Most basements are cool and dark places where a musty smell doesn’t seem out of place. However, you shouldn’t detect a sewer smell, and if you do, something is definitely wrong. But what?
In this post, we identify five possible causes of sewer smells in the basement, in order of probability. And if you don’t have a toilet in the basement, you can eliminate two of these possibilities. Plus, we’ve included some tips to eliminate sewer smells and unpleasant odors.
Bonus: We’ve rounded up some of the best basement odor eliminators so you don’t have to hold your nose while you’re working to fix the problem.
Family Handyman
A Dry Water Trap is Causing the Sewer Smell
The most likely cause is a P-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. However, that water will probably also evaporate, so here’s a tip from professional plumber Jimmy Hiller:
“If you slowly pour 2–3 tablespoons of cooking oil in, it will sit on top of the water and keep that water from evaporating so fast.”
Sewer Smell May Be Caused by a Missing Cleanout Plug
Check for a cleanout plug inside the floor drain to get rid of a sewer smell in the bathroom or laundry room. 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, and that’s why you’re getting the sewer smell.
Sometimes someone will remove a plug to clean the sewer line and forget to replace it, or the plug might not have been installed in the first place. Either way, this is an easy fix: Buy a replacement plug at a hardware store and screw it in.
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Dry Toilet Trap
It’s less likely, but the water in the toilet trap also could have evaporated. This is a possibility of you hardly ever use the toilet. Of course, simple flushing will restore that water and should eliminate the foul smell. In the future, try to remember to go down and flush that toilet once of twice a month.
Family Handyman
Leaky Wax Ring
A bad wax ring seal between the toilet flange and the base of the toilet can release sewer smell into the bathroom. If the bathroom is in the basement, there’s your culprit. The wax ring may be leaking because the toilet rocks or because the flange is mounted too far below floor level for the wax to reach the bottom of the toilet.
If that’s the case, you’ll have to remove the toilet and replace the wax ring. Here are some tips from Hiller and master plumber Aaron Adams for doing the job efficiently:
Tips for replacing a wax ring
“Turn off the water to the house,” says Adams. “It sounds simple, obvious, and possibly unnecessary, but you wouldn’t believe how many homes we’ve seen flooded from a broken supply line.”
“Once you have the water turned off, flush the toilet to drain the water from the tank to the bowl,” he continues. “Then use a shop vac to remove residual water in the toilet P-trap. This avoids a mess when you lift the toilet away from the drain.”
When you get the toilet off, Hiller recommends getting every scrap of the old wax ring off. “Even a tiny chunk left behind can cause leaks later.” A stiff putty knife usually does this job efficiently, and have plenty of paper towels handy for disposing of the old wax.
Hiller continues: “If your flange sits low, use an extra-thick wax ring or a ring with a rubber funnel.”
Once the new ring is in place and you set the toilet back in position, Hiller recommends sitting on the toilet for a few minutes. “Your weight helps squash the wax for a better seal.”
Finally, says Hiller, “always do a couple test flushes and look for drips before you say you’re done.”
If the toilet still rocks, the flange is probably mounted too high. To stop the rocking, use plastic shims between the stool and the floor and caulk the joint.
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Cracked Lines Causing Sewer Smell
A more serious possible cause of the sewer smell in your house could 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.
Two gases — methane and hydrogen sulfide — are responsible for the sewer smell, and these gases are both flammable and noxious. However, at the concentrations present due to a plumbing leak, they probably aren’t dangerous — not unless the basement is sealed and the gases have been leaking for a long time.
“What you should worry about,” warns Hiller, “is why it smells. A leak, a cracked pipe, or a dried-out drain that lets in gas and moisture can lead to mold, water damage, bugs, or a sewage backup that wrecks your basement.”
Will bleach help?
No. Bleach can kill pathogens around a dirty drain opening, but that’s not the problem here. The smells come from the sewer, and you can’t pour enough bleach into a drain to eliminate those, nor would you want to. They are a byproduct of normal waste decomposition, and if you interfere with that, you risk ending up with a clogged sewer or septic system.
About the Experts
Jimmy Hiller is the CEO and President of Happy Hiller, a company specializing in plumbing, heating, cooling, air quality, and electrical services.
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