It’s never a pleasant surprise to get close to the drain of your sink or your shower only to have your nostrils assaulted by an overwhelming bad smell.
“Oh no,” you think. “What is this smell?!”
And just like that, you have so many questions. Why does your sink or shower drain smell so bad? Is this normal? How can you fix it?
First things first: No, your sink or shower drain should never smell bad, so if the odor is bad enough that you notice it, there’s probably some sort of problem. And depending on what the cause of the odor is, the solution might be fairly simple or may require a visit from the plumber.
Start by determining what the smell is.
The main culprits for a smelly drain are mold, biofilm, or sewer gases, and most people will be able to determine the likely cause of the smell just by doing a quick assessment.
Is it mold? If your shower or sink drain is flowing slowly and water is left standing for several minutes, this could mean you have a clog caused by debris like soap scum, oil, mineral deposits, dirt, and hair. These clogs, if they persist long enough, can be a cause of mold because they hold so much wetness and moisture in a dark place without a lot of airflow. This mold and mildew can then become a source of unpleasant odor.
Is it biofilm? Like mold, biofilm is another type of buildup. At first glance, it could even be mistaken for mold. But biofilm tends to look a little slimier. It’s the result of bacteria and decay, and it’s not unusual for it to form just out of sight inside bathroom fixtures like sink drains or shower drains. Biofilm, which tends to smell like rotten eggs, might not be something you see at a casual glance. However, if you take a look at the drain with a flashlight, you should be able to identify it just beyond the edge of the drain.
Is it sewer gas? If you don’t see any sign of biofilm but you’re still noticing a strong rotten egg smell coming from your drain, this might be because sewer gases are making their way up. This is a pretty gross realization for most people. Your plumbing is actually designed to stop sewer gases from rising to the surface of your drain. Every drain has something called a P-trap, which is a curved portion of pipe that holds just enough water to block off any vapors rising from down below. If sewer gases are making their way up, you might have a problem with your P-trap.
Addressing mold or biofilm.
If you determine that your smelly drain is the result of mold or biofilm buildup, the first step is to address the debris that’s causing the mold or biofilm to grow.
Use a snake down your drain to clear the pipe of any large debris. After you’ve done that, use a combination of baking soda and vinegar, followed by a flush of hot water, to remove any gunk trapped in your pipes. The chemical reaction of the baking soda and vinegar will also kill any mold or bacteria.
Once you’ve flushed sufficiently with hot water, you can pour bleach down the drain to eliminate remaining mold and bacteria.
Be careful, however—if you mix bleach and vinegar, it will produce toxic chlorine gas! Be sure all the vinegar has been flushed away before using bleaching.
Addressing an ineffective P-Trap
If you suspect that the smell might be sewer vapors, then it’s time to address why your P-trap isn’t working. In some cases, the solution might be a simple fix—maybe it’s a sink or a shower that you don’t use very often, such as the ones in a guest bathroom, so all of the water sitting in the P-trap has evaporated and all the vapors that would normally be blocked off are making their way up. Flush the drain with water and see whether that solves the problem. If the smell goes away, then make a regular point of flushing the drain at least once a month even if nobody is using that sink or shower.
If the problem persists, then you may have a crack in your P-trap that is preventing water from collecting the way that it should. P-traps are typically made from heavy-duty material, but it is not unusual for them to wear down as they get older. If you have a leak in your P-trap that is allowing sewer vapors to rise into your home, it’s especially important to address this. Not only is the smell gross, thus making you and other members of your household uncomfortable, but also the methane and nitrogen hydrogen sulfide gases could be a threat to your health.