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How to Eliminate Basement Odor and Sewer Smells

This article shows five reasons why your basement could smell like a sewer. If you have a smelly basement and can't locate the source, we'll show you the most likely culprits.

By the DIY experts of The Family Handyman Magazine

Finding the source of the basement smell

If you notice a foul sewer smell in your basement, here are the five possible causes in order of probability:

  1. You have a water trap under a floor drain, laundry tub or wash basin that has dried out from lack of use. Water in any trap under unused drains will eventually evaporate. That would allow sewer gas to come up through the drain into the room. Solve that problem just by dumping a pitcherful of water into the drain to restore the trap water. If you follow that up by pouring a couple of tablespoons of cooking oil into the drain, a floating seal of oil will keep the water from evaporating so fast next time.
  2. Check for a cleanout plug inside the floor drain. Remove the grate that covers the opening and make sure there's a plug inside the drain bowl. If the plug is missing, there's a direct path for sewer gas to bypass the water trap. Sometimes, the plugs are removed to clean sewer lines and not replaced. Buy a replacement plug at a hardware store.
  3. It's less likely, but the water in the toilet trap also could have evaporated. Weeks of disuse could cause this. Of course, simple flushing will restore that water.
  4. Another culprit could be a bad wax ring seal between the toilet flange and the base of the toilet. This wax ring can occasionally leak, sometimes because of a rocking toilet that has broken the seal. With a leak in this seal, sewer gas will find its way out from under the toilet. If that's the case, you'll have to remove the toilet and replace the wax ring. If the toilet rocks, use plastic shims between the stool and the floor and caulk the joint. This will ensure that a rocking toilet doesn't ruin the new wax ring.
  5. A more serious prospect would be a broken or cracked sewer line or even a loose connection joint in the ceiling or buried in a wall. If you've addressed the four easier possibilities, use your nose to start investigating, starting with all visible joints. If you can't detect the leak, contact a plumber who specializes in hunting down leaks.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

You won't need any tools to locate the source of the smell.

Required Materials for this Project

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No materials are needed.

Comments from DIY Community Members

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October 05, 6:31 PM [GMT -5]

Another possibility I discovered for anyone who has gone through this and still not found the source:
We just bought a house, and during the move in, someone hit one of the black PVC drain pipes coming down from the floor above, I think from the kitchen.

On that pipe, near the floor there was a T, and coming straight out from the T was a 8 to 10 inch length of pipe with an orange cap on it. This pipe was was not glued into place, and when it was bumped, it opened up a bit, and allowed some smell to come up. Additionally, the orange cap was cracked. A little duct tape over the pipe connection, as well as the cracked platic cap, and problem solved.

Hope that helps!

October 31, 9:59 PM [GMT -5]

I cannot thank you enough for this simple and well directed article. I've been struggling with basement order for months in a newly constructed house. I have been trying to explain this to the builders who have insisted the problem stemmed from a pulp mill in the next community over, my cat's litter box, to the cat's peeing in the basement corner or the P-Trap for the washer. It was actually the drain pipe, which has been leaking in the unfinished basement bathroom, but had a plastic bag stuffed in it and so could not drain itself. Far from being resolved, I finally have a lead.

October 31, 9:59 PM [GMT -5]

I cannot thank you enough for this simple and well directed article. I've been struggling with basement order for months in a newly constructed house. I have been trying to explain this to the builders who have insisted the problem stemmed from a pulp mill in the next community over, my cat's litter box, to the cat's peeing in the basement corner or the P-Trap for the washer. It was actually the drain pipe, which has been leaking in the unfinished basement bathroom, but had a plastic bag stuffed in it and so could not drain itself. Far from being resolved, I finally have a lead.

October 21, 3:43 PM [GMT -5]


When the water in the trap portion of your drain evaporates, this removes the natural water seal and opens your home to disease carrying bugs and sewage gas smell. H2GOFlo.com and the theInterceptor-Plus replenishes this water periodically, creating the required barrier, that blocks off the back flow from the sewer system from having direct access into your home.

March 03, 2:17 PM [GMT -5]

I just want to thank you for this article. For months I had a foul sewage smell in the basement. The clean out is in the wall (finished). I thought the smell was leaking out of that cleanout plug, not a good seal. so I fixed that but it didn't fix the problem. I called plumbers for 2 months. Persistently. For some reason I couldn't get either of them to come over, flakes. Then I came across your post. I wondered if it might be something under the carpet, I removed the carpet in the corner of the room and there was a floor drain under the carpet there. It was all dried out and rusty shaving filled it. I noticed when I flushed the toilet in the other room, there was a gust of sewage coming out of this floor drain. Vwaaala! I cleaned out the trap with vacuums and filled it with water and cooking oil as you suggested. Still not sure if this fixed the problem, but it would make sense. I know nothing about plumbing, this article was very helpful, the picture in particular. Thanks, S

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