We've recently moved into an old house. We changed part of the soil pipe in the basement, which actually makes the flow more straight-forward/unobstructed. I've marked this in the attached diagram.

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Since when, we've noticed when someone flushes toilet on first floor, it sucks the water out of the traps/u-bends on the top floor. And then the top floor smells like a sewer. Presumably the gases have been released.

I figured we need to add a plumbing vent. Basically attach a pipe to the top of the soil pipe, on top floor, and run it through the roof. I've marked proposal in attached diagram.

I just wanted to check if i'm on the right track?

I haven't opened up the wall to get at the soil pipe yet, so i'm not sure what's waiting for me. Plus, I'm a little worried about the roof work. I suppose there's no way to do the work entirely from the inside?

I don't have any scaffolding, and i'm certainly not confident enough to go out on the roof without it. Even then, I think i'll have to hire someone to do this.

Thanks for any advice. James

  • first floor is ground level toilet? Dec 17, 2020 at 18:17
  • No, you can't fit roof flashing from the interior.
    – isherwood
    Dec 17, 2020 at 19:56
  • Are you sure the vent is not plugged it should not be inside the home that is bad. But I have found old cast that had short horizontal runs of 1-3’ plugged with rust, clean the crud out of the pipe and they drained much better.
    – Ed Beal
    Dec 17, 2020 at 20:28

2 Answers 2


Did you replace your pipe and keep the 2% slope? I think your fix will work. Assume your main stack is 4" and your toilets are on 3? You might be over the critical trap arm length for the basement toilet but that would just siphon that toilet.

Even if this wasn't a problem you need a vent for your main stack and it needs to go through the roof. You don't want the potential for sewer gasses to build up and vent into your house.

Good explanation of the critical trap length. https://www.jlconline.com/how-to/plumbing/maximum-length-for-fixture-drains_o

  • I need to double check all of this. 4" sounds about right. Not sure what the slope is, or whether joins are 3" for toilets. Need to dig.
    – BigJim71
    Dec 17, 2020 at 19:54
  • when you say "critical trap arm length", what does it mean? I've marked the distance on diagram now. About 8m (26'). Is that too long? I'll take a photo or two.
    – BigJim71
    Dec 17, 2020 at 19:56

If the diagram is close to reality, the lowest floor toilet needs a vent, the top floor shower needs a vent, and yes, the main stack needs a vent (and it's very surprising that it does not have one, if that's indeed the case.)

I personally don't like them (.vs. an actual outside vent pipe) but an AAV (mechanical vent, "studor" which is a brand name of one that's become shorthand for the things, etc.) will provide venting so long as it's in good condition and beats siphoning your traps, perhaps as a stopgap while you consider how the roof penetration is to be done. I dislike them because they are prone to needing to be replaced form time to time when they quit working, while pipes to outside are more reliable, in general.

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