What sizes of DWV pipe are commonly used for (very) small residential homes?

Question

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.

Answer 1

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.

Answer 2

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.