The kitchen sink is draining slowly after the water runs for 10-15 seconds. I've stuck a snake down the cleanout and nothing is there. However, if I leave the cleanout cover off (which is just under the sink), the sink is draining fine, with no backup, and certainly not out of the cleanout hole.

This leads me to believe it's a venting issue, as it backs up as soon as I put the cleanout cover back on. I've gone into the attic, and the vent for the sink is by itself until it connects with the bathroom vent (T junction) just a few feet below the exit through the roof. The bathroom sinks, bathtub, and shower on the other side of the vent T junction are all fine and not backing up. No other things in the house are backing up.

I went up on the roof to ensure there was no snow covering the vents and even poured 4 liters of hot water down the pipe to ensure it wasn't some ice/snow blocking it. Still draining slowly.

Before I get up in the attic and start cutting open the vent pipe to see if something is stuck in there, can someone tell me I'm looking in the right place? I can't think of anything else to troubleshoot before I start cutting some pipes.

  • dead rodent in the pipe, get a snake camera and inspect, do not cut
    – Traveler
    Commented Feb 9, 2023 at 23:40
  • That is what I'm assuming. I do have a snake camera, but if it is something in there, then what? I still need to snake it out somehow, and I'd rather not push it down into the drain! I can't go up from the cleanout under the sink either due to the angle down connector. :(
    – et071385
    Commented Feb 9, 2023 at 23:55
  • Best practice is not to push it down, but sink vent is probably 1 1/2 inch, main drain/vent is 3 or 4 inches.
    – crip659
    Commented Feb 10, 2023 at 0:07

3 Answers 3


A T-Joint just under the roof exit implies, like shown in the sketch below, that would be a horizontal section in the vent pipe with either scenario.

enter image description here

A horizontal section can be problematic because it is a spot where debris or birds or animals that got into the vent could end up creating a clog. From your description it sounds like a horizonal section may very well be the part of the vent that branches off toward your kitchen.

  • It's more an upside down T, with two horizontal sections connecting and then going up out through the roof, if that makes sense. One of the horizontal sections is JUST for the kitchen sink, so it makes sense if there's a clog in there. I'm going to drill some small holes in a few sections on the weekend and stick my snake camera in to see if I can locate the blockage.
    – et071385
    Commented Feb 10, 2023 at 0:54
  • @et071385 - Yeah, two horizontal sections is even more problematic. :-( I updated my sketch to show two scenarios.
    – Michael Karas
    Commented Feb 10, 2023 at 2:45

So I went up in the attic and drilled a few holes in various spots to look for a blockage. Turns out the blockage was at the last bend before the horizontal section that goes to the sink. It was blocked with ice!

The reason pouring hot water down the vent from the roof didn't work is because the horizontal section that goes left for the bathroom is above where it's frozen, so it just hit that and flowed down the vent for the bathroom.

enter image description here

  • Instead of drilling holes into pipe that now need to be repaired/replaced, it might have made sense to just feel the pipes for cold spots. Well, lesson learned for next time. Just start with a heat gun.
    – FreeMan
    Commented Feb 10, 2023 at 18:38
  • 2
    Also, the fact that water was able to sit there means that your vent does not maintain proper slope (1/4" fall for every 1' run). If it was properly sloped, whatever rain/snow that got in would run down the vent and into the drain system below, eventually to exit the house. I suppose you _might have gotten snow packed into there that formed ice, but that seems unlikely considering it's basically undisturbed and that the snow would have had to melt & refreeze to form ice and should have drained away in the "melt" phase.
    – FreeMan
    Commented Feb 10, 2023 at 18:39
  • The odd thing is it's only a little bit of ice in the elbow. I think snow got in there and got compacted then had a few freeze thaw cycles. I'm in Eastern Canada where that's pretty common. Explains why it didn't just melt and flow down. Best I can tell the slope is correct. Also, ice never crossed my mind untill I got near the elbow and the pipe made a cracking sound when I grabbed it.
    – et071385
    Commented Feb 10, 2023 at 20:49

Push a copper wire though your vent pipe and leave it in there. It will conduct enough heat from the controlled environment to keep that area of the pipe from freezing That trick works well with condensation drains as well 🤠

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