Full Flow Plumbing & Drainage

why do my drains smell

Why Do My Drains Smell? Common Causes and Solutions

Do you notice an unpleasant smell coming from your drains? It’s not unusual to experience this issue, and it can be caused by various factors.

One of the most common reasons for smelly drains is the buildup of bacteria and organic matter in the pipes. This can lead to a buildup of slime and gunk that emits an unpleasant odour.

Another reason for smelly drains is the accumulation of hair, soap, grease, food, and other debris in the pipes. This buildup can cause a blockage, leading to slow-draining water and an unpleasant smell.

If you have used chemical drain cleaners to unclog or clean your drains, the chemicals mixed with the organic matter can often make the smell worse.

Fortunately, there are ways to fix smelly drains. In this article, we will explore the common causes of smelly drains and provide you with tips on how to get rid of the unpleasant odour. By following these steps, you can ensure that your drains remain clean and fresh.

Why Do My Drains Smell?

Here are the most common drain smells we encounter and why this can happen:

Like Rotten Eggs

If the smell coming from your drain resembles rotten eggs, it’s likely that you have a problem with your plumbing system’s venting. Plumbing vents are designed to allow air to escape from your pipes, preventing pressure buildup that can lead to blockages and other issues.

When your plumbing vent is blocked or not functioning correctly, it can cause a rotten egg smell from your drains.

Like Sewage

If your drains smell like sewage, you may have a problem with your sewer line. A broken or damaged sewer line can cause raw sewage to leak into your home’s plumbing system, resulting in a strong, unpleasant odour.

Other potential causes of a sewage-like smell include clogs in your drain or sewer line, which can cause waste to back up into your home’s plumbing system.

When it Rains

If you notice a foul smell coming from your drains only when it rains, it’s possible that you have a problem with your home’s drainage system. Heavy rain can cause your drains to become overwhelmed, leading to backups.

If your home’s drainage system is not properly designed or maintained, it can lead to water pooling, which can also cause bad smells.

At Night

If your drains only smell bad at night, it’s possible that the problem is related to your home’s ventilation system. When your home is sealed up tight at night, it can lead to a buildup of moisture and odours, which can cause your drains to smell bad.

Common Causes of Drain Smell

If you are experiencing a foul odour emanating from your drains, you are not alone. Smelly drains are a common problem that many homeowners face. Here are some of the most common causes of drain smell:


Mould, a type of fungus, thrives in moist environments like your drains. It accumulates over time due to the combination of water, soap scum, and organic matter that pass through the drain.

The musty smell emitted is a result of this mould growth. Mould releases microscopic spores, giving off an unpleasant odour but can also cause health issues like allergies and respiratory problems.

You can try using the baking soda (bicarbonate soda) and vinegar method to kill the mould. Pour half a cup of baking soda into the drain, followed by half a cup of vinegar. Let it sit for about 20 minutes, then rinse it with hot water.

Failing that, mould removers are available in most supermarkets and are designed to kill mould and remove its stains. If the mould returns quickly after cleaning, or if you notice it’s spread to other areas of your bathroom, there could be an underlying issue, such as leaks or structural damage.


Biofilm is a microbial community that thrives in a slimy, glue-like substance. This substance, which they produce, allows them to stick to various surfaces, including your shower tiles and inside the drain. The biofilm acts as a protective layer for these bacteria, making them resistant to cleaning agents and antibiotics.

A characteristic of biofilm is its musty, mildew-like smell. This is due to the metabolic by-products of the bacteria within it. Biofilms can harbour dangerous bacteria such as Salmonella and E.coli, which can pose serious health risks.

Removing biofilm is generally straightforward and inexpensive, but requires a bit of elbow grease. The first step is to physically remove as much of the biofilm as possible using a brush or scraping tool. By repeatedly moving the tool in and out of the drain, you can dislodge a significant amount of biofilm.

After physically removing the biofilm, the next step is to disinfect the area using an antimicrobial chemical. This will kill any remaining bacteria and help prevent future biofilm formation. Spray the chemical onto the area and let it sit for a few hours before rinsing.

If you’re not confident handling biofilm on your own, give us a call at Full Flow Plumbing and Drainage. Our Brisbane plumbers can professionally clean your drains and remove any deep and hidden biofilm.

Trapped Debris

One of the most common causes of drain smell is trapped debris. Over time, hair and soap scum, food particles, and other debris can accumulate in your drains. As this debris decomposes, it smells.

To prevent trapped debris from causing a smelly drain, it is essential to clean your drains regularly. You can use a plunger or a drain snake to remove any clogs. Additionally, you can use a mixture of baking soda and vinegar to break down any buildup in your drains.

Sewer Gas

Another common cause of drain smell is sewer gas. Sewer gas is a mixture of methane, carbon dioxide, and hydrogen sulphide.

Sewer gas can enter your home through your drains if there is a problem with your plumbing system. This can occur if there is a blockage in your sewer line or if your sewer line is damaged.

To prevent sewer gas from entering your home, it is essential to have your plumbing system inspected regularly by a professional plumber. We can identify any issues with your plumbing system and make the necessary repairs.

Dry P-trap

The P-trap is a U-shaped pipe that is located under your sink or shower drain. The P-trap is designed to hold water, which creates a seal that prevents sewer gas from entering your home.

If the P-trap dries out, it can no longer create a seal. This can occur if you have not used a sink or shower drain for an extended period.

To prevent a dry P-trap from causing a smelly drain, run water through your drains regularly. You can pour a small amount of vegetable oil down your drains to help keep the P-trap from drying out.

Professional Solutions for Drain Smell

If the DIY methods do not work, it might be time to call in a professional plumber. We use specialised tools and techniques to identify and fix the root cause of smelly drains such as:

CCTV Drain Camera Inspection

A CCTV drain camera inspection is a non-invasive way to identify the source of the smell. We insert a small camera into the drain and navigate it through the pipes to locate any blockages or damage. This method is particularly useful for identifying issues that are not visible to the naked eye, such as tree root intrusion or cracked pipes.

Drain Cleaning

At Full Flow Plumbing and Drainage, we use specialist tools and equipment to effectively clean your drains without the need for chemicals.

Drain Replacement

If the issue is caused by damaged or corroded pipes, we may recommend replacing the affected section of the drain, or if possible, relining the drain.

Drain replacement is a more invasive solution that involves digging up the affected area and replacing the pipes. While it may be more expensive than other solutions, it is a long-term fix that can prevent further issues down the line.


Smelly drains are a common problem that can be caused by various issues. Some of the most common causes include a dry P-trap, clogged vents, mould growth, and a buildup of hair, soap scum, and residue in the drain trap.

To fix the problem, you can try running the taps more often to keep the P-trap from drying out, clearing any clogs in the vents, cleaning out any mould growth in the pipes, and removing any buildup in the drain trap. You can also use baking soda and vinegar or boiling water to kill bacteria and loosen any objects stuck in the drain.

It’s important to remember that prevention is key when it comes to smelly drains. Regular maintenance and cleaning can help prevent these issues from occurring in the first place. By taking care of your drains and keeping them clean, you can avoid unpleasant odours and keep your home smelling fresh and clean.

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