Ask the Builder: Sources of Sewer Gas

Ask the Builder: Sources of Sewer Gas

Two months ago I received a desperate text from a young woman who’s the cantor at my church. All of a sudden she was smelling noxious sewer gas in her condominium. She had moved in just two months before and for weeks there had not been a problem.

If you’re a new reader, you may not know that in addition to being a builder, I’ve been a master plumber since 1981. I was attracted to plumbing work because drain, waste, and vent pipes along with water lines present intriguing three-dimensional riddles that must be solved as you build a home. I love it when someone poses a riddle, even if the riddle is presented by inanimate objects like pipes.

Not only did this woman with the voice of an angel smell sewer gas, but down in the basement she had also noticed a water stain in the ceiling above her fitness equipment. I made a trip over to her home and within minutes I had figured out the issue.

“Did anything different happen here just before you started to smell the odor?” This is the first question I ask when I begin my sewer gas investigations.

“Well, I had new flooring installed upstairs,” she replied.

“Let’s go up and see what’s going on,” I said. Once we climbed the stairs, I headed to that part of her condo that was above her fitness room. Lo and behold, I ended up in a half-bathroom.

When I asked her if the flooring installers had removed the toilet, she answered, “Oh yes, the installer disconnected the toilet and reinstalled it again an hour later once the new flooring was down. He said I might need a plumber to come in, but didn’t say why.”

There’s no might about it. The flooring contractor broke the seal between the toilet and the drain pipes that prevents water and sewer gas from escaping into this woman’s condo. He failed to install a new wax gasket to account for the extra thickness of the laminate flooring he put down. Each time the toilet was flushed, water sloshed around and leaked out through the broken seal. Sewer gas was constantly seeping out from under the toilet.

The red arrow points to the white toilet flange that’s currently buried under two layers of laminate flooring. It was supposed to be installed on top of the flooring! (Tim Carter)
The red arrow points to the white toilet flange that’s currently buried under two layers of laminate flooring. It was supposed to be installed on top of the flooring! (Tim Carter)

It took me just 10 minutes to fix the problem. But the astonishing thing I discovered when I pulled the toilet up was that the original plumber had made a grave mistake. He had installed the toilet flange directly on top of the sub-floor of the condo. I was looking at a toilet flange buried beneath two layers of laminate flooring!

A toilet flange is a special plumbing fitting that is permanently attached to the drain pipe under the toilet. It is supposed to be screwed to the floor and it also has slots that accept brass bolts used to connect the toilet to the flange.

The flange flares out and a flat surface is at the top. Wax gaskets placed on this flange provide the all-important seal between the underside of the toilet and the flange. When installed correctly, wax gaskets can last for hundreds of years never allowing water or sewer gas to escape. It’s mission-critical for the flange to be installed on top of the finished floor.

Just two years ago I had to solve a similar problem. A man in Massachusetts hired me to drive hours to his home because his wife was extremely ill. Several doctors were unable to figure out why she felt so bad. Her husband thought it might be sewer gas but was not sure.

I met there with a local plumber and a company that does smoke testing. Once I had the smoke tester start his machine, it produced vast amounts of harmless Hollywood smoke that issued from the bottom of two toilets in the house. Both bathrooms had been recently remodeled. A tile contractor installed new tile on top of the old tile.

I oversaw the plumber install the correct amount of wax rings on top of the toilet flanges and reset the toilets correctly. We did a second smoke test and all was well. Within a day the woman started feeling much better.

After this happened I decided to create a small how-to guide to help folks like you solve your own sewer-gas dilemmas. There are many sources of sewer gas, and I explain how to locate them. You can save hundreds of dollars solving the problems yourself instead of calling in a plumber.

You can get the how-to guide by going to the following page of my website. Be SURE you type the word GO followed by a period before the word askthebuilder (

Tim Carter is the founder of He's an amateur radio operator and enjoys sending Morse code sitting at an actual telegrapher's desk. Carter lives in central New Hampshire with his wife, Kathy, and their dog, Willow. Subscribe to his FREE newsletter at He now does livestreaming video M-F at 4 PM Eastern Time at (C)2022 Tim Carter. Distributed by Tribune Content Agency, LLC.
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