Work out whether it is one appliance or several
Fill the bath, the basin and the kitchen sink a few inches and let each go in turn. If only one is slow, the blockage is in that appliance's own waste pipe and you can reasonably tackle it yourself. If two or more are slow, they share a length of pipe that is restricted and the fault sits beyond the point where they join.
In a bathroom, the basin, bath and shower usually join before they reach the soil stack, so all three being sluggish points at that common branch rather than at any one trap. In a kitchen, the sink often shares its waste with a dishwasher and a washing machine, and a machine that has been discharging detergent and fat into a cold pipe for years is a very common cause.
Clear the trap first
The U bend under the sink catches almost everything. Put a bucket underneath, unscrew the two collars by hand, and lift the trap out whole. Expect grey sludge, hair and food debris. Wash it out, check the seals are still seated, and refit it hand tight rather than with grips, because overtightening a plastic collar cracks it and you will be back under the cupboard with a leak.
Refit and test before you go any further. If the sink now runs freely, you are finished. If it fills the trap and then backs up, the restriction is in the pipe beyond it.
Plunging, and doing it properly
A cup plunger works on hydraulic pressure, not on suction alone, and it only works if the system is sealed. Block the overflow slot with a damp cloth, run enough water to cover the cup, and use short firm strokes rather than one heavy push. Twenty seconds of consistent strokes shifts more than a single effort.
In a double sink, block the second plughole as well or you will simply push water from one bowl into the other. This one detail is the reason plunging a kitchen sink often appears not to work.
What to avoid
- Caustic soda and strong chemical cleaners, which sit in standing water and create a hazard for the next person to open the pipe.
- Wire coat hangers pushed hard into a plastic waste, which scratch the bore and give future debris something to catch on.
- Boiling water straight into a plastic trap, which softens the fitting and can distort the seal.
- Repeated heavy plunging after a leak appears, which usually means a joint has already been disturbed.
The signs that it is not the sink at all
If the sink gurgles when the washing machine drains, if the external gully outside the kitchen window bubbles or backs up when you empty the bowl, or if a downstairs toilet is affected at the same time, the restriction is out in the main drain and no amount of work under the cupboard will help.
Grease is the usual culprit in that stretch. Kitchen waste cools quickly in an external pipe, and a decade of warm fat going down as liquid leaves a hard layer on the pipe wall that narrows the bore to a fraction of its diameter. Rodding punches a hole through it. High pressure jetting is what actually strips it back, which is why we jet rather than rod when a kitchen run is involved.
Keeping it clear afterwards
Scrape plates properly and let fat set in a container rather than washing it down warm. Fit a cheap plughole strainer in the kitchen and clear it daily. In a bathroom, pull the hair out of the basin waste every few weeks rather than waiting for it to slow down, because hair bound with soap is remarkably strong once it has been in place for months.
For a house that has had repeated kitchen blockages, an annual jet of the kitchen run costs less than two emergency call outs and takes under an hour.
If you have to call someone out
A blocked waste that has resisted a clean trap and proper plunging is not a stubborn sink, it is a pipe problem, and continuing to work at it usually ends with a disturbed joint and a leak in the cupboard. At that stage the useful thing is equipment that reaches beyond the trap.
We jet kitchen and bathroom runs back to full bore rather than punching a channel through the deposit, and where a kitchen has blocked more than once we put a camera down afterwards so you can see whether it is fat, a lipped joint or standing water causing it. There is no call out charge and we quote before starting.
Questions we get asked
Do baking soda and vinegar work?
They fizz and can loosen light soap film, but they do not remove hardened fat or hair. They are harmless to try and rarely sufficient on their own.
Why does my sink gurgle after I clear it?
Gurgling is air being drawn past a partial restriction, so the pipe is flowing but not fully clear. It usually returns to a full blockage within a few weeks.
Can a blocked sink cause a smell?
Yes. Trapped food debris and a fat layer both hold bacteria, and a trap that has run dry lets sewer air back into the room.
Is it worth calling someone for one slow sink?
If the trap is clean and plunging has not worked, yes, because at that point the fault is in pipework you cannot reach safely.