Updated 16 Oct 2026gully trap7 min read
What is a gully trap?
Quick answer
A gully trap is a drain inlet with a water seal that stops sewer gas escaping, usually one that takes waste water from a kitchen, a wash area or the ground. In a Singapore home it normally means the kitchen or service yard floor trap; in PUB's Code it also means a heavier cast iron trap used at markets, bin centres and car washing areas.
Three things people mean by gully trap
Search results for gully trap mostly come from the UK, Australia, New Zealand and India. They describe something you will not find on a Singapore sewer pipe. The word covers three different things.
| Meaning | Where | What it is |
|---|---|---|
| Kitchen or service yard floor trap | Singapore homes, in trade talk | A normal floor trap with a 50mm water seal. Shops sell it as a floor trap or gully trap |
| Cast iron gully trap | Markets, bin centres, sheltered car washing areas | A heavy-duty trap to a PUB standard drawing, before the inspection chamber |
| Outdoor gully | UK, Australia, New Zealand, India | A trapped grate in the ground outside the house, often taking rainwater, or acting as an overflow relief |
The first meaning is the one most Singapore homeowners need. Norika, a Singapore grating supplier, sells its 150 size cast grating as a "Floor/Gully Trap Grating", and a Shopee listing sells a "UPVC Floor Gully Trap". Same part, two names.
Why the outdoor gully does not apply here
Australian guides describe the gully as an overflow relief: if the sewer chokes, sewage spills out of the gully in the yard instead of into the bathroom. UK and Indian guides have it collecting rainwater as well as kitchen water.
PUB's Code rules both of those out for a sewer connection. Section 4.1 says open areas such as backyards, courtyards, air wells, balconies, void decks and any other area that receives rain must not be connected to the sewers. Section 4.3.11 says a floor trap must not sit in an open area receiving rainwater.
Singapore keeps rainwater and sewage apart. So an open grating in your yard or on your balcony should lead to the rainwater drain, and a trap on the sewer side should be under a roof.
Do not drain a wash area into the rain drain
The rule works both ways. Waste water from washing goes to the sewer through a covered floor trap. If a yard grating takes rain, do not route a washing machine or sink into it.
Advice to clear leaves from your gully, or to check its height against the indoor drains, comes from those other systems. For a Singapore flat, ignore it.
The kitchen gully in a flat
This is the trap the kitchen sink and the dishwasher drain into. In many flats it sits inside the sink cabinet, so the sink pipe drops straight down into it. In the service yard, the washing machine hose goes into it. PUB's Code requires both machines to discharge into a floor trap, and says a kitchen sink must not drain into a floor trap in a toilet.
It is built like any other floor trap: a grating, a riser, a water seal of at least 50mm and an outlet of at least 100mm. Our floor trap guide goes through the parts and the covers you can buy.
How it differs from a bathroom floor trap is what goes down it. A bathroom trap gets hair and soap. The kitchen gully gets grease, rice, coffee grounds and hot detergent. The service yard gully gets lint.
PUB's Code never uses the word gully for a home trap; it calls them all floor traps. Our floor trap vs floor waste guide covers the rest of the naming mix-up.
Why a kitchen gully smells or chokes
These are the gully-specific causes. For the clearing steps, see how to clear a floor trap choke.
- Grease past the trap. Oil from the sink cools and sets along the pipe beyond the water seal. The gully can keep draining while grease builds up beyond it. Hot water and detergent shift fresh grease; old grease needs a drain machine.
- Food in the seal. Rice and scraps settle in the bend and rot. HDB asks owners not to put cooking oil, food waste or solids down floor traps.
- A gap around the sink pipe. If the sink pipe goes through a hole in the grating, or the grating has been left off to make room for it, smells and cockroaches come out around it. Close the gap rather than leaving the trap open.
- The machines run at once. A washing machine or dishwasher pumps water out faster than a half-choked gully can take it, so it floods only on the drain cycle.
- A dry seal in a second kitchen. Wet kitchens or yards that are rarely used lose their water. Pour a cup in every week or two.
If water comes up the kitchen gully when you use the sink, the choke is at or past the gully, not in the sink trap.
Cast iron gully traps at markets, bin centres and car washes
This is the only place PUB's Code uses the word gully trap, and it is denser reading than the home case.
| Where | What the Code requires | PUB drawing |
|---|---|---|
| Market | Floor sumps at every stall, waste sumps along service corridors, then a cast iron gully trap in a sump connecting to the last inspection chamber. Upper-storey market stacks join the gully sump at ground floor. Drain-lines from the waste sumps at least 250mm | PUB/WRN/STD 045C |
| Bin centre or refuse chute | Sullage water goes to the sewer through a cast iron gully trap with a strainer | PUB/WRN/STD/044C |
| Car washing area in a car park | Must be sheltered and kerbed so rain stays out. Floor waste or floor trap, then a cast iron gully trap before the inspection chamber | PUB/WRN/STD/043C |
Car washing bays at businesses that use oil, such as motor workshops, go through an oil interceptor instead. Food shops and hawker stalls drain through floor traps and waste sumps into a grease trap, and their floor trap covers must be bolted or screwed down.
In an HDB estate, the bin centre is common property, which the town council maintains. A smell from that gully is the town council's to fix, not yours.
If your gully needs work
The kitchen and service yard gully inside your flat is yours to maintain, as are the pipes from it to the common stack. If neighbours on the same stack have slow drains too, call the town council first.
Clearing a choked gully does not need a PUB Licensed Plumber. Typical Singapore prices are $50 to $120 to clear the trap, $130 to $220 with a drain machine on the branch pipe, and $150 to $350 for renovation debris. See floor trap choke and kitchen sink choke.
WhatsApp us a photo of the gully with the grating lifted and the sink cabinet open, 24/7. We tell you the likely cause and a price range, and the price is agreed before work starts.
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Frequently asked questions
What is the purpose of a gully trap in plumbing?
To let waste water into the sewer pipe while a water seal stops sewer gas, smells and pests coming back out. In a Singapore home it does this for the kitchen sink, dishwasher and washing machine. Overseas versions often also take rainwater, which PUB's Code does not allow on a sewer connection.
Which is better, a P-trap or an S-trap?
A P-trap, where there is a choice. Its outlet goes out sideways and holds its water better. An S-trap's outlet goes straight down, and without venting it is more likely to siphon its water out. Most HDB toilets drain through the floor, an S-trap layout, with the trap built into the bowl.
What is the purpose of a floor waste gully?
Floor waste gully is mostly an Australian term for a grated floor drain in a bathroom or laundry that takes floor water to a trap. In Singapore, PUB's Code calls the untrapped kind a floor waste and requires it to drain into a floor trap above the water seal.
What is the size of a PVC gully trap?
In Singapore homes a gully is a floor trap, so the same PUB rules apply: an outlet of at least 100mm and a water seal of at least 50mm. Replacement gratings for them are commonly sold in 100mm and 150mm sizes. Measure your old grating before buying.
Does an HDB flat have a gully trap?
Not the outdoor kind. What people call the gully in a flat is the floor trap in the kitchen or service yard, often hidden inside the sink cabinet. It works the same way as a bathroom floor trap.
Why does my kitchen gully smell?
Usually rotting food or grease in the trap, a gap around the sink pipe where it enters the gully, or a water seal that has dried out. Clean the trap, close the gap and pour in water. If it also drains slowly, grease has built up past the trap and needs a plumber.
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