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Old 02-01-2011, 02:08 AM   #1
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replace cast iron soil stack

We are re-doing our bathroom, and when we removed the wall we found the wall was built with extra studs laid flat against all the existing studs to build the wall out an extra 1.5" to accommodate the soil stack. Our bathroom is very narrow, and we would like to gain the extra inch and a half. Will a 3" pvc pipe fit within the wall (sounds like O.D. of a pvc 3" pipe is 3.5"), or will we still need more room since that will leave no extra space between the drywall and pipe? The soil stack currently vents the toilet, and bathroom sink that you can see attached to it in the picture. The shower also connects, but it also has its own vent.

If a 3" pipe will fit, could I also drop the sink drain below the floor, and then hook into the vent stack there? I believe it would be about 4' away from the p-trap for the sink. I have been searching, and it sounds like the minimum distance might be 3'?


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Old 02-01-2011, 07:18 AM   #2
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3" PVC will fit, and is used in modern day drain/vent schemes.


When its all said and done there is usually more said than done
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Old 02-01-2011, 07:19 AM   #3
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Local code may allow 3", but I would not advise changing it from 4".

I seen wood lath plaster and galvanized supply plumbing, which dates the house aas pre 1940's. I would definitely get rid of the galvanized supply, however. Cast iron is supposed to be good for about 100+ years. Replacing it with 4" PVC and fernco hubs might give you a little more room. But the plumbing will be much nosier.
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Old 02-01-2011, 06:54 PM   #4
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You could use 3", in fact, the code I run with would allow 2". However, I still need vents that are collectively equivalent to 4"(or bldg drain size) at their termination. I'm assuming this 4" is about the only vent stack in your house, except maybe the kit sink. So, I would leave it.
Not sure what your plan is for the lav. Sounds like an illegal s trap configuration. Can you explain more?
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Old 02-03-2011, 02:08 AM   #5
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Thanks for all the replies and also pointing out the s-trap. I looked into it, and that was probably where I was heading with it. The main reason we are replacing the stack is to be able to remove the extra studs turned flat against the existing studs to open the bathroom up a little more.
I imagine, to be able to get the 3" pipe in the wall, I could not have any other joints or fittings in the wall since they would stick out farther. Would my other option for the lav be to have a vent go directly up once in the wall, and then connect to the stack in the attic before exiting the roof? That way I could then have the drain drop down through the floor and connect with the soil stack there?
To answer one of the other questions, the shower has its own vent, along with the kitchen sink.

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