September 5th, 2023 by Shauna Hammond

We have all likely experienced foul-smelling drains, from the smell of dampness, mould, or the pungent rotten egg odour that more often is associated with drains. Not only does this make the room where the offending drain is located unpleasant. Sometimes it can be a quick and easy fix, but that is not always the case. In this blog, we will take a look at some of the most common causes of bad-smelling drains and how you can fix them.

What Causes Drains to Smell?

There is a wide range of things that can cause your drain to smell, after all when you think about how frequently we use them, from sinks, toilets, bathtubs and showers – in fact, it isn’t a push to say that your drains get used multiple times, daily. It is understandable that given the frequency of use and their purpose bacteria and other contaminants can easily build up resulting in bad odour forming over time, so the most important thing to do is identify the cause of the odour so that you can correctly treat it. So, let's take a look at some of the leading causes of smelly drains and how to identify if this is the issue you are facing.

A Build-up of Organic Matter

One of the biggest causes of odour from a drain especially in bathroom and kitchen sinks is a build-up of organic matter, this may not necessarily cause a blockage which is why it can be difficult to tell there is a problem until the distinct odour of rotting food or even rotten eggs is predominant.  A build-up of organic matter can include small food particles, grease, hair and soap scum from bathroom and kitchen sinks or showers – as mentioned these small particles are not always enough to cause a blockage however they can become stuck to the inside of the pipes they will then begin to rot and creates gas as a by-product and this in turn causes the foul odour.

The best way to tell if the smell from your drains is being caused by a build-up of food particles and other natural matter is by the distinct rotten egg smell or the sickly-sweet smell that you often find with rotting food. The good news is that this is a problem that is fairly easy to treat in a couple of different ways. You can use a mixture of baking soda and vinegar this will expand in the drain absorbing odour while the vinegar will help to break down the stuck particles making it easy to flush them away – this will not only remove the odour but will also help to remove the source too. It is a good idea to keep up with this cleaning routine periodically to keep pipes clear and prevent any foul odours from returning.

Blocked Drain Pipes

Most commonly bad odours from your drain can be caused by a blockage in the pipe, this blockage can be a result of organic matter such as hair, soap scum and food particles mentioned above, or other item that shouldn’t be going down the drain such as rings, hair clips, plastic and many more. Blockage in pipes can result in odour for a number of reasons including organic matter decaying or the foul odour may be a result of standing water stuck in the pipe as a result of the blockages. Not only does the cause odour but it can also invite unwanted guests into your home such as drain flies, which are not only unsightly but can also be quite a nuisance to get rid of too.

It is easy to identify if the issue is a result of a blockage as you will notice the water is either draining slowly or not draining at all – the smell of standing water is quite recognisable too. So, with these easily identifiable factors, it should be fairly obvious if this is the problem. There are again a few ways this can be dealt with, if you think that you may have lost something important like a ring down to the drain, then unscrewing the u-bend and manually removing the blockage is probably the best method, however, if there is nothing you need to salvage from your drain you can use a plunger to free the blockage so it can then be rinsed away, or alternatively you can use products such as Pro-Kleen Sink and Drain Blitz that will break down the build-up of organic matter making it easy to be rinsed away.  

1L Pro-Kleen Sink and Drain Blitz Opener and Unblocker

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Dry Trap

Another cause of foul odour which can be a little harder to treat and identify is when the u-shaped pipe under your sink or shower known as a P-trap runs dry. These pipes are designed to hold a small amount of water inside and this water acts as a barrier preventing sewer gasses and odour from entering your home, if this water is to leak or runs dry, this allows the odours and gasses to enter your home and can create a foul smell that using drain cleaners or baking soda and vinegar can not remove. P-traps are more likely to run dry in sinks that are not frequently used, so if you move into a property that has been vacant for a while you may encounter this problem.

In some cases, this can be easily resolved by leaving a tap running for a few minutes at regular intervals this will restore the small amount of water needed once again creating the barrier to block out odour, however on some occasions this may not resolve the issue and a professional may need to be contacted to replace or repair this pipe.

Mould and Mildew

It may come as a surprise that mould and mildew can develop in our drains and pipes, but considering that they thrive in cramped and damp places it is understandable that drains and pipes are actually the perfect place for them to develop. It can be difficult to promptly deal with mould colonies in pipes and drains since they can not be easily seen the way they can be on walls for example. You can usually tell if you have an issue with mould if you notice the water is draining slower than usual and if your pipes have a distinct musty smell that you find with mould.

If you suspect that you have mould in your pipes there are a few methods you can use to try and treat this such as pouring thick bleach down the plug and leaving this to work for around one hour before rinsing away with boiling water, this can then be followed up with a mixture of baking soda and vinegar to remove any remaining build up and absorb any lingering odour, leave this mixture to sit between 10-15 minutes before rinsing with more boiling water.

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We hope this article has helped you understand what can cause your drains to smell, how to identify the cause and how best to treat it. We know that bad smells can be very frustrating, but we also want you to know that there are things you can do to stop them and it does not need to be a difficult task either! If after following the above tips you notice that odours or poor draining persist it may indicate a more complex issue and a professional plumber should be contacted.