First, I'm not sure that this is what's happening but I suspect this is the problem. On occasion my wife and I find cockroaches in our house. This isn't very frequent (I'll find one once every few months) and we also have an exterminator who comes and sprays once every month.

However these random cockroaches tend to always be found near or in our bathrooms and, to my recollection, we've never found one in our kitchen or dining room - places where vermin could most certainly find food.

I've heard that if you lose water on your sewer line, vermin can come in through the sewers. Is this true? If so, what can I do about it? I also suspect that this may be happening because there have been a few cases where on the back end of our house (near a spare bathroom that is infrequently used) we will get a sewage smell. The smell is quite rare, and we've checked for leaks around the toilet, yet we can't find any problem with the toilet itself.

Are we losing pressure on the sewage line, which is suctioning the water from the toilet bowl and allowing vermin into our home?

If this is the case, this is super-nasty. Please advise.

6 Answers 6


The smell of sewage is definitely a bad sign and means that either a trap has gone dry or a connection somewhere is bad releasing sewer gasses. Maybe this is related to the vermin, maybe not, but its definitely something that needs a resolution.

When checking traps, check:

  1. Are all your toilets full of water?

  2. Do you have any sinks or fixtures that are very infrequently used (eg: basement laundry sink)?

  3. Do you have drain and trap in your basement floor? Does it have water in it? Is there a trap primer hooked up to it (this is a small pipe that is connected to a frequently used fixtured so that when you run the fixture, a small amount of water flows into the trap and keeps it full)

Other things to check:

  1. Do you have a sewer clean-out inside your house? If so, is the cap done up tightly? Also check the gasket on it - they can dry up and start leaking sewer gasses due to the poor seal.

  2. Bottom of toilets - flush each one a couple times and look for small amounts of water near the base. This would indicate a failed wax ring. Make sure to flush a number of times because if the leak is very small, it might not be noticable after a single flush.

  3. Vents on roof - are any drain vents near air intakes (sucking the sewer gasses back in to your house)

  4. Signs of a sewer line blockage - gurgling sinks, slow draining fixtures, sewage coming out of the lowest drain in your house (often the floor drain in your lowest level)

I had a situation in my house where one of the connections in the clay sewer had sunk by an inch or so. This would cause stuff like toilet paper to start collecting resulting in a partial blockage. Overtime, it would get more blocked until finally the pressure of the water combined with atmospheric pressure would cause it to push through. During this process, it would suck all of the water out of my basement floor trap and I'd go from a full trap to a completely empty trap very suddenly. Another interesting thing here was that when running a camera through my sewer, I noticed spider webs in them, so that definitely seems to indicate that some critters might come up from the actual sewer itself.

Ultimately my point here is that it can be a lot of things and if you can't find the cause on your own it is advisable to call a plumber - the sewer gasses are not something you want to be breathing in all day.

  • 1
    I would also add: Check that there is a trap at all. It's possible renos were done by someone who didn't know better and didn't have a permit/inspection.
    – gregmac
    Aug 9, 2012 at 21:40

An unused drain or toilet will eventually dry out. The easy solution is to just run a little water periodically (on the order of every other month). If you still smell sewer gases, then you have a failure in the drain line somewhere. One common location is the wax seal under the toilet. You should also make sure nothing is nesting in the exhaust vent.

That said, I've never seen vermin come up a drain line when I'm working on it. I would start looking for crevices where they could be nesting. For example, they could be hiding behind a cabinet or coming through holes where plumbing lines are installed.

  • Also, if the vent for the toilet is compromised (e.g., clogged at or below roofline), it could result in suctioning of water out of the trap. But unlikely bugs through pipes. They often come in through small gaps along side of pipes as BMitch says.
    – bib
    Aug 8, 2012 at 16:17

To prevent the water from evaporating from the trap of an unused toilet, lift the seat and cover the bowl with plastic wrap (cling wrap).

  • 11
    This tip also works well on frequently used toilets, especially if you don't like the person that frequently uses it.
    – BMitch
    Aug 8, 2012 at 15:51
  • Thanks, but this isn't quite that useful in my case. This toilet is, in fact used. Of the two in our house, this one, however, is used probably once every 2-3 days, rather than 3-5 times a day, like the other.
    – RLH
    Aug 8, 2012 at 15:53
  • 1
    @RLH: Then it shouldn't dry out. On the occasions where it has smelled, was it dry? Aug 8, 2012 at 16:03
  • Well, I just started to "connect the dots" about a day ago. I can't recall, because the smell only comes and goes about every couple months-- which could correlate with the exists of the roaches. Unfortunately, I can only base this off of recollection-- not hard fact.
    – RLH
    Aug 8, 2012 at 16:09

While all of these answers are excellent, keep in mind that a badly maintained bathroom vent could be bringing in insects from the outside too.

  • By bathroom "vent", am I correct that you're referring to an exhaust vent, and not a plumbing vent?
    – BMitch
    Dec 17, 2012 at 20:25
  • Yes, I'm referring to an exhaust vent. We had one exhaust vent treated quite badly by a AC install crew (upstairs central bathroom, no window, not near any entrance), and although I can't prove anything, after fixing it we didn't seem to encounter as many bugs. If your traps aren't dry, then the plumbing vent should also be behind the water trap.
    – Edwin Buck
    Dec 17, 2012 at 20:38

My two cents, I have a condo left unused for several months in the summer. In addition to having an exterminator he told me that roaches need a water source not food, so that is why you are seeing them in the bathrooms. He suggested that all drains, sinks, shower have rubber stoppers over their tops and the drains closed while not in use. For the toilet he suggested wrapping tightly with saran wrap, however I caution if you use plastic wrap be sure you place an old towel first over the bowl to absorb moisture if you live in a high humidity climate or you will have mold on the toilet then wrap up. This has eliminated all roaches in my Florida condo. They also sell pheromone sticky traps: put a few behind the toilet and in the tub or sink. Good Luck


As a professional exterminator my advice to keep those American Roaches out is, buy 5 pounds of boric acid. Boric acid can be found at Walmart or home depot. Remove your clean out cap and dump 2.5 pounds in the clean out. Then get on your roof and dump the remaining 2.5 pounds down the sewer pipe vent. The boric acid will coat the sides of the pipes where the roaches are and kill them. Boric acid is a organic substance and won't huts sea life when flushed into the sewer.

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