How to fix a leaking pipe under the sink (2026)

Overview

A leak under a sink is often fixable without major plumbing work, but the right repair depends on whether the leak is coming from the waste assembly or the water supply. Waste-pipe leaks usually show up only when the tap is running or the sink is draining, and they are commonly caused by loose compression nuts, badly seated washers, or a cracked trap. Supply-line leaks can drip even when the sink is not in use and may come from flexible connectors, isolation valves, or the tap shanks above them. The safest approach is to dry everything thoroughly, reproduce the leak, and trace the highest wet point, because water often runs along a pipe and makes the actual source easy to misread. Once you know the source, make a like-for-like repair: tighten a loose joint gently, reseat and replace a washer or seal if needed, or swap out a damaged trap or flexi hose. Avoid makeshift fixes such as heavy sealant on compression joints or over-tightening plastic fittings, as these often make leaks worse. If the leak involves a cracked fitting inside the wall, corrosion on a fixed metal pipe, or you cannot isolate the water, it is usually time to call a qualified plumber.

Who this is for

Homeowners or tenants dealing with a minor, accessible leak under a kitchen or bathroom sink, especially from the trap, waste pipe joints, flexible tap connectors, or isolation valves.

What you’ll need

• Bucket or washing-up bowl
• Old towels or cloths
• Torch
• Dry kitchen roll or tissue
• Adjustable spanner or suitable slip-joint pliers
• Replacement trap washers or seals that match the existing fitting
• Replacement trap or waste section if cracked
• Replacement flexible tap connector if the supply hose itself is leaking
• Gloves
• Mild cleaner for wiping grime away before inspection

Before you start

Clear the cupboard so you can see all joints. Work out whether the leak is from the supply side or the waste side: if it leaks only when water is running or draining, suspect the waste; if it drips all the time, suspect the supply. Locate the isolation valves or the property stop tap before undoing anything. If electrical items are stored under the sink, unplug and remove them first.

Step-by-step

1. 1

   Dry the area and pinpoint the leak source

   Place a bucket underneath, then wipe every pipe, nut, valve and hose dry. Run the tap briefly and drain the sink while watching with a torch. Use dry tissue around each joint and along the underside of pipes to find the first point that gets wet. If the sink is not in use and water still appears, check the supply hoses and shut-off valves instead of the trap.

   Why: You need the true source before repairing anything, because water often travels along pipework and drips from a lower point that is not actually faulty.

2. 2

   Isolate the water if the leak is on the supply side

   If the leak is from a flexible connector, isolation valve, or the tap connection, turn off the local isolation valves if fitted. If there are no working isolation valves, turn off the main stop tap for the property. Open the tap to relieve pressure and catch any remaining water in a bucket.

   Why: Taking pressure off the line prevents sudden spraying and lets you work safely without water continuously feeding the leak.

3. 3

   Tighten only obviously loose joints carefully

   If a slip-joint nut on the waste pipe or trap is visibly loose, hand-tighten it first, then give only a small further adjustment with pliers or a spanner if needed. For metal supply connections, support one side with one tool while turning the nut with another if possible. Do not force plastic fittings or keep tightening if resistance increases sharply.

   Why: Many leaks are simply from movement or slight loosening, but over-tightening can distort washers, crack plastic nuts, or damage threads and create a worse leak.

4. 4

   Dismantle and inspect the leaking joint

   If tightening does not stop the leak, undo the suspect joint over a bucket. Check the washer, seal or compression ring for splits, flattening, twisting, dirt or poor seating. Inspect the mating surfaces for cracks, burrs, cross-threading or dirt, and clean them before reassembly.

   Why: A joint seals because the right parts are aligned and compressed evenly. Dirt, damage or a twisted washer prevents that seal from forming properly.

5. 5

   Replace worn washers, seals or damaged pipe sections

   Fit the correct replacement washer or seal for that trap or connector, making sure it sits the right way round for the fitting type. If the trap body, waste arm, flexi hose or valve is cracked, corroded or deformed, replace the damaged part with a matching type and size specified by the manufacturer or retailer. Reassemble squarely and tighten evenly.

   Why: Worn sealing parts rarely recover once damaged, and cracked or corroded components will continue to fail even if temporarily sealed from the outside.

6. 6

   Test the repair in stages

   Dry the area again completely, then restore the water or run the tap slowly. Check the repaired point first, then fill the basin partly and release it to test the waste under a stronger flow. Wipe around each joint with tissue to detect tiny fresh drips.

   Why: A staged test helps you catch a slow seep before you put everything back, and a full drain-down tests the trap and waste under more realistic load.

7. 7

   Recheck after a short period of normal use

   Leave the cupboard clear for a while and inspect again after the sink has been used normally. If there is still dampness, repeat the tracing process rather than tightening every joint at random.

   Why: Some leaks show only after repeated use, and random tightening can create new faults while hiding the original one.

Why this works

Under-sink pipework usually relies on mechanical compression: a nut squeezes a washer or seal evenly between two correctly aligned surfaces. Leaks happen when that compression is too loose, uneven, dirty, worn or applied to a damaged component. Restoring clean alignment and replacing failed sealing parts re-establishes the water-tight joint.

Common mistakes to avoid

• Trying to fix the drip without first finding the highest wet point
• Over-tightening plastic trap nuts until the washer distorts or the fitting cracks
• Using lots of sealant or tape on joints that are meant to seal with a washer
• Replacing only the visible drip point when the real leak is higher up
• Cross-threading a nut during reassembly
• Ignoring a cracked trap or corroded valve and attempting a temporary patch

Troubleshooting

Compare your options

Alternatives

• Use a temporary self-fusing repair tape only as an emergency short-term measure on a low-pressure drip while you arrange a proper replacement
• Replace the complete under-sink trap assembly if several joints are worn or misaligned instead of repairing one section at a time

Pro tips

• Photograph the pipe layout before dismantling anything.
• Keep old washers until the repair is finished so you can match replacements.
• Use tissue rather than just looking, because very small leaks can be hard to see.
• If a plastic nut will not start easily by hand, back it off and try again to avoid cross-threading.
• Support pipework while tightening so you do not twist connected parts out of alignment.

Safety notes

• Turn off the water before dismantling any supply-side connection.
• Keep electrical appliances and extension leads away from the wet cupboard area.
• Wear gloves when handling dirty trap contents or corroded fittings.
• Be cautious with old metal pipework, which can fail further if heavily stressed.
• If you smell sewage gases strongly or find blackened, damaged waste fittings, ventilate the area and inspect carefully before continuing.

Legal & regulatory notes

Simple like-for-like repairs on accessible traps and tap connectors are usually straightforward household maintenance, but work on fixed supply pipework, concealed leaks, or any installation that must meet local plumbing rules should follow local regulations and manufacturer instructions. If you are in a rented property, check your tenancy terms before replacing plumbing parts.

Cost considerations

The cheapest successful repair is often a new washer or trap seal, but repeated small fixes on cracked or badly aligned pipework can cost more in time and water damage than replacing the affected section outright. Hidden leaks can also damage cupboards, flooring and nearby units if left too long.

Frequently asked questions

Sources & references

Guidance on this page is traced to documented sources. Last checked 24 September 2026.

• Fluidmaster · manufacturer

  Supports manufacturer-led guidance that plumbing repairs depend on correct identification of leaking components, proper washer/seal fit, and like-for-like replacement parts.

• Oatey · manufacturer

  Supports general trap and drainage repair principles, including correct joint assembly and use of the proper sealing components rather than improvised fixes.

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