That Lovely Sewer Gas Smell

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I was just in my basement and I noticed a rotten egg smell. Then I realized it was coming from the main floor bathroom, too. What’s going on and what do I do about it? 

What you’re smelling is sewer gas, which, in most cases, is a mixture of ammonia, methane, carbon dioxide, hydrogen sulfide and a little sulfur dioxide. You’re most likely getting a whiff of the sulfur compounds, which are among the most toxic of this fairly toxic group of gases.

The first thing you need to do is ventilate your house. Next, figure out where the gas is coming from. It’s possible that sewer gas has backed up into your house because one or more of your sewer traps have opened up. This is a fairly common problem in basements, where floor drains and even utility sink drains go unused for months at a time. If the water contained in the U-shaped trap evaporates, the open trap allows gas to flow freely from the sewer into the basement. If this were the sole source of your bad smell, all you would need to do to fix it is pour a bucket of water into the floor drains in the basement and run some water in the utility sink — about a quart would do the trick.

However, because you also reported smelling gas in your main floor bathroom (and because it’s winter), I suspect your problem is a little different. If you’ve had a number of extremely cold days in a row, it’s possible that your sewer vent stack has iced over. That can happen if the water vapor in the sewer gas condenses and freezes inside and around the stack’s opening. (Look for the stack on your roof — it usually looks like a 4-inch pipe sticking out of the shingles.) The stack is designed to vent sewer gas to the atmosphere and to equalize pressure in your home’s drain lines, which helps keep your bathtub’s drain from gurgling when you flush the toilet, among other flow enhancements.

If the vent stack freezes shut — or comes really close to it — pressure can build up and cause sewer gas to work its way through the drain traps and into the house. Fixing ice-blocked sewer stacks isn’t terribly difficult, but be extremely careful when working on your roof, especially in wintertime. Essentially all you need to do is access the stack from the roof and knock off the ice cap, or pour hot water into the stack to melt an internal plug. If the problem persists every winter, you could insulate the vent stack where it passes through unheated attic spaces. In an extreme case, you could install an electric heat tape between the stack and the insulation (be sure to check fire codes) or have a new, freeze-proof stack installed.

In any case, I would suggest you attend to the problem ASAP to avoid headaches or worse from breathing the toxic gases. If you are unable to perform an immediate fix yourself, call your local plumber’s emergency line or call a neighbor who can do the work for you today. If there’s no immediate fix in sight, you might see about bivouacking with a friend until your house has been aerated.

— Oscar H. Will III, editor at GRIT magazine 


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