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Plumbing in the Home/gas from lateral line entering basement


Hi Louie, My mom's house is connected to the city sewer line, and about a dozen times a year the basement fills with sewer gas. The basement is finished living space with concrete walls, but just a crushed stone floor that has a wooden joist and plywood floor above it. According to the previous owner who renovated the entire house he may have connected some interior perforated ground water drainage pipes to regular sewer line inside the house. My assumption is that there is no P-trap in the lateral line, so gas from the city sewer line is able to enter the the home sewer pipes through the lateral line, then it enters the basement through the perforations in the drainage pipes.

I have checked all the p-traps to be sure they have water, and we had a plumber do a smoke test to determine the location of the leek. He found the leek to be from under the wood floor where the drain lines probably are.

I feel that we could install a P-trap inside the basement at the  end of the interior sewer. This would prevent the gas from entering the house through the perforated drain pipes.

What is your advice? If you think this the way to go, how do I install the P-trap, and dose it need any additional venting?

Thanks for your help.


Wow Paddy.  What a question!!!!  I knew this would be fun.... :)  Well, first off, you seem to know a LOT about this issue.  Congrats.  You explained it as well as an articulate plumber would have explained it.  Rare, very rare indeed you are so well versed in plumbing as well as this particular problem.  Sure does make my job a lot easier.

Ok, connect the triple backflow check assembly, to the interior swing valve, while mating it with a ductile 8.66mm flange, using teflon vs dope and then sweat in a double junction by-pass cross IN line (not out) to the 4 inch main 9.96 feet from the foundation opening...  If you follow those directions'll have achieved nothing.... :)

Ok, all kidding aside.. if possible with me, I am not a big fan of main sewer lines traps.  They just cause issues.  No such thing in the south...but I hear they are used a lot up north....not sure WHY really....but they are.

The previous owner was INSANE to hook up the french drain system to the sewer and the words "i might have" can be assuredly mean he DID!!!

IF THIS PROBLEM is related to that, no promises it is, but it sure does sound like it, then yes, the ONLY solution is to trap the main line after the connection point of the french drain system.  This will NOT however, prevent sewer smell from the rest of the house going into the basement, but it will sure as heck help.  Just make sure this 4 inch P-trap is accessible at all times and ALWAYS tell anyone who comes out to snake it you have this trap or it could cause problems with the sewer machines!!!

When installing it, don't glue it, but use 2-4 inch ferncos on either side of it, so it can be removed if needed.  If exposed great, if underground, build a box or something so you can get to it. (one thing you didn't mention was whether it was underground or exposed in the basement, but I am assuming it's all underground, under the crushed stone, or there would have been no way to drain the basement into the sewer)

Don't worry about venting, everything before that point is vented and you SHOULDN'T have a problem....but if you wanted to play it safe, put in a 4x2 tee after the trap (between street and trap) and bring up a 2 inch pipe straight in the air into the basement and put a studor vent on it to play it super safe.....but I REALLY don't think it's going to matter.  I only say that in case it's a MAJOR project digging up and getting down to the main, you wouldn't want to do that again, so at least bringing up a pipe will have it "vent ready" should it turn in to an issue :)

Plumbing in the Home

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Louie The Plumber


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I am a 35 year Master Repair Plumber. I have owned my own repair company for over 20 years. My company has been the number one rated company on Angie's List for 5 years in a row. I have received the Angie's List Super Service Award for 5 years in a row. We are also the mosted rated company. We have almost a straight A rating across the board (99%) A's.

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