The pool of water sitting inside your floor trap is what blocks the sewer, and once it dries out or clogs with debris the smell and the roaches come straight up the pipe.
Get the water seal back and clear the debris out, and both problems go away together. The causes are short, and so is the fix.
The water in the trap is what stops the smell
Under the grating in your bathroom floor there is a bend that holds a small pool of water. That water is a physical barrier across the pipe. Sewer air cannot get past it.
Leave the bathroom unused and the water evaporates. HDB names this directly: a toilet not in use for a long period “may cause the foul smell to break through the water seal level inside the floor trap”.
The other cause is what collects inside. HDB lists “Foreign particles are trapped in the floor trap” as a cause of foul smell. Hair, soap and grease sit in there and rot, and that smells on its own whether the seal is intact or not.
Source: HDB, Fixing Choked Floor Traps in HDB Flats, last updated 22 September 2025.
Clear it out, then keep water going through it
HDB gives the fix in two steps: “Remove the foreign particles, then flush and clean the floor trap”.
Lift the grating off. Pull out the hair and the soap sludge by hand. Flush the trap through, clean the grating, put it back.
Then keep the trap wet. HDB’s line is “Flush the toilet regularly”, and for most bathrooms that is the whole of it. If your toilet bowl is being changed out in a renovation anyway, sort the floor trap out in the same job, while the floor is open.
Going away for a while? HDB’s tip is to “Seal the floor trap cover temporarily when it is not in use for an extended period”.
What not to pour down it
This is the part most people get wrong.
HDB: “Avoid pouring dirty liquid or oil down the floor trap”.
It says it twice. In the section on chokes: “Avoid pouring cooking oil or discarding food waste and solid items down the floor trap or into the sink”.
So no oil. No cooking fat or food waste, and not the dirty water from mopping. Water and soap, nothing else.
If the trap is choked rather than smelly, HDB’s instruction is “For serious cases, engage a licensed plumber to clear the choke”.
Why the cockroaches come with the smell
Same mechanism. The water in the trap is a barrier across the pipe. When it dries out, or when debris stops the trap working, the pipe is open, and whatever is in it has a route into your bathroom.
Which is why the smell and the roaches show up together, and why they leave together.
Every step above is ordinary drain maintenance. It works on the roaches because the route they use is the pipe. Take the debris out. Flush and clean the trap. Keep water going through it. Cover the trap if the bathroom is going to sit unused. Close the pipe and the route is closed.
Picking a replacement
Cleaning fixes a dirty trap. It does not fix a damaged one. If you have cleared the debris and the trap still will not hold water, or the body or the grating is cracked, it needs changing rather than cleaning.
The easiest time to do that is when the floor is already open. A renovation will put you there, and so will upgrading works under HDB’s Home Improvement Programme.
Our floor trap range is here. If you want a hand picking one, the team at either Singapore showroom will go through it with you.
Common questions about a smelly floor trap
Why does my floor trap smell bad?
Two causes, and HDB lists both. Debris is trapped inside it and rotting, or the water seal has dried out because the bathroom has not been used. Clear the debris out, flush and clean the trap, then keep water going through it.
Why does it smell worse after I have been away?
The water in the trap evaporates while nobody is using the bathroom, and once that pool is gone the pipe is open. HDB describes it as the foul smell breaking through the water seal level inside the floor trap. Before a long trip, HDB’s own tip is to seal the floor trap cover temporarily.
How do I stop cockroaches coming up the bathroom drain?
Close the pipe, which means getting the water back in the trap and keeping it there. That pool of water is a physical barrier, so a dry or clogged trap is an open route. Take the debris out, flush and clean the trap, use the bathroom regularly so the water stays topped up, and cover the trap if it will sit unused. It is the same fix as the smell.
Do you need to buy anything to keep cockroaches out?
Start with the seal and the debris, because the barrier is the water already sitting in the trap. Buying comes into it when the trap itself is the problem. If it is cracked, damaged or no longer holding water, a replacement is the answer.
When is it time to replace the floor trap instead of cleaning it?
When cleaning stops working. If you have cleared the debris and the trap still will not hold water, or the body or grating is damaged, a clean will not bring it back. A renovation or a HIP upgrade is the natural moment to change it, since the floor is open anyway.
Visit our showroom. Lavender: 116 Lavender Street, #01-03, Pek Chuan Building, Singapore 338730. Tampines: 1 Tampines North Drive 1, T-Space, #01-18, Singapore 528559. Both open every day, 11am to 8pm. Or call +65 8089 2811.
