A sink usually drains slowly because something is partly restricting the flow, such as a build-up of grease, food, soap or hair in the plughole, trap or waste pipe. Sometimes the cause is further along, in a shared waste pipe or the drain outside, so the slow sink is only the first place it shows.
Working out which of these is more likely can help you decide what to do next. A slow sink does not always mean the blockage is in the trap.
Build-Up Near the Plughole
In kitchen sinks, fat, oil and grease from cooking can cool and stick to the inside of the waste pipe, catching food scraps as they pass. In bathroom basins, hair, soap and toothpaste can collect around the plug and in the trap below.
Build-up often happens slowly, so the sink may drain a little more slowly each week before it becomes noticeable. Not every kitchen sink blockage is caused by grease, though. Food waste, coffee grounds and small objects can also play a part.
Problems With the Trap or Waste Pipe
The trap is the curved section under the sink that holds water to stop smells coming up from the drain. Because of its shape, waste can collect there. The waste pipe beyond it can also become restricted, especially if it runs a long way or has several bends.
Sometimes the way the pipe was fitted plays a part. A waste pipe that does not fall properly towards the drain may let waste settle over time. A pinched or damaged section can also slow the flow.
When the Problem May Be Further Along
If other fixtures are also slow, such as the bath, shower or a second sink, the restriction may be in a shared pipe or in the drain outside. A gurgling sound from the sink when another fixture is used can also suggest the problem is beyond the sink itself.
Kitchen sinks often share a waste with a washing machine or dishwasher. If the sink fills when an appliance empties, the shared waste may be restricted.
What You Can Safely Do
Check whether the problem affects just one sink or several fixtures, and whether it is getting worse. This can help a plumber understand where to look.
Clearing debris from the plughole and using a plughole strainer can help. Avoid pouring fat or oil down the sink, and do not mix drain cleaners or use them repeatedly, as some products can be harmful and may not solve the problem.
If the sink keeps slowing down, drains very slowly or starts to back up, it may need checking. Our page on blocked sink repairs in Poole explains how sink blockages are assessed and cleared.
If your sink is draining slowly and you are not sure why, a plumber can look at the cause and explain the options.