That dip in the drain pipe under the plumbing fixtures, called a “P-trap,” is your first line of defense against sewer gas seeping up into your home from the drain. The pool of water it holds is a simple, effective seal. But there are a few other ways sewer gas can leak into your home. Here’s our list: 

1) A loose wax seal between a toilet and the floor can allow a small opening that will let sewer gas get out, since it is behind the internal trap of the toilet. Some wastewater leakage at the floor will also occur; but, it may not be immediately obvious while it is slowly spreading under the floor covering. To do a quick check, stand over the toilet bowl with it between your legs and see if you can jiggle the toilet slightly by pressing gently from side to side at your knees. If you feel any movement, the seal needs to be tightened down or replaced.

2) In an older home with elevated wood floors, the cast iron drain pipes suspended under the floor in the crawl space may have rusted through at the bottom of the pipes or at pipe connections, and be leaking sewage onto the ground under the home. If you stick your head in the access opening under the home, you can recognize quickly if that’s where the smell if coming from—and it’s time to call a plumber. Also, there will be a mini-pyramid of dried sewage under the section of pipe that is leaking.

3) If a waste drain pipe has fractured under the the concrete floor of your home, that can also allow sewer gas to rise up through cracks in the floor slab. it may not be as easy to this one track down though, until sufficient wetness permeates the floor slab.

4) Sometimes the cause is the trap of a plumbing fixture that has been abandoned, such as an old washing machine drain, or a fixture that was roughed-in but never installed, like piping for a future laundry sink that never happened, shown below.

5) Or the smell may not be sewer gas. If the stink happens after you have opened a hot water faucet, it could be due to anaerobic bacteria in a water heater that has not been used for a while (such as while your were away on vacation). They feed on the minerals in the the tank and cause a rotten-egg odor that is not exactly the same as sewer gas, but equally annoying. Letting a hot water faucet run until the tank has been flushed out will usually fix the problem. As long as you continue to use hot water on a regular basis, the smell will not reappear.

While that’s everything on our list of possibilities, we also consulted our plumber James Freeman, of J.W. Freeman Plumbing, in Gainesville, and he added a few more suggestions:

6) A bad air admittance valve or auto vent under a sink could be the problem. It is essentially a diaphragm that is constructed to open when a drain is trying to suck in air to keep a neutral air pressure needed to allow the drains to flow freely. It closes after admitting the necessary air, but when the diaphragm membrane ruptures it allows a two-way flow of air and sewer gas can escape.

7) A bad waste arm (the pipe behind the trap that connects the drain to the waste stack pipe in the wall) can also leak sewer gas into the wall cavity.

8) And, last, the flexible corrugated pipes that are used by some handymen and do-it-yourselfers as a tailpiece under a sink can be the culprit. Although they don’t allow sewer gas to come up through the drain, they can create a similar problem. “Sometimes funky-smelling stuff can develop in the ribs,” according to James. “Because it’s above the trap, it can push smelly air into the room when you run the water.” 

    Also, see our blog post Should I call a plumber or septic tank contractor when my septic tank backs up into the house?

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?  

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a galvanized nipple?

• What are the pipes sticking out near my water valves?

• How do you accurately find a broken water pipe leak under the floor slab?

• What is the difference between water pipe and sewage (waste) pipe? 

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

• Is a hot water faucet handle required to be on the left? 

• What is a dielectric union? 

• What's that powdery crust on the pipe connections at the water heater? 

• How can I tell what type of plumbing pipe I have?

• Why is there a flexible accordion pipe under the sink? 

• What is the difference between PVC and ABS plumbing pipe?

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.