A bathroom drain smell is one of those problems that seems to arrive overnight. The room looked clean yesterday, and today there is a sour, eggy or sewage-like odour every time you open the door. In this guide we explain where that smell usually comes from in a Singapore bathroom, how to clean the floor trap and hair catcher properly, how to keep the water seal working, and how to tell when the problem is really one for a licensed plumber.

The good news is that most drain odours in HDB flats and condominiums have simple causes. Hair, soap residue and grit collect just below the grating, start to rot in our warm, humid air, and the smell rises straight back into the room. A dried-out water seal in a bathroom that nobody uses is the other common culprit.

A small share of cases are pipe faults that need a professional. Femme5 cleans homes but does not do plumbing or pest control, so everything below is cleaning and checking a household can safely do itself, plus the signs that tell you to call someone qualified.

Why bathroom drains smell: how floor traps and water seals work

Every bathroom floor in Singapore drains into a floor trap, the round or square fitting under the grating near the shower. Below the grating is a bend that always holds a small amount of water. That water is the seal. It sits between your bathroom and the sanitary pipes, and stops air from the pipes coming back up into the room.

The Ministry of Sustainability and the Environment explained this in a written parliamentary reply on sanitary systems. Singapore’s sanitary systems are designed as closed systems, and the water seals in toilet bowls and floor traps act as barriers that keep foul air out of homes. Regular water flow from sinks, basins, showers and toilet flushing keeps those seals topped up.

That gives you the two main reasons for a floor trap smell:

Something is rotting in the trap. Hair bound together with soap and shampoo residue, skin cells, sand from shoes and bits of grime build a slimy layer around the grating and in the trap. In warm, damp conditions it breaks down quickly and smells.
The seal has gone. If water has not run through a drain for a while, the water in the trap can evaporate. With no water in the bend, nothing stops air from the pipes rising into the room.

HDB’s home care guide on fixing floor traps lists exactly these two causes for a foul smell: foreign particles trapped in the floor trap, and a toilet left unused for a long period, for example during an overseas holiday, so that the smell breaks through the water seal.

First, find where the smell is really coming from

Spend two minutes working out the source before you scrub anything.

Floor trap. Lift the grating and smell directly above it. A strong odour here points to buildup in the trap.
Basin. Smell the plughole and the overflow hole separately; both collect slime.
Toilet base. A smell where the bowl meets the floor can mean a faulty seal, which is a plumbing job.
Other sources. Damp bath mats, wet towels and ceiling mould produce musty odours often mistaken for drains. Our guide to removing smells from a room covers those.

If the smell is sharpest at the floor trap or basin, carry on with the steps below.

How to clean a smelly floor trap step by step

This is the job that fixes most cases. Wear rubber gloves, open the window or switch on the exhaust fan, and keep children away from the area while you work.

Lift off the grating and any removable hair catcher. Do not force fixed parts.
Pull out hair and sludge with a gloved hand, old chopsticks or a plastic drain hook. Bag and bin it rather than rinsing it back down.
Scrub the grating and catcher in a pail of warm soapy water with an old toothbrush. Much of the smell lives in the slime on the underside.
Brush the trap walls as far down as you can reach comfortably with a bottle brush.
Flush with a pail of water to rinse away grime and refill the water seal.
Disinfect if needed. HDB’s floor trap guide suggests cleaning bathrooms with diluted household bleach at 1 part bleach to 99 parts water. Use that weak dilution, pour it around the trap, then rinse with clean water after a few minutes.
Refit the grating properly. HDB advises keeping the grating closed so hair and solids cannot choke the floor trap.

Clearing the catcher weekly takes two minutes and prevents both odours and slow drainage. Our weekly bathroom cleaning routine shows where it fits alongside the toilet and shower.

A word on chemicals

Never mix bleach with ammonia-based cleaners or with acids such as vinegar or toilet descalers, because the combination releases toxic gas. Use one product at a time, rinse between products and keep the room ventilated. Chemical drain cleaners are strongly caustic: follow the label exactly, wear gloves and eye protection, and never combine them with anything else. A fully blocked drain is a job for a plumber, not a stronger chemical.

Keeping the water seal topped up

If the odour comes from a rarely used bathroom, or appears after a trip, the seal is the likely problem. The fix costs nothing.

Run water weekly. Pour a pail of water into the floor trap, run the basin tap for a moment and flush the toilet in any bathroom nobody is using. HDB specifically advises flushing toilets regularly.
Cover the trap before a long trip. HDB suggests sealing the floor trap cover temporarily when it will not be used for an extended period, for example with cling film you remove on your return.
Refill on your return. Before you unpack, run water through every floor trap, basin and toilet in the home.

Mosquitoes matter here too. Drains and traps with standing water can become breeding spots. The National Environment Agency’s list of mosquito breeding prevention measures advises covering rarely used gully traps and installing anti-mosquito valves, and HDB’s guide suggests an anti-mosquito device for floor traps as well.

Basin and shower drain odours

Toothpaste, soap and hair collect in the basin plughole, stopper and the trap underneath.

Clean the stopper and overflow with a narrow brush and soapy water, then rinse.
Clear the bottle trap if you are comfortable doing so. HDB’s guidance on sinks and wash basins explains that a bottle trap below the basin can be unscrewed anticlockwise so trapped solids can be removed. Put a pail underneath first, as it will be full of dirty water, and hand-tighten it back carefully afterwards so it does not leak.
Fit a strainer. HDB recommends a strainer so solids are not washed into the waste pipe.

When the smell is a plumbing problem

Cleaning will not fix a fault in the pipework. If you have cleaned the floor trap and basin thoroughly and refilled the seals, and the smell is back within a day or two, stop and look for these signs:

a persistent sewage odour, not just a musty or soapy one;
water draining slowly from several fittings at once, or gurgling when the toilet is flushed;
water rising out of the floor trap, which HDB attributes to solids or grease choking the trap;
dampness, stains or leaks around the toilet base or on the ceiling below;
neighbours noticing the same smell.

In the same parliamentary reply, the Ministry of Sustainability and the Environment advised owners who notice sewage odour or leaks from toilet bowls or sanitary pipes to promptly get help from licensed plumbers. PUB runs an online search for licensed plumbers so you can check who you are hiring.

If you live in an HDB flat, it also helps to know who is responsible for what. HDB’s guide to pipes says flat owners maintain the sanitary branch pipes in their flats, while the Town Council maintains the main sanitary pipes. If you suspect the problem is in the common stack, for example because several units are affected, contact your Town Council. Condominium residents should go to their managing agent.

Habits that stop a floor trap smell coming back

Clear the hair catcher after every few showers, or at least weekly.
Keep the grating closed, as HDB advises.
Do not pour cooking oil, food waste or dirty mop water with grit into bathroom drains. HDB’s guide warns against pouring dirty liquid or oil down the floor trap.
Run water through unused bathrooms weekly.
Keep the bathroom ventilated with the window open or the exhaust fan on after showers. Humidity speeds up the breakdown of anything left in the drain, and also encourages mould, which our guide to removing bathroom mould covers.

Frequently Asked Questions

Why does my bathroom drain smell only at night or in the morning?

Odours from buildup in the floor trap collect in still air when the bathroom has been closed up for hours. Cleaning the grating, hair catcher and trap walls, then running a pail of water through, usually solves it. If the smell persists after a thorough clean, have a licensed plumber check the trap and pipes.

Can I pour bleach down the floor trap to stop the smell?

Bleach does not remove the hair and slime causing the smell, so clean the trap first. HDB suggests a weak dilution of 1 part household bleach to 99 parts water for routine bathroom cleaning. Ventilate the room, wear gloves, rinse afterwards, and never mix bleach with ammonia, vinegar, toilet descalers or other cleaners, because that can release toxic gas.

My flat was empty for a month and now the bathroom smells. What happened?

The water seals in your floor traps and toilets have most likely dried out or dropped, letting air from the sanitary pipes into the room. HDB lists a toilet left unused for a long period, such as during an overseas holiday, as a cause of floor trap odour. Run water through every trap, basin and toilet, and cover the floor traps before your next trip.

Does Femme5 fix choked or smelly drains?

No. Femme5 does not do plumbing or pest control. Bathrooms are part of our home cleaning and deep cleaning services, but plumbing faults are not. A choked drain, a sewage leak or a smell that returns after cleaning needs a licensed plumber, or your Town Council or managing agent if the problem is in common pipes.

How often should I clean the floor trap?

Clear hair from the catcher weekly, more often if several people with long hair share the shower, and give the grating and trap walls a fuller scrub every few weeks. Run water through rarely used bathrooms weekly so the seal does not dry out.

Conclusion

Most bathroom drain smells come down to buildup rotting in the floor trap or a water seal that has dried out. Cleaning the grating and trap, then flushing with a pail of water, fixes the first. Running water through unused bathrooms and covering traps before a long trip fixes the second.

When a sewage odour returns after a proper clean, or comes with slow drainage, gurgling or leaks, treat it as a plumbing issue. Call a licensed plumber, and in an HDB block, check with your Town Council if common pipes could be involved.

A clean drain stays that way when the whole bathroom gets regular attention. When you need an extra pair of hands, see our part-time home cleaning or deep cleaning options, or talk to the Femme5 team for a quote.

The Femme5 Team
Home cleaning team, Femme5 Agency
Femme5 provides part-time home cleaning, deep cleaning and post-renovation cleaning for HDB flats, condominiums and landed homes across Singapore.

Femme5 Cleaning Team

Written by the home cleaning team at Femme5 Agency Pte Ltd, which provides part-time home cleaning, deep cleaning and post-renovation cleaning across Singapore.

How to Fix a Bathroom Drain Smell: Floor Traps, Water Seals and When to Call a Plumber