After reading this related question and check that the wax seal is ok, I assume I have a problem with the trap, but where's the trap? The outline of the plumbing in my home (a bit simplified) is like this:

enter image description here


  • Blue: the main pipe that comes from the bathroom to the city wastewater system through the house basement.
  • Yellow: City wastewater (I can't touch it).
  • Red: The problem.


  • When I bought the house, there was a bad smell in the bathroom. I assumed it was because nobody lived there for months.
  • After some weeks, the bad smell didn't dissapeared, so I checked the bathroom (everything right), then I opened a manhole I found in my garden (the red part). I found it dry and had a strong bad smelling.
  • After asking to a professional plumber, we decided to make a trap there with a pipe. Something like this: enter image description here

  • This get rid of the bad smell for 3 months, but after it eappeared again. Then we cleaned the strap and the bad smell dissapeared again for another 3 months.

Of course, that's not a good solution for 2 reasons:

  1. We have to clean the trap (at least) 4 times every year.
  2. Worst: after a couple of month the main pipe is full of water because it doesn't have enough slope. I'm afraid a day it can explode and flood the basement.

So, the question is: how can I fix this? There are no other traps in the house, should I make one? Or there is a simpler way to fix it?

  • 3
    Normally you shouldn't be able to smell what is in the blue main sewer pipe. The traps should all be upstream of that. That your fix removed the smell means that there is a drain's trap that is dried out/an open cap for a future proof hookup. – ratchet freak Oct 28 '15 at 15:17
  • As for where the traps are, there is usually one in the basement floor, ideally near the water heater. Can't find it? Is there flooring anywhere in the basement? – Steven Oct 28 '15 at 19:36
  • That's the problem. I can't find any trap. May be it's hidden between the basement and the bathroom floor? I think I have to dig up ground.. :/ – Ivan Oct 29 '15 at 7:17
  • 5
    The trap for the toilet is built into the toilet. You can often see it in the shape of the lower, rear portion of the bowl casting. – isherwood Feb 26 '16 at 15:59

On top of looking for a dried out trap I would check your vents. I had a squirrel build a nest in a vent pipe and plug it so when you flushed it pulled the water out of the other traps and let the smell in. I had to make a cap with vent holes to keep the little bugger out.


That's a very odd setup & I'm surprised by the Plumber's actions. If I were the Plumber, I would've piped a trap in the manhole hole & dropped a pipe to even just make loose contact or within 25 to 50mm of the city's pipe. This would either eliminate the quarterly cleanouts or drastically reduce the need for them.

And definitely, put traps under your sinks & even under the toilet if it's a straight shot situation now. But yes, the red box is your main & initial problem.

If you're not allowed to do any of the above, then your manhole cover needs to be turned into a manhole grate (this should be a requirement & figured out VERY long ago), not only for ventilation but also so rainwater can clean out the red box for you. After or along with that, put a trap absolutely wherever you want (just 1) in the blue line.

  • 2
    Are you implying that there are no P-traps on the sinks now? That's very unlikely. Are you also suggesting that OP add a second trap immediately downstream of a toilet, which has its own trap? That's probably not even allowed by code. – isherwood Feb 26 '16 at 16:02
  • Astonishing actually, isn't it? But Yep, that's how he starts & ends his question. You'd think 100+ year old technology would've made it around the globe by now. – Iggy Feb 26 '16 at 16:06
  • 4
    I think the OP misunderstands what a trap is. He's in Spain, not some third-world jungle. Virtually every toilet has one built in. – isherwood Feb 26 '16 at 16:09
  • I think he knows what he's talking about. He says he thought the stink was just the toilet (or plumbing) not being used for a long while. But then, says usage didn't change anything & only cleaning the outdoor pit made any difference at all & was the only stink recurrence & stink removal repeat solution. I find it hazardous as well, but I honestly don't know much about Spain, except the women...now that's good stuff. – Iggy Feb 26 '16 at 16:24
  • A 2nd trap on a toilet is crazy in my opinion. I would be looking for a lack of venting. I think @isherwood is correct that the op may not understand what a trap is. I know when in Ireland they called them u bends. – Ed Beal Dec 19 '18 at 15:24

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