using a 3 inch vent pipe for sewer drain

[Davey Smith]

Bought an old house with one bathroom on the first floor. There is a large closet on the second floor that is directly above the first floor bathroom. There is a 3 inch cast iron vent pipe (that goes through the roof) that is tied into the kitchen sink and the existing first floor bathroom and it is connected to the 4 inch drain pipe in the basement.

I want to add a new bathroom in that second floor closet that is above the first floor bathroom. The 3 inch vent pipe is located in a "chase" that rises up through the second floor closet (future bathroom).

Is it possible to use that existing 3 inch vent pipe also as a drain pipe for the new fixtures that will be installed in the new second floor bathroom? It would also continue to function as the household vent pipe as well.

My builder says it is possible. He is a top-notch home builder and remodeler.

Any thoughts on using the 3 inch vent pipe as a drain pipe (as well as a vent pipe)?

Thanks

 

[MACPLUMB]

While your top notch home builder is OK but is not a Plumber, that is Illegal, you can run a sewer pipe down beside that pipe
and tie in the 4" sewer in the basement, then tie your vent into that existing vent though roof

 

[Sponsor]

 

[MACPLUMB]

The reason that is Illegal is because while you are flushing the toilet, and running other fixtures on the second floor then it is no longer "VENTS" the first floor fixtures that are also being used at the same time !

 

[Terry]

A vent can never be used as a drain for a floor above.
Take the new plumbing from above, and tie it in below, lower than those vents. Dumping waste down a vent will siphon the traps. It's the entire reason we have venting. Tell your "General" to stick to things he knows, or hire someone that really does.