Can Defect in Main House Stack Cause Sewer Gas in Basement ?

[mitchell357]

My sister's house (split level - 16 years old) has a very bad sewer gas stench emanating from the basement laundry room. There is a floor drain in the room as well as the laundry trap itself. In addition, there is a bathroom directly next door. This is not the first time that the sewer smell has occurred, but now it is just about unbearable!

All of the traps in the house have been filled and refilled with water, yet there is a strong sewer gas smell in the basement. Note that none of the traps get sucked dry by everyday usage. We have torn off the drywall to expose the laundry room pvc, but cannot see any obvious bad pvc joints. We have also now duct taped shut the pipe where the laundry pipe drains into.

There are two clean outs, which upon examination are not obstructed. We have stuck a tape measure from the roof down into the vent pipe, and it goes down freely to 21 feet, which appears to be the correct distance down to the basement where the stack goes into the slab.

Here's the interesting part - we have poured about 2 gallons of water into the vent from the roof. An observer in the basement can see water running on the outside of the stack, in the basement area that has stack exposed.

The area where the stack runs thru the main floor joists is extremely tight, but you can see two joints, one probably connecting in the main floor tub - the other is a short piece of pipe (90 degrees offset) that is capped off. There are no splices in the attic, and the outside of the pipe up there does not appear to be wet.

Is it possible that the welds in the joint(s) have gone bad and are causing sewer gas to seep down into the laundry room ? I always assumed that if you have water in a trap, sewer gas cannot escape, is this true ? Is there any alternative to tearing into the drywall on the main floor to check status of the pvc stack ?

Thanks in advance - Mike

[jimbo]

It is true that sewer gas will not get through a trap that has water in it. Are you sure the floor drain is trapped?

If you are very careful pouring the water into the vent on the roof so that it all goes into the pipe, and then you have water leaking out somewhere in the basement, I believe you have answered the question. Finding the spot will be more difficult.

[mitchell357]

Ah yes, opening the wall showed the problem. Two joints loose. Apparently the pvc was installed too tight, settlement of house over years caused joints to pull apart?