While painting, my wife discovered some mold on top of the baseboards, and being curious I investigated. What I found pulling the baseboards is they were covered in black mold on the backside, as well as the bottom of the drywall. On further investigation, the source was the joint of a 3" vertical DWV pipe, which had a slow (but consistent) water leak.

I cut out drywall to access the joint, then cut out an 8" length of the pipe, including the joint section. I bought two Fernco shielded couplers for 3" PVC (P3005-33), and a short section of 3" PVC pipe. When I went to replace, I realized the vent pipe was NOT 3" PVC Schedule 40 pipe, but something smaller (I measure about 3.25" OD). I thought it was Schedule 30, but one supply store said 3" PVC wouldn't go down that far.

For the life of me I can't find this small of 3" pipe at any of the big box stores, or local plumbing supply stores (I have one plumbing supply store left to go ask, though), let alone the shielded couplers I was planning to do to make this repair.

So my questions are:

  1. How would you repair this vent pipe? (about 8" run that is currently cut out)
  2. Where can I get shielded couplers and repair pipe, that is 3.25" OD? Anywhere recommended online?
  3. (an aside) Is there any problem with leaving the vent pipe open currently? We won't be using the bathroom until everything is repaired.

UPDATE Thanks for the help. I found from a local plumber that it was an SDR 30 pipe, which I found at a local plumbing store (big box stores didn't carry). No shielded couplers, but used some slip couplers and seems to work well. But now water coming up from the slab where the pipe comes through... will create new question.

  • 2
    Sched 30 should be 3.25 inch OD. It's specified as such in ASTM D 2949. 1. You might be able to use regular 3" couplers with Sched 30 (not sure) 3. For sure make sure you plug up the vent so that sewer gases don't escape into your home.
    – Edwin
    Dec 7, 2015 at 21:51
  • I tried the pipe in the the regular 3" couplers (for Schedule 40), they are just too big for me to believe they'll give a tight seal when I torque them down (I didn't try though, cause I wanted to return them).
    – Michael
    Dec 7, 2015 at 22:08
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    Sorry, I just realized you may have meant 'regular' as in the white PVC couplers instead of banded Fernco couplers. I did find Lowe's has such a coupler for ASTM D2949 (thanks for the spec). Most things I read online recommended repairs like this to use the banded flexible couplers, that the regular couplers even well installed often leak. Your experience?
    – Michael
    Dec 7, 2015 at 22:55
  • I thought that PVC schedule changes came from the ID of the pipe? Same OD, same couplings? Dec 8, 2015 at 6:04
  • I think thats true for most pipe types, and its true for PVC Schedules 40 & 80. Not sure why 30 is different, but its definitely standardized for smaller OD (3.25" OD) than either Schedule 40 or 80 (3.5" OD) (look up the ASTM D 2949 spec that Edwin provided). Perhaps there are Schedule 30 that are 3.5" OD also? I don't know
    – Michael
    Dec 8, 2015 at 11:28

2 Answers 2


The smaller diameter pipe is because it is specified for in-wall use. The socket end of a standard 3" PVC pipe coupling is 3.97" which is wider than a 2 x 4 stud wall cavity, which is 3.5". The 3.25" pipe OD on the in-wall pipe allows for a smaller coupling which can fit inside the wall cavity.

Depending on the size and type of leak, you might have been able to seal it up without having to cut it out. I put a nail for a picture frame through a drain line once. To repair it, I scraped some shavings from some scrap PVC, mixed it with a little bit of PVC solvent until it formed a paste and then filled the nail hole with it. Note, that I would never do this on a pressurized line.


Make sure the bands on the couplers are overly loose & spread some dishwashing liquid inside them to see if they go on then. If not you can put them in the oven at 200-degrees for 5-minutes to get the rubber much more pliable...re-coat them with the soap & try again.

Other than that, go back to the supply store with your cut-out piece & get 1 or 2 slip couplers along with primer & cement cans to do it up right. Then, return those banded couplers. Done right does frequently end up being cheaper as well as much better.

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