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Community and Q&A

‘Sweating’ PVC drain pipe in basement in raining day

Bigtree2021 | Posted in General Questions on

1. Notice this ‘sweating’ only in raining day.

2. This drain pipe conected with toilet upstairs. 

3. Only see the ‘sweating’ from the orange circle and above(wrapped with insulate) 

My questions: does this mean the roof vent boot has a problem on the roof?

Please help.

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  1. Expert Member
    NICK KEENAN | | #1

    My bet would be rainwater is leaking. Either around the roof penetration, or the pipe itself is leaking and rainwater is getting in and leaking out. The second case would have to be a big hole to be noticeable.

    1. Bigtree2021 | | #2

      Thanks for your advice.

      Just thinking should I call the roofer first or call the plumber first... ...

      1. Expert Member
        NICK KEENAN | | #3

        Can you get up on the roof? Pouring some water down the pipe would tell you.

        1. Bigtree2021 | | #5

          I wish I had that ability :)

          1. Expert Member
            NICK KEENAN | | #7

            OK, do you have the ability to inspect the vent from below the roof? You might just be able to see where the water is coming in.

  2. Expert Member
    BILL WICHERS | | #4

    Use some tape and aluminum foil (or rigid(ish) plastic sheet, whatever is handy) to make a skirt around that pipe. The idea is to create a drip edge. Condensation is not likely to result in significant amount of runoff from that drip edge. A roof leak IS likely to cause a lot of runoff.

    Some rolled up paper towels make a good cheapie water gauge. I have caught many a very slow leak by wrapping a paper towel or two around a suspect joint and waiting a few days to see if it the paper towel gets wet. If you need a control group, wrap a paper towel around a different section of pipe :-)

    BTW, condensation will occur both above AND BELOW the drip edge. Roof runoff will be noticeably less UNDER the drip edge, since the drip edge will direct it away from the pipe at the location of the drip edge.

    This will help you figure out what is going on here. You can put drip edges In multiple locations (or paper towels) too. Whichever one is wettest is likely to be closest to the leak.


    1. Bigtree2021 | | #6

      Great, I'll give it a try and wait for another rainy day! Many thanks for your experience.

  3. tommay | | #8

    If that pipe is in the basement and that cleanout goes into the floor and out to the street, it's not likely coming from the roof. I would check to make sure the line isn't clogged and full of water. Having cold water sitting in the pipe ( not being sure of what the conditions are in the immediate area) surrounded by warmer air would cause the pipe to sweat. Just tap on the pipe and make sure it isn't full of water or loosen the cleanout slightly to see if any water comes out.....A three or four inch pipe can hold a lot of water so it may not be apparent that there is a clog if your water usage is low and water is still possibly flowing slowly out the main drain.

  4. walta100 | | #9

    When I look at the photos of your pipe I see condensation on the outside of the pipe.

    I think your vent pipe has setup a stack affect so warm air is moving out the top of the pipe as it does so it draws cool air in the bottom of the pipe from the underground pipe. When the outside of your pipe happens to be cooler than the dew point of the air around the pipe water condenses on the outside of the pipes and drips down the pipe.

    One solution is to lower the dew point in your home with a dehumidifier.

    Another solution would be to stop the flow of air in the pipe by putting a trap in the pipe leading to sewer main out in your yard.

    It could not hurt to slow the flow of air around the outside of the pipe by sealing the gaps around the pipe near the top.


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