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Since very recently, our washing machine's drain pipe started overflowing. It has worked fine for the last couple of years but for some reason it doesn't anymore. When the washer dumps only one load of water the pipe instantly overflows onto the floor. We checked for clogs and cleaned it out but found nothing.

NB: When I say "we" I mean me and the other tenants, as we share a washing machine and dryer.

I'm not able to take pictures at the moment, so I drew a rough idea of how it's all connected. I added some numbers to indicate points of interest, I've explained them below the image. It's not quite to scale but I did pay attention to the relative positioning of elements (like how the trap is at the lowest point). There's also a tough plastic clamp attached to the washer's hose to keep it in place at the top of the pipe.

Drain schematics

  1. Unused pipe, what's this even for?
  2. This is a plastic cap with a white plastic ring in it, but parts of this ring seems to have broken off (judging by the rough edges).
  3. There seems to be a small leak here (droplets of water fall down).

The "neighbour" is my downstairs neighbour and he's the only one on the ground floor. Everyone's toilets and showers etc still drain fine, if it matters.

I'm not entirely sure what influences the ability to siphon properly, but I know air (both excess and absence of) is a big part of it. It seems like a simple fix so calling an expensive specialist is something we'd like to avoid.

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    Does the cap, item 2, come off? If so, it is intended for cleaning the trap it is connected to. You will want a bucket or whatever water container you can fit under it.
    – Andrew Morton
    Jan 28, 2017 at 18:32
  • Yeah it does, that's actually one of the ways we checked for clogs (and when we noticed the broken ring inside of it).
    – Sahbi
    Jan 28, 2017 at 20:51
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    That's not the only place a clog can occur. You should snake down the drain line as far as you can. The trap is really there just to keep sewer gases from coming up and into your living space...and smelling aweful. The item you refer to as #1 is a vent pipe. That is a pipe that goes up and out of the house/apartment/etc that allows air to come into the drain line to help aid in the water going down. If this pipe is plugged, then no fresh air can get in. This creates a vacuum and would allow the water to come back up and out the drain. It could be either of these places that you have a plug.
    – Jeff Cates
    Jan 29, 2017 at 1:54
  • If your neighbor has the same issue, and if they run their washer and drain, does it come up your drain? If not, and it does come up theirs, then you have a plug down the drain line. If it backs up into your drain, then could have either a plug drain line or a blocked vent pipe.
    – Jeff Cates
    Jan 29, 2017 at 1:55
  • Well, the "vent pipe" doesn't run outside, it's the same height as the rightmost pipe. But I suppose it still does its job of allowing air in. We haven't found any clogs from the top of the pipe to the first turn after the trap (since we didn't have anything flexible enough to make the entire trap). Guess we can give it a shot with a proper drain snake. And the downstairs neighbour doesn't have any issues with backflow when he's showering or whatever. It's merely the (shared) washer that does this.
    – Sahbi
    Jan 29, 2017 at 11:04

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Alright, it took a bit of effort but we managed to take apart the drain construction (without breaking anything of course) so we could get deeper to unclog. Everything from the hose entry up to the neighbour's drain was already checked. The clog seems to have been a bit farther ahead, right about where my "diagram" stops. Lots and lots of gooey black gunk came out, I think we emptied about 3 or maybe 4 full buckets worth of the stuff.

We ran a couple quick tests with the washing machine: "rinse" program followed immediately by "centrifuge and pump" as soon as it stopped drawing water. This went perfectly fine for about 3 times (the vent pipe stayed dry the entire time), so we decided to pour a little drain unclogger down the top to get rid of any residual gunk. Then we ran the 2-program cycle twice again to rinse it out. My normal laundry is being washed as I'm typing this, so thanks for the tips and additional information on how drains work. =]

tl;dr: babby's first plumbing job was a success.

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