Background: I have zero plumbing experience, hence the sanity check. Everything I describe here is brand new to me.

My house is in a hilly area in Silicon Valley. All of the houses in this neighborhood have drains distributed around the yard, which drain to pipes under the sidewalk just above a storm drain. I only moved in recently; the house is 20 years old. A couple weeks ago we had a historic storm, and water collected deep enough that my house was very close to flooding. The drains in the yard did not really appear to be taking in much water, and the pipe at the street was putting out only a trickle. But I ended up being saved by luck; the drain in the front yard closest to the street was also the lowest, and was operating in reverse, with a torrent of water coming out of that drain and then running across the surface of the front yard toward the street, ironically dumping over the sidewalk directly above the pipe it was supposed to be going through. The next day I poked a stick up that pipe, under the sidewalk, and felt resistance... so I dug there.

Here is the aftermath of what I found:

enter image description here

The white pipe is coming out of the yard. The right black pipe is going under the sidewalk (the left black pipe was unused, which is normal in this neighborhood). They were connected with corrugated pipe and hoseclamps (one end still barely visible in that photo after I destroyed the middle section), and the corrugated pipe and the entire black pipe were filled with a giant rootball that had entered under the hoseclamps:

enter image description here

Reconnecting the pipes without another cheesy corrugated section proved difficult. There was both an elevation and a slope mismatch. The yard side was lower and sloped UP maybe 10deg. The street side was higher and sloped DOWN maybe 10deg. Here is what I ended up with:

enter image description here

I used two Fernco 3" flexible pieces, a simple straight one, and also a cleanout tee. They are connected with a 3" long piece of ABS. The cleanout is a nice bonus, but the real win here is that the length of that piece gave me even more flexibility for the alignment problem.

Here is the main question though. The white pipe is some weird thing which I believe is schedule 30. Incredibly thin-walled (0.070"), so it amazes me it hasn't had roots penetrate it yet. But its OD is 3.25", compared to the Fernco IDs and the black pipe OD (which are all 3.5"). At first I was worried this would be a problem. But:

(1) I ran a simple test to see if the rubber would contract nicely down to that smaller diameter. I overinserted the straight Fernco coupling onto the white pipe (so that I could see the mate more clearly) and cranked down the hose clamp. The result:

enter image description here

While it does look like there are small gaps, this is only because the rubber bows up away from the clamps. I am pretty convinced that at the clamps, there is a very good seal (and flushing 50gal loads down the drain after full assembly seemed to confirm this). The circular shape of the cross-section is distorted a bit which doesn't seem ideal, but I think this may have more to do with the thin wall than with the diameter mismatch.

(2) Ironically, the looseness of the Fernco <-> 3.25" mate was kind of critical in allowing me to actually assemble this entire mess with the huge starting misalignment. I don't think I'd have been able to get it all together if the white pipe had 3.5" OD. Basically I was able to overmate the entire "patch" assembly onto the white pipe with a pretty crooked alignment, which allowed me to get the other end of the tee slid back onto the black pipe and tighten that clamp, after which I tightened the crooked clamp onto the white pipe, which slowly straightened itself out, leading to the desired bend in the flexible coupling.

My major concern is if there is any issue with using the 3.5" ID Fernco coupling with the 3.25" OD thinwall pipe. I want to prevent future root invasions.

  • I don't really see a specific question in here which is required before we try to provide an answer. Based on your description I'm not sure if the changes you made are going to work until you try them. You might want to hook it up and run a garden hose into the lawn drains and see if you get good flow. I would try to correct the negative slope you referred to. It may not be easy but otherwise water will just sit between the lawn drain and the upslope.
    – HoneyDo
    Nov 11, 2021 at 5:06
  • @HoneyDo I updated the end to make my question more clear. The drainage works fine (as I mentioned, I flushed 50 gallons through it). Even though there is a small upslope in this section, it is all far lower than the upstream sections. I just want to make sure the mate between the schedule 30 pipe and the Fernco fitting will not allow future root invasions because of the small diameter mismatch (which appears to disappear when the clamps are tightened and the rubber contracts). My changes did not affect the overall path of the water, only the connections in this small segment.
    – The111
    Nov 11, 2021 at 5:25
  • 2
    Seems like a reasonable repair. This is ground water drainage. In climates that get more rain you typically have a sump that the rainwater or ground water would enter first to help prevent sediment or anything else from entering the city service. The sump inflow pipes are typically higher than the outflow pipe but really as long as there is general slope it probably doesn't matter too much that you have a belly in your pipe - that portion will likely have water sitting in it during the rainy season but you have a cleanout should root get in again. Not worth the trouble to dig and slope it. Nov 11, 2021 at 6:45
  • I'd try to to snake / roto-rooter the whole thing before you bury it again. You may have gotten dirt and other blockages in the line.
    – gbronner
    Nov 11, 2021 at 13:28
  • 1
    @gbronner no other clogs upstream, as I mentioned I flushed 50gal and it flowed smoothly. Also this section is 5ft from the curb exit, I can always snake from there if I need to.
    – The111
    Nov 11, 2021 at 19:40

1 Answer 1


What you assembled will be a good match for the conditions and allow for a clean out. You are correct the fittings don’t fit well on the drain pipe because of its smaller diameter. Since there were roots in the area one thing I have done is pack rock salt around the patch (yours is well beyond what many contractors would have done) the salt will prevent roots from immediately trying to get back in and is a natural way without harsh chemicals. It won’t take a lot of salt a couple of handfuls really helps. But what you have should work fine.

  • Thanks Ed for your input!
    – The111
    Nov 11, 2021 at 19:39
  • FWIW, and for anybody else reading in the future: I actually spoke to Fernco customer service today, and they said sizing up or down 1/4" on the mate is no issue. Though I have no idea how anybody would upsize. Those fittings would not be easy to stretch!
    – The111
    Nov 12, 2021 at 2:26

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