What's causing the sewage smell in my laundry room?

Question

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?

Answer 1

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!