We think we may have figured out the sewage smell in basement, but I am a little confused. When we open up the floor there are several pipes: one is a large vent line which goes outside, two sewer lines going into ground both have threaded caps, and what is confusing is there is a large pipe that appears to be connected to the drain hole just outside the basement. Within that drain pipe there is a flap that I think goes to the main sewer, is this some kind of rainwater overflow flap made to open if our drain gets too much water so that some can go directly to the city line? I think this is where the smell may be coming in. We are going to try to replace the 5 inch bolted cap that covers the top of the drain line to see if that helps, but it seems difficult to get a good seal as it is not threaded. Any ideas?


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  1. I have a smell coming from my drain under the stoop also. The drain is closed up with dirt, gravel, etc. A neighbor said theirs smelled until they had it cleaned out, so I am going to try that.

  2. The under floor box contains the trap for your main drain.

    It has two caps (usually iron) on both sides of a U-shaped “trap” to prevent sewer gas from coming in from the street. Just like the one under all your your sinks. It doesn’t run down, its just a catch basin. That main drain runs straight out to the street, its not a vent. The trap may need to be cleaned from time to to time, and we usually ask the roto rooter guys to do that while they are there to get the tree roots out 2x year.

    i think many houses have the outside drain under the stoop that runs into the main drain (ours does). I am guessing that it is unlikely your smell is coming from there. That pipe does not carry waste just usually rainwater.

    To have a separate stoop drain that went directly to the street sewer, it would need its own separate trap (which would need its own cleaning as well). Much easier to use the house trap since the water you get from the stoop drain is not significant. And the city has to service only one connection to the street (huge in maintaining sewers in a city this size)

    I am not certain of the description “within this pipe appears to be another pipe with a flap “. I am not visualizing this, except to think that if the sewer backed up human waste wouldn’t be showing up under your stoop.

    Regardless, the underfloor box is a cesspit of sorts. Probably years of #1 and #2 backup (that’s what the pit is designed for). If you have a tree out front and don’t roto-root it, you might be familiar with all the foul things that enter this pit.

    The pipes in the pit sweat in summer months, and generally a sweating and musty cesspit is not going to smell wonderful.

    MY POINT IS……..first take meticulous care that this pit cover is literally airtight with a thick solid lid. Rubber or weather stripping seals, etc.

    The caps on the trap are supposed to be just tapped with a rock to “sit on top” because if they don’t release easily, the sewage would backup at the next highest point, probably a toilet somewhere. (i’ve had this happen in a previous house….trust me, its much better staying in the pit!)

    That trap pit is going to smell, seal it up heavy and air tight, and see where that gets you. Also, they have flood sensors you could place down there so you know every time something backs up i highly recommend it,

    Smells can be very hard to track, start with the pit lid.