I've been living in a fixer-upper for a while... I've been fixing a lot that was previously done wrong.

I've finally gotten around to ripping out our first floor bathroom (two full bathrooms) and I'm noticing some plumbing issues. Mainly that there's what seems to be a vent pipe terminating into my wall. So yeah, there are no proper plumbing vents in this house. Surprisingly I haven't yet had any problems, but now that I've got a bathroom torn out it seems like I should fix it correctly.

I've made a diagram of the existing... situation. The large pipe is 4" PVC, and the smaller pipes are 2" PVC — I don't have wall access into the second floor bathroom at this point, but from what I can tell by the first floor, everything is draining into a single 4" pipe.

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Edit: looking at the existing work some more, it just seems better to redo the entire waste line system. Here's my plan. Any red flags here?

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  • Great question! Just wondering, where and when was your house built? Dec 24, 2015 at 17:39
  • the house was built in the 1920s — it's a 1.5 floor cape in New Hampshire. It was gutted and renovated in 2007... they pulled out all the old plumbing at the time, didn't get permits, and left me with this mess. Surprisingly the plumbing has been fine for 2 years despite the obvious issues. Dec 24, 2015 at 18:57

3 Answers 3


I am not a plumber but I think your plan is just fine according to these tables:



A 3" pipe is plenty for the number of DFU's in your stack and horizontal run. The 2" vents on the sinks are probably even oversized and so they should be plenty.

The only issue I see is you can't take sink through the roof as a 1.5". At least not here in the north where it could frost over.

Have you considered tying the kitchen vent into your main vent and avoiding another roof penetration? Just a thought.

The only time plumbers around here use 4" in a single family home is for the main sewer exit from the house, and then only a few feet of the exit itself.

Good luck!


Unlike what most people think, a vent is not always needed. You said that in the old situation, all small pipes were 2". Often, 2" pipes are used to prevent the need for vents. I installed all of the plumbing in my house, using 2" pipes going to 4" where they come together (to be more precise, I used 50mm and 100mm pipes, which are the metrical aquivalents in Europe). I have no vents at all, mainly because they are known to sometimes cause a stink where the vent reaches the surface (which is why they normally go to the roof, where you never go, and therefore never smell it. Only I have no areas where I can let them surface without the risk of causing a stink.) So I checked it out, and there is no real need for vents, as long as you use pipes that are significantly bigger than the drain of the appliance connected to it (eg the sink or washer, which have 32mm or 1,26" connections). 2" inch pipes are big enough, as they allow the water to move in a tumbling motion when going down (the water volume is reduced by the smaller outlet of the appliance). Therefore you never build up a negative pressure in the pipes... If you use 1,5" however, you will build up negative pressure, and therefore you need a vent.

  • Professional plumbers will likely disagree, but your explanation seems to make sense. Here in the Philippines (in single family homes), all main pipes including the toilet pipes are 4" and the sink pipes are 2", which in comparison to US sizing is quite large. I don't see roof penetrations anywhere on middle class homes, and our plumbing engineer did not indicate venting on the submitted building permit plan. Our builder included two main stack vents for 4 bathrooms, and we requested one roof penetration. Almost no other venting was added, and so far everything seems to work fine.
    – ChrisW
    Apr 8 at 12:41

Looks pretty good. I'd suggest a 3" vent for the washer (for a top loading unit's dump), a 2" vent for the kitchen's dishwasher dump & 4" for all of the 3", but I get the framing issues so really no problem there. You can vent through a sidewall if you're 4-ft (check your codes) away from any doors or windows (the vent just needs to be higher than that branch's sink). It really dedicates the venting a whole lot better & I hate roof penetrations.

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