Why is there a hole in my basin, and a bend under my sink?
Two of the most common questions we get are about bits of plumbing everyone sees and nobody explains: the small hole near the top of the bathroom basin, and the curved pipe under the sink. Both are there for good reasons. Here's what they do, what goes wrong with them, and what to do when they smell or block.
The hole in the side of the basin: the overflow
That small hole or slot near the rim is the overflow. If a tap is left running with the plug in, water drains out through the overflow instead of spilling onto the floor. It also lets air into the waste so the basin drains faster when the plug is pulled.
- Smell from the overflow: soap, toothpaste and hair build up in the overflow channel and go stale. Flushing it with hot water and cleaning the opening usually helps.
- Water coming out of the overflow: if water backs up out of the overflow when the basin drains, the waste or trap below is partly blocked.
- Basins without an overflow: some modern basins don't have one and use a waste without a plug, or a pop-up waste that can't seal fully.
The bend under the sink: the trap
The curved pipe under a sink or basin is a trap. It might be an S bend, a P trap, or a bottle trap on modern basins. Its job is to hold a small amount of water in the bend all the time. That water seal stops sewer gas and smells coming up through the plughole into your home.
S trap
The pipe drops down into the floor. Common in older homes.
P trap
The pipe goes back into the wall. Common in newer homes and wall-mounted basins.
Bottle trap
A neat cylinder under modern basins that does the same job and is easy to clean.
What goes wrong with traps
- Blockages: hair, soap, food and fat collect in the bend. Most traps can be undone and cleaned.
- Sewer smell: if the trap dries out from lack of use, or the water is sucked out by a venting problem, smells get in. See gurgling drains.
- Leaks: loose or cracked joints drip into the cupboard. Old metal traps can corrode through.
- Dropped items: a ring down the plughole often ends up in the trap and can be recovered.
When to call us
If a basin or sink drains slowly after cleaning the trap, smells persist, or water comes back up the overflow, the problem is further down the pipe. We clear it, check the venting and replace worn traps and wastes, with a fixed price before any work starts. See blocked sinks.
Why locals call us
- Local to Matraville and the southern Eastern Suburbs
- Free on-site quotes, with a fixed price before any work starts
- Lifetime labour warranty on our workmanship
- 100+ five-star Google reviews
- Licensed plumbers and gasfitters (Lic. 398171C)
Common questions
What is the hole in the side of my basin for?
It's the overflow. It stops the basin spilling onto the floor if a tap is left on, and lets air in so the basin drains faster.
Why is there an S bend under my sink?
It's a trap that holds water to stop sewer gas and smells coming up through the plughole.
Why does my basin overflow smell?
Soap, toothpaste and hair build up in the overflow channel. Cleaning and flushing it with hot water usually fixes it.
Why does my sink smell like sewer?
The water in the trap may have dried out or been sucked out by poor venting. Run the tap, and if the smell returns, get the venting checked.
Related help: blocked sinks, gurgling drains, drain flies, taps and toilets and our blocked drains guide. Local pages: Matraville, Maroubra, Pagewood and Coogee. Browse all our plumbing guides.