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Old 09-26-2011, 11:41 PM   #1
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Basement plumbing layout advice

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:

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I have two pipes coming out of the floor:

Basement plumbing layout advice-stack_sm.jpg

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.

Basement plumbing layout advice-upstairs_sink_toilet_sm.jpg

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:

Basement plumbing layout advice-upstairs_tub_front_sm.jpg

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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.



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Old 09-27-2011, 03:09 AM   #2
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That ABS pipe coming up out of the floor is the vent for the toilet. And it looks like there used to be a shower also? It's drain vent is probably connected to that one. Both are under slab.

For drains to work correctly, they must be able to get air behind the water going down. Especially toilets, which will pretty much fill the pipe at initial flush. That ABS pipe was their attempt to vent the toilet and shower.

That is why it goes up to the tub drain. Trying to get as close to that existing tub vent as possible. But it should not be connected to the drain. It should connect to the vent, above the drain. Although that would have been much harder. Some would argue it should connect to the tub vent above the level of the top of the tub.

And then there is sizing. Every drain - toilet, sink, tub, shower, etc. - needs a vent. and the more vents connected, the bigger the final vent pipe should be.

That sink down there is draining into the ABS pipe also. They call that a 'wet vent'. Meaning putting drain water down a vent pipe. Although not against code, it has restrictions and limitations. (actually, that shower drain is probably a wet vent also, which would further limit using another one, in the same vent.)

So, given all that. question 1 answer is ; the vent for basement toilet
question 2; The plastic (abs) pipe does nothing for draining the tub. question 3; Probably not. ques 4; Yes, with other changes.

Does the house have a second story? How hard to get another pipe up to roof? What I'm getting at, that pipe coming out of basement floor should go straight up and connect to main vent stack ABOVE where upstairs toilet connects.

With that done, you could take out the steel tub drain, (not the vent) and then connect tub trap to ABS pipe (re-attaching vent) running up inside joist to wall and over to stack, where you would cut stack and attach.

Sure sounds simple, but working with that old cast iron is not.

And be aware. That stack of cast iron pipe going up through the roof is HEAVY. It must be supported before cutting. Not always easy.



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Old 09-29-2011, 09:56 PM   #3
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Thanks for the information. That was enough to know that I wanted to consult with a plumber on connecting to the cast iron. I also need to move the location of the toilet out from the wall and change the location of the shower drain, so again a plumber may be helpful in planning that.

It looks like I have some concrete busting in my future! :-)

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