If a bad odour is drifting up from your drains, you are not alone. A drain smell is one of the most common household plumbing complaints, and it usually has a straightforward cause. This guide walks through the seven most likely reasons your drains smell so bad, how to fix each one, and when it is time to call a professional.
Most of the time a drain smell is caused by harmless bacteria or trapped gunk. Occasionally, though, it points to something more serious such as a sewer gas leak, so it is worth working through the possibilities methodically.
7 common causes of a bad drain smell
1. Bacteria and gunk build-up
The most common cause is simply a build-up of bacteria and debris inside the pipe. Food scraps, hair, soap and grease all collect on the pipe walls over time and start to smell. This often goes hand in hand with slow drainage, so a smelly drain is frequently an early warning of a developing blockage. A thorough clean usually clears both problems, and for a stubborn case it is best to use a professional drain cleaning service. See our guide to blocked drains if the water is also draining slowly.
2. A dry P-trap
Every drain has a U-shaped section of pipe called a P-trap, which holds a small amount of water to block sewer gas from rising into your home. If a sink, shower or floor drain has not been used for a while, that water evaporates and the gas comes straight through. The fix is simple: run the tap for a minute to refill the trap. To prevent it happening, run a little water through every drain at least once a month.
3. Clogged or missing vent pipes
Your drain pipes are connected to a network of vent pipes that let fresh air in and allow waste gases to escape, usually through a pipe that runs up and out of the roof. Together these form the drain-waste-vent (DWV) system. If a vent is blocked, damaged or missing, gas can be forced back through the drains instead, causing a persistent odour that no amount of cleaning will fix.
4. A smelly sump
If the bad smell is coming from a basement or sump area, check that waste water is not draining into the sump pit where it can stagnate. Water from a washing machine running into a sump, for example, can build up and turn foul over time.
5. Grease in the kitchen drain
Fat and grease that cool and congeal in the kitchen drain produce a distinctive rotten or musty smell. Never pour grease down the drain. As a simple home remedy, pour two teaspoons of bicarbonate of soda down the drain, follow with vinegar, then flush with boiling water to help dissolve and wash away the build-up. For persistent grease, especially in a commercial kitchen, a professional clean is the better option, and our grease trap cleaning page explains how we handle it.
6. A missing or faulty drain trap
If a fixture was installed without a proper trap, there is nothing to stop sewer smells entering your home. Fitting or repairing a drain trap is a job for a qualified plumber, but it is a permanent fix for what can otherwise be a constant nuisance.
7. Water contamination
Sometimes the smell is in the water itself rather than the drain. Sulphate-reducing bacteria in a hot water cylinder, for instance, can create a rotten-egg odour. If you suspect the water supply, have a professional check your pipes and water quality.
When to call a professional
You can tackle a dry P-trap or a greasy kitchen drain yourself. But if the odour persists after cleaning, comes back quickly, or smells strongly of sewage, it usually points to a blockage, a venting fault or a damaged line that needs expert attention. Reaching for shop-bought chemicals is rarely the answer, our guide to the dos and don’ts of drain cleaners explains why, and how to unblock a sewage drain covers more serious cases.
For more on the gases involved and why they should not be ignored, see the sewer gas overview on Wikipedia. Drain Blasters provides professional drain odour removal across Cape Town. If a bad drain smell will not go away, get in touch and we will trace it to the source.
