plumbing connecting the tub to the stack:
The tub has it's own vent through the roof as indicated in the pictures. What I don't understand is why the smaller plastic pipe from the floor is routed around to T into the tub drain line. The T is angled up, so water can't flow that way.
So my questions are:
1) What is the smaller plastic pipe coming out of the floor used for?
2) What is the purpose of the plastic pipe to draining of the tub?
3) Should the basement sink be draining into the plastic pipe?
I'd like to relocate the tub drain to improve headroom in the bathroom.
4) Is it possible to drain the tub into the plastic pipe?
5) If not, what are some suggestions on how I could move that tub drain to get it away from center of the bathroom ceiling?
I'm open to any suggestions and/or criticisms.