Why Do My Drains Smell?

dog smelling a drainIf 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 or even a dead animal in the walls close to the 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.

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 gasses 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 the smell is coming from a shower drain, 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.

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 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 which 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 build up, however, a bad smell or even a drain clog can result.

If the smelly drain is in the shower, the usual cause is a build up of hair in the drain trap. 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 clog. If the drain cover does not come off easily you will need to call a plumber.

With kitchen sink buildup the usual culprit is allowing too much grease and food to go down the drain. Try these fixes:

  1. Pour a kettle of boiling water down the drain.
  2. If that doesn’t work, sprinkle ¼ cup baking soda in the drain and allow it to sit for a few minutes. Follow up with ¼ cup white vinegar and wait again. After a few minutes, add hot water.

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 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.

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 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.

Learn More About BioOne