View Full Version : This Old House Part IV

11-26-2004, 07:28 AM
I cut into a 4" drainpipe, planning to run a line up to the third floor for a bathroom. Guy in the home center says you can run two bathrooms off a 3" line, which has me looking lustily at the vent pipe which runs exactly where I need it to be. Why can't you just tap into that and call it a drain below and a vent above? (Don't tell the city). I know it's not done this way, but why wouldn't it work?

11-26-2004, 08:49 AM
Sewer gasses.

11-26-2004, 09:09 AM
But the water would drain all the way to the main line, thus making it a vent once again. The toilet would be offset about 3-4 feet and has a trap in it, so I don't see how this could be a problem.

Oyvind Snibsoer
11-26-2004, 09:42 AM
IIRC, that's the way it looks in my house. The pipe that the 1st floor WC drains into runs all the way over the roof. It's thus a drain pipe below the WC, and a vent pipe above just as you've described. If you think about it, the vent pipe actually has to hook up to the drains somewhere in your house; it's all in the same system.

11-26-2004, 12:02 PM
Um, the drain stack is the vent above the highest input ... each individual drain has a p-trap to stop gases from entering living spaces ...


[ 11-26-2004, 12:02 PM: Message edited by: brad9798 ]

11-26-2004, 04:22 PM
What you described is a "wet vent" and it can be done as long as the 3" has the capacity to drain all the appliances and fixtures.Wet venting is fairly common in retrofits and older homes.The toilet has a trap built in so gases shouldn't be a problem.
In a perfect world each fixture has a dedicated vent,but in the real world this is rarely done or even necessary.

11-26-2004, 05:01 PM
Thanks. As it turns out, it's a threaded steel 2" line - oops. I could cut into it but it might clog where it enters the main line, so I guess I'll go back to Plan A, run a 3" PVC line alongside it. That will enter the main line only a few feet from where the vent enters. Will that provide sufficient venting?

George Roberts
11-26-2004, 08:18 PM
Victor ---

Buy a code book. It will tell you how far from a trap a vent can be. It will also answer most other issues you might raise.