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One of the DIY projects I sometimes contemplate is replacing the ~45 year old galvanized steel drain pipe in my house with PVC DWV pipe and fittings. There is at least one slow, dipping leak I currently have a bucket underneath. There are also other connections which have been "repaired" by wrapping them with black electrician's (?) tape. My feeling is that even I could do a better job than what is there now.

One of the things I thought I would do if I ever actually did this was use slightly larger pipe. However, while browsing the big box store websites, I got the impression that the most common DWV pipe they seem to offer is either 1 1/2" or smaller and 3" or larger. In particular, 2" PVC … the next step up from 1 1/2" pipe … doesn't seem to be offered in the bulk 10' lengths which the other diameters are available in.

Is my impression about this correct or am I just looking in all the wrong ways? If it is true that 2" is not as popular a flavor then, well, why would that be the case? What (obvious?) aspect about how DWV installations "work" am I not understanding/grasping?

Below is some additional background about my home for context.

My understanding is that most of the plumbing & electric in my house was put in by the previous owner when it was built in ~1950. The only drains in the house are for

  1. The kitchen sink.
  2. The bathroom sink.
  3. The shower/bath tub.
  4. The toilet.

The toilet sits right over the main outlet pipe leading to the septic, so there is essentially no additional drain pipe for it. All of the other drains feed into this main septic outlet underneath the toilet. There is only 1 vent pipe and it also connects to this septic outlet pipe.

The other drain pipe consists of about 32' of galvanized steel pipe.

The main drain line is (unvented) 1 1/2" pipe from the kitchen sink's trap to outlet to the septic. The path is ~6' from the trap to a "T" with a plug on one end. The other end of the "T" runs ~13' to reach the outlet to the septic.

The bathtub drain has a straight run of about 5' of 1 1/2" pipe into another "T" into the 1 1/2" to the septic outlet.

About 8' feet of 1 1/4" pipe is used to connect the bathroom sink to the 1 1/2" pipe.

Basically, because the setup is not that big and most of it can be directly accessed, every time I consider making a spot fix … during which I may break some other weak spot … it seems to make more sense to just yank most of the existing pipe and replace it.

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Find a better store, if the ones you are going to don't have 2" PVC in full sticks (or possibly look higher up in the store you are in, and get someone to drag it down.)

In MY house, the only place you'll find 1-1/2 or 1-1/4" is the tailpieces on the sinks - as soon as it hits pipe, it goes up to 2", because it's not that much more expensive and it greatly reduces the odds of having any issue with it. I don't recall having any trouble finding it in full sticks. The main line is 4" (not 3") for the same reason. Builders looking to squeeze maximum profit don't do that sort of thing, but builders who plan to live in the building for a while sometimes do.

In retrofitting, you might need to use 1-1/2" due to space constraints. In any case, if you are redoing the pipes, you should probably bring the venting up to current code while you are at it.

  • About venting. Partly I'm thinking long & hard about this before actually doing anything because I see NO way to vent the kitchen sink "to code". The sink is underneath windows jutting out in a rectangular bay (?) about 4' deep by 9 1/2' across. The vent pipe is not supposed to go horizontal until it is > 6" above the sink/counter. I don't see any way to do that except putting a vent pipe in the middle of kitchen floor or counter. I suspect this is part of why it has never been vented. If I can figure out how to phrase it, I may ask this as a separate question. – irrational John Aug 22 '15 at 18:48
  • @irrationalJohn There's actually a specific method to deal with island sinks (which are common) which would probably be applicable to your "effectively an island" sink. There's also language in most versions of the code permitting otherwise inappropriate horizontal runs where unavoidable, if they are not tied in with other things and are built with drain fittings. There seems to be much more local variation in plumbing codes than electrical codes, so check your local one carefully. Here's an island sink sheet from California: fremont.gov/DocumentCenter/Home/View/7197 – Ecnerwal Aug 22 '15 at 19:04
  • Thanks! Hadn't thought about that. I'll have to look through it to see if there is a fit to my wacko situation. FWIW and possible future reference, here's a link to a google images search venting island sinks. Appears there maybe be a number of different possibilities to consider. – irrational John Aug 22 '15 at 19:22
  • One of the suggestions is to use an AAV (Air Admittance Valve). I was definitely considering this originally, but I had assumed that even an AAV should be at least 6" above the sink, but if it can sit underneath the sink then going that way would be a lot simpler. – irrational John Aug 22 '15 at 19:28
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The issue is not 'what is in favor' but what the building code requires. Here is a general rule: toilet: 3" or 4" washing machine and shower: 2" sinks: 1 1/2" nothing smaller. combined vent through roof must be equal or greater than size leaving the home to sewer or septic. This is just a guideline - I take it you are not hiring a plumber to do this work. At least use pick up a plumbing help book from the literature section of that box store.

  • Code sets minimums. Exceeding those is both fine, and what @irrationalJohn seems to want to do if he can find the pipe for it. – Ecnerwal Aug 22 '15 at 1:30

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