Calendar 18 September, 2023 (Updated 21 September, 2023)

Smelly Drains: Main Causes And Cures

Nobody wants the smell of a blocked drain, particularly inside the home. And it’s not only sewers that can produce stomach-turning odours; wastewater from sinks, dishwashers and washing machines can be pongy too, while the smell from roof gutters, downpipes and surface drains has a unique and unforgettable bouquet.

Fortunately, most of the time it’s easy to find the cause and straightforward to rectify the problem. Let’s look at why we sometimes get these nasty niffs and how to banish them.

How you will know your drains are blocked

There are three main symptoms of a blocked drain and they all involve your senses.

Drains look like they are blocked

A partly blocked drain will often become apparent if the water level in a toilet, sink, basin, shower or bath drops much slower than usual. If the drain is completely blocked, the water won’t drain away at all; this can be particularly distressing if a flushed toilet overflows.

Drains sound like they are blocked

In addition to not emptying quickly, a partly blocked drain may make strange gurgling noises. If your coffee percolator is turned off and no family members are gargling their mouthwash, it’s likely to be the drains.

Drains smell like they are blocked

A drain doesn’t have to be completely blocked for it to smell bad. As soon as matter starts to accumulate in the pipework rather than flushing right through, it becomes a breeding ground for the bacteria that make these dreadful smells. That’s why a bad smell is often the first indication that your drains need attention.

What might be blocking your drains

The likely cause of a blocked drain will depend on where the waste is coming from.

Toilets causing blocked drains

Sewage by itself doesn’t generally block drains; it’s everything else we flush down the toilet that causes the problem. One of the biggest culprits is wet wipes; even the ‘flushable’ variety. This is because while they are formulated to break down in contact with water, the time it takes for them to disintegrate is far shorter than the time it takes them to flush down the sewer, meet an obstruction and start causing a blockage.

Unfortunately, some people regard the toilet pan as a general-purpose waste disposal unit for sanitary products, cotton buds, food leftovers and even copious amounts of toilet tissue. This is worsened by the fact that most toilets these days have a ‘water-saving’ flush.

Sinks and dishwashers causing blocked drains

The prime suspect for drain blockages coming from kitchen sinks and dishwashers is, unsurprisingly, food waste, and the worst culprits are those who pour waste oils and fats down the drain. These quickly solidify and create the notorious fatbergs that have hit the headlines in recent years.

Baths, basins and showers causing blocked drains

Blocked drains in the bathroom are most commonly caused by an accumulation of hair and shampoos, soaps and conditioners. While these are good at cleaning our hair and skin, they’re not so good at keeping the pipes shiny.

Washing machines causing blocked drains

Lint, unemptied contents of pockets and fabric softener are the biggest culprits here and as more and more machines are designed to work on energy-efficient cycles that are shorter and cooler, this situation can only get worse.

Why your drains might be smelly

All water that passes through a drain contains substances that either are or have the potential to be smelly.  Sewage is an obvious one, as are food scraps from kitchen sinks and dishwashers, but there’s also lint that accumulates in washing machines and rotting moss, leaves and other vegetation from our roofs.

If a drain is working properly, there should be sufficient volume and force of water to wash all this disgusting stuff far down the drains and into the main sewer or wastewater systems. As an added precaution, traps are built into the drainage system to create an airlock that prevents smells from wafting back up the pipework – you’ll see them under any sink or basin, bath or shower and behind a washer or dishwasher.

Where your drain may be blocked

Once again, this often depends on what the drain is connected to. In the case of a toilet, it is generally the result of flushing the wrong things down the pan or not flushing enough water. The blockage may occur in the trap immediately below the toilet pan, but more often than not it will be further downstream where the flow has less force.

Washing machines feature filters on the water outlet that will collect lint and foreign objects. This is designed to be easily and regularly removed and cleaned.

Most other blockages close to home will be found in the trap directly under the sink, basin, bath or shower, or wall-mounted behind a dishwasher. 

How to unblock your drain

You will need to start with identifying the location of the blockage, which often as not will be in the trap. Except for the trap on a toilet, these are mostly easily unscrewed (place a bowl under the trap to catch any spillages) and manually cleared out.

Use air or water pressure

If you can’t (or don’t want to) remove the trap, you could try using a plunger; a deep rubber cup mounted upside down on a wooden handle. Run enough water into the sink or bath to cover the cup and place its open mouth over the drain to create a complete seal. Push rapidly and firmly down with the handle, and this will force water through the trap, hopefully carrying the blockage with it.

If a simple plunger doesn’t work, you could try a more advanced and powerful version that uses compressed air or a handheld jet pump that uses high-pressure water to clear the blockage.

Mechanically dislodge the blockage

If the above methods aren’t successful, a drain snake may be the answer. This is a coil of steel wire with a corkscrew-shaped end. As the tip of the snake reaches the blockage it is rotated clockwise, burrowing itself into the waste and dislodging it one bit at a time. One advantage of a drain snake over a plunger or jet is that it can reach easily around corners to clear blockages.

Use a drain-unblocking professional

Still no success? If this is the case it may be time to call in the professionals. Unless you know what you’re doing and have the right equipment, you could quickly cause expensive damage to your drains.

Professionals use a variety of tools to unblock stubborn drains, ranging from manually or electrically operated commercial waste unblockers to kinetic water ram kits and drain rods.  Drain rods are equipped with a selection of end attachments including a corkscrew, harpoon, brush and hook to loosen and remove all kinds of blockages and in professional hands, they will quickly have your drains running clear again.

What if your drain isn’t blocked but still smells?

If a drain smells bad but still lets water drain through the trap, this can be caused by trap failure.

What is trap failure and how can it be resolved?

The purpose of a drain trap is to create a section of pipe which is permanently filled with water. As water and waste pass through the trap the level in the trap remains constant, forming an airtight seal between the upstream and downstream sides of the trap. The water in the trap is regularly replenished and therefore doesn’t have enough time to stagnate.

While waste matter in the drains can and will decompose, creating unpleasant gases and foul smells, the trap prevents these from returning to the house.

However, a trap will be unable to work properly if that airtight seal is removed. This can happen in two ways.

Possibly the most common reason for this is that the water in the trap evaporates, often if no water has been flushed through the trap for a while. This can happen much sooner in warm weather. 

To resolve this problem, run water through the trap for a while until the contents have been replaced with fresh water. Allow the room to air well, and the smell should disappear.

Another cause of trap failure is siphoning. This is most commonly a result of a poorly installed drainage system; a run of pipe that is too long, a pipe gradient that is too steep and a pipe connection with a soil pipe being too short are all common examples. The force of the water downstream from the trap becomes so strong that it sucks the water out of the trap. This is what causes drains to make gurgling noises.

Installing an anti-siphon trap should resolve this problem without having to completely reinstall the drainage system. Also known as anti-vac traps, these feature a valve that opens when siphoning starts, drawing air into the system to regulate the air pressure and maintain the water level in the trap. As this is a one-way valve, it prevents any gases or smells from escaping.

If the small can’t be traced back to a trap, it could be lurking in a gully outside the house. Wastewater from basins, baths, showers, sinks and appliances often empties into an open gully just outside the house before continuing its onward journey to the main drains. This can be a collecting point for particles in the wastewater mixed with outdoor debris such as dead leaves. 

If the gully looks congested, lift the gully grate and clear away any debris from the grate and the gully, Flush water into the gully to ensure it is emptying freely and replace the grate.


We hope you have found this information helpful and interesting. If you have any further questions, you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected].

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!

Vicki James Drainfast Team Portrait

Written by
Vicki James

Sales & Marketing Coordinator

Vicki is a vital part of the marketing team; from reporting to copywriting, she ensures we complete projects on time.

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