Bathroom drain smells like sewage: bicarbonate of soda and hot water (and what to avoid)

A bathroom drain can start giving off a smell similar to sewage even when there is nothing wrong with the sanitary system. Hair, soap residue, skin oils and other organic matter stick to the inside of the drain and the trap, creating a film that holds grime and encourages bad odours. Bicarbonate of soda and hot water can help lift this build-up, but one point matters: boiling water should be avoided in PVC pipework.

Why does the drain start to smell?

While you are showering, loose hair, soap, skin cells and body oils travel away with the water. Some of this debris catches on the grate, settles in bends in the pipe, and sits in the trap. Over time, it forms a thin layer of residue that can produce an unpleasant smell and may also slow down how quickly the water drains.

When the odour is coming from this surface build-up, cleaning the drain usually makes a noticeable difference. If hair has collected, though, it is best to pull it out by hand or with a suitable tool, because bicarbonate of soda will not dissolve large clumps of hair. Home mixtures can help with minor restrictions, but they do not replace physically removing a bigger blockage.

• Remove hair and any obvious debris from the grate;
• Clear any dirt you can reach at the drain opening;
• Use bicarbonate of soda to help loosen residue stuck to the sides;
• Finish with hot (not boiling) water if the pipework is PVC.

How to use bicarbonate of soda and hot water in the drain

Once you have taken out the hair, add a small amount of bicarbonate of soda to the drain and let it sit on the dirty area before rinsing through with hot water. The aim is to help detach residue and move the loosened dirt along the pipework, not to rely on it to dissolve a tightly packed obstruction on its own. Some plumbing-maintenance guidance also suggests bicarbonate followed by hot water as a routine option to help manage odours in certain drains.

Why pouring boiling water is not a good idea

Temperature matters most with PVC installations. Drainage systems made from this material can have a maximum operating temperature of around 60 °C, depending on the pipe specification. Water straight after boiling is close to 100 °C, which exceeds that limit.

For that reason, using hot water rather than water at a rolling boil is safer, especially if you do not know the age or type of the pipework. Any at-home cleaning should also be straightforward, without combining different chemicals inside the drain.

• Let boiled water cool down a little before using it;
• Avoid repeatedly pouring water at roughly 100 °C into PVC;
• Do not combine chemical drain unblockers with other mixtures;
• If water is pooling, remove the hair blockage first.

What about caustic soda - can it be used in pipework?

Caustic soda, also known as sodium hydroxide, needs far more caution. It can cause serious burns to skin and eyes, and it releases a lot of heat when it comes into contact with water. Although PVC can be chemically resistant to different concentrations of sodium hydroxide, incorrect domestic use of caustic drain products introduces risks that are not present when you simply clear a drain mechanically.

• Never mix caustic soda with other cleaning products;
• Avoid any contact with skin and eyes;
• Do not increase the amount in an attempt to speed up unblocking;
• Call a professional if the blockage persists or water backs up.

What if the smell continues even after cleaning?

Even a perfectly clean drain can still smell of sewage if the trap has dried out. The water sitting in the bend of the trap acts as a physical barrier that blocks sewer gases from coming back into the room. If a bathroom, cloakroom or floor drain goes unused for a long time, that water can evaporate and allow gases to re-enter the space.

In this situation, bicarbonate is not needed: simply run or pour water into the drain to restore the water seal. If the smell returns quickly even when the trap is full and the drain is clean, the cause may be a sealing issue, poor ventilation, or another installation problem-where inspecting the pipework is more helpful than continuing to pour products down the drain.

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