Signs to look for
- A puddle or damp patch in the cupboard under a sink or basin
- A drip from the trap or waste pipe when the sink empties
- A swollen or stained cupboard base
- Drain smells near the sink, which can point to a dried or damaged trap
What to do first
Dry everything and watch
Empty the cupboard, dry the pipes with a towel and lay dry kitchen paper underneath.
Test the waste
With the plug in, fill the sink part way, then pull the plug and watch. A leak that appears only as the water drains is on the waste side.
Test the supply
If the paper gets wet while nothing is draining, the leak is more likely on the tap connections or supply pipes. Close the isolating valves under the sink if you can.
Stop using it
For a waste leak, avoid using that sink, or the dishwasher or washing machine if they drain into it, until it is repaired.
If drain chemicals have been used, do not undo the trap. The water inside can burn skin and eyes. Keep water away from sockets and appliance plugs in the cupboard.
Possible causes
These are common causes, not a diagnosis. The actual fault is confirmed on site.
Loose or misaligned joint
Plastic waste fittings use nuts and seals that can loosen or shift, especially if something in the cupboard knocks them.
Perished or missing seal
Washers and seals inside the trap and waste joints harden and crack with age.
Cracked trap or pipe
Plastic can split, often after being knocked or overtightened.
Appliance connection
Washing machine and dishwasher waste hoses joining the sink waste can leak at the connection.
Partial blockage
A slow drain can back water up and push it out through a weak joint.
How WHP would assess it
- Confirm whether it is a waste, supply or appliance leak.
- Check the trap, waste joints and any appliance connections for damage and alignment.
- Clear any partial blockage contributing to the leak.
- Run the sink and any connected appliances to check the repair.
We explain any call-out or diagnostic charge before booking, and agree the repair and its cost with you before carrying it out.
What affects the price
Every repair is different, so we do not publish fixed prices for this work. These are the things that make the most difference:
- Whether the existing fittings can be re-sealed or need replacing
- Access in the cupboard and whether appliances need moving
- Whether a blockage needs clearing too
- Unusual or designer waste fittings
Before you call
Useful to send us
- A photo of the pipework under the sink
- A short video showing when it leaks: while draining, or all the time
- Whether a washing machine or dishwasher connects to it
- Whether drain chemicals have been used recently
Get professional help when
- It leaks all the time, not only when draining (likely a supply leak)
- The trap or pipe is cracked
- The cupboard base or floor is getting wet or damaged
- The sink is also draining slowly or smells
Common questions
Why does it only leak when I empty the sink?
The waste pipes are not under pressure, so they only carry water while something is draining. A leak that appears only then is almost always on the trap or waste pipe.
Can I just tighten it?
Plastic waste nuts are designed to be hand-tight. Overtightening with tools can crack the fitting or squash the seal. If hand-tightening does not stop it, the seal or fitting probably needs replacing.
Sources
Safety and responsibility details checked against these sources on 12 October 2026. Water supplier guidance can change. Check your own supplier’s current advice.