Plumbing – What’s causing the sewage smell in the laundry room

drain-waste-ventpipeplumbingsmell

I noticed my laundry room has developed a strong sewage smell over the last two days. I suspect it may have to do with vent blockage, but I ran a garden hose to the vent in the roof last night and had my wife listen near the drain in the laundry room, and she reported hearing a slow trickle of water – so it's not completely blocked up.

  • I haven't washed anything particularly dirty that might put a lot of debris down the drain pipe recently.
  • The washer has been used regularly, so the drain pipe trap shouldn't be dry.
  • I'm on a septic system, if that matters. None of the other bathrooms or sinks in the house have any smell coming from them. I sniffed all the vent pipes while on the roof and could detect a faint level of the same laundry room smell coming from all of them, but I think that's normal.
  • I checked in the attic and discovered the vent pipe makes a 15-20 foot horizontal run before exiting vertically through the roof.
  • The slope on the horizontal run actually dips down and back up a few inches over its distance, but the issue just surfaced after living in the house for 7 years. I suppose it's possible it's acting as a trap for venting gas, but I don't know why it would start doing so spontaneously. (The water I ran down the vent from the roof before noticing the lack of proper slope could easily exacerbate this possibility, so I plan ensuring it's drained tonight.)

So my forward plan tonight is as follows:

  • Snake vent pipe.
  • Pour water down drain pipe to fill trap if it's empty.
  • Cut vent pipe in the attic and snake down into drain pipe from there.
  • Reroute attic vent pipe to have a minimum 1/8" per foot slope at all points.

Any other suggestions for remediation?

Best Answer

Well, I'm not sure which of the following actions fixed it, but the smell seems to be dissipating after letting the room sit overnight afterwards:

  • I cut the vent pipe off in the attic right above where it came out of the ceiling. Then I drained any stagnant water due to the "U-bend" in it.
  • I snaked down from the roof to the cut location, with no obstructions found. (Due to more luck than planning, the run was within a few inches of the 25' length of my auger!)
  • I snaked from the vent cut location in the attic down as far as my auger would reach, and don't think there was anything there, since it came back clean again.
  • I rerouted the vent pipe in the attic so that any horizontal runs had at least a 1/4" per foot slope upwards so any sewer gasses wouldn't be trapped.
  • I snaked from the laundry room drain pipe as far as I could reach, once again no obstructions detected.
  • I borescoped the drain pipe and saw that the trap was full of water as it should be. I added a gallon or so of water anyway, then maybe a cup of vinegar to help with any smell that might be in the pipe itself.

After all that, the smell dissipated overnight. Unfortunately, I didn't find any obstructions, and discovered the trap was full of water. The only thing I really "fixed" was the vent pipe slope, but that's been in that configuration for 7+ years without any trouble, so I still don't know what actually worked!

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