Why Do My Drains Smell?

Are you smelling rotten eggs, a sewage smell or just any other unpleasant odors?

If you’re getting a sewer smell coming from one of your sink or shower drains, it is likely one of two causes:

  1. Something is wrong with the trap.
  2. A buildup of organic matter.

In very rare cases it can be something else, like mould, a broken sewer line or even a dead animal in the walls close to the sink drain.

Let’s hold our noses and explore the most common causes in more detail. Meanwhile, run any exhaust fans and open any windows in the room. If the problem is actual sewer gas, it’s not only unpleasant it can be dangerous if it builds up too much.

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1. The Sewer Gas Trap

All drains have – or should have (more on this later) – a low bend called a trap. The bend faces downwards, and water collects in there, blocking the sewer gases from emerging from the drain. Gravity holds the water in place, and it is refreshed every time you run the tap. Usually in Ottawa, we have P-shaped traps like the one pictured below.

P Trap Diagram
Image courtesy Wikipedia (public domain).

There are several things that could be wrong with the trap.

1. There may be a leak from the trap, allowing the water to escape.

To see if a leak is the cause, check for signs of water under the sink. If there are bathroom sink smells, check for signs of a leak on the floor below. Examine the ceiling and walls and look for discolouration or distorted textures.

2. The water could have simply evaporated from the trap if you haven’t used any water in weeks. This can be a nasty surprise for people who come home from long vacations.

If there are no signs of a leak, try pouring some water down the drain to fill the trap. You’ll have to wait a while to see if it worked, as the room will already be full of the smell.

3. In some very old homes, it’s possible a trap was never installed. Only a plumber can confirm this for sure and install one if needed.

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2. Organic Matters

If the trap is fine, you may have a buildup of organic sludge in the trap and/or on the inside of the drain pipes.

Over time residue from all of the dish soap, toothpaste, food scraps, grease and other things we put down our drains collects on the inside of the pipe. It can also build up on the inside walls of the plumbing, attracting bacteria that feed on it and multiply. This can be a good thing, as long as it stays in check and doesn’t narrow the diameter of the pipe too much. If there is too much buildup, however, a bad smell or even a drain clog can result. You will probably need a drain inspection from a plumber.

If the smelly drain is in the shower, the usual cause is a build-up of hair in the drain trap. You may also experience an overflowing floor drain. If it’s easy to lift the drain cover you may be able to reach in with pliers and pull out the hair yourself. Don’t use your fingers as there could be sharp edges. The important thing is not to push it down – the clump may catch somewhere else further down in your plumbing and start a sink, shower and bathtub clog. If the drain cover does not come off easily you will need to call a plumber.

Try these fixes:

  1. Pour a kettle of boiling water down the drain.
  2. If that doesn’t work, sprinkle a cup of baking soda in the drain and allow it to sit for a few minutes.
  3. Follow up with a cup of white vinegar and wait again. After a few minutes, add hot water.

It’s important to note:

Boiling hot water from the kettle could crack fixtures or deform piping under extreme circumstances. A sudden temperature change can cause cracking. Most plumbing systems are not designed to take those high temperatures, modern plastics could be weakened, deformed or broken.

With kitchen sink buildup the usual culprit is allowing too much grease and food to go down the drain. Pour baking soda and vinegar, boiling water as instructed above to handle it. That baking soda and vinegar trick will sometimes need hot vinegar in order to get your kitchen drain opened.

We don’t recommend people try and open the traps below sinks and clean them out themselves. We’ve been called in too many times to rescue DIYers who got stuck halfway through the job, or who were unable to reattach the trap tightly enough to prevent leaks. Save yourself some grief and call a licensed master plumber.

Whatever you do, please don’t use Drano or other harsh chemicals. They don’t just attack organic buildup, they attack pipes too. Drano weakens pipes and can lead to leaks if used often enough.

3. Dirty Garbage Disposal

While the convenience of a garbage disposal a great, its possible for food is caught in the disposal blades and rotting food which may be why your kitchen sink drain smells. This can be caused by not running enough water each time you put food down the sink, or by dumping a lot of food all at once.

Cleaning it out with coarse salt, ice cubes, cold water and lemon peels may fix the smelly sink. If it doesn’t, your disposal may need to be replaced.

4. Clogged Vent

Ottawa building codes make sure there’s at least one main vent of at least four inches in diameter where all fixtures are vented. This allows for the air in the piping to “vent,” so the water can move through the pipe freely. If the vent is clogged, it can slow the water from draining properly and then sewer gases can move into your kitchen and house.

A blocked vent can also create a vacuum in the piping. If large enough, it can suck water from the traps which then allows sewer gasses to enter the home through a dry trap.

5. Unused Drain

As we mentioned, your sink drain smells when it hasn’t been used in a while, this is typically because of a dry p trap. The p trap is a part of your plumbing system that holds water and the base of the curve in order to keep the nasty smell away. However, if it hasn’t been used in a while, the water in the p trap will evaporate and you suddenly don’t have protection from your smelly sink drains. If your home is very old, you may not have a p trap at all- though that would be noticeable quickly.

Similarly, if you have food particles building up in the pipes or organic matter such as hair, soap, shampoo, and other stuff you typically wash down your sink drain- when it dries up it’s not long before you have a stinky sink on your hands. If this is the case, your smelly drain is well on its way to becoming a clogged drain, especially if you are hearing some gurgling when running water.

6. Not All Bacteria Are Bad

Germophobes may not like this idea, but your home’s plumbing actually benefits from some bacteria, just like your body uses some bacteria to help you digest food. As mentioned above, these bacteria digest the organic slime that builds up in the pipes and actually do you a favour.

If you’re looking for a natural way to help the process along, BioOne is a natural drain treatment that adds non-smelly bacteria to your drain. They go to work eating all the gunk in there, and keep your drains clear. Best of all BioOne doesn’t damage your plumbing. A capful in each drain every month will keep them free of buildup, as long as you’re not putting too much hair or food down the drain.

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