A kitchen sink leaking underneath is easy to overlook at first. You may notice a damp smell in the cupboard, a swollen cabinet base or a small puddle around cleaning products. Left alone, even a slow leak can damage kitchen units, flooring and nearby walls, while creating the conditions for mould to develop. The good news is that many sink leaks have a clear cause, but finding the exact source before damage spreads is the priority.

First, limit the water damage

Empty the cupboard beneath the sink and place a bowl, tray or towels under the pipework. Do not store food, electrical items or household chemicals in the wet area. Dry the cabinet base as well as you can, then line it with dry kitchen roll or tissue. This makes it easier to see where fresh water first appears.

If water is running steadily, avoid using the sink until the fault is repaired. Turn off the isolation valves on the hot and cold supply pipes if the leak is coming from a flexible hose, tap connection or pipe joint. These small valves are usually found beneath the sink and can often be closed with a flat-head screwdriver. If there are no isolation valves, turn off the main water supply to the property.

A leak from the waste pipe will normally stop when you stop using the sink. A leak from the water supply may continue under pressure, so it needs quicker attention.

Where is the kitchen sink leaking underneath?

The position of the water usually gives the best clue. Dry the pipes and fittings, then run a small amount of water into the sink while watching closely with a torch. Start with the plughole and waste pipe, then test the taps and supply connections. Do not fill the bowl immediately, as a slow leak can be easier to trace when water is introduced gradually.

Water around the plughole or basket strainer

If water forms directly below the sink bowl, the seal around the plughole may have failed. The fitting can loosen over time, or the sealing compound may deteriorate. This often leaks only while water is sitting in the bowl or draining away.

A loose fitting may be tightened, but overtightening can damage the seal or crack parts of the assembly. If the plughole is old, corroded or difficult to access, replacement is usually the more dependable repair.

Drips from the trap or waste pipe joints

The curved section of pipe beneath the sink is the trap. It holds a small amount of water to prevent odours from the drain coming back into the kitchen. Loose compression nuts, worn washers, a split trap or a poorly aligned waste pipe can all cause leaks here.

Check whether the leak happens only when the sink drains. If it does, the trap or waste connection is a likely source. Sometimes a hand-tightened nut is enough to stop a minor seepage. However, if a seal is worn, a pipe is cracked or the pipework is under strain, tightening alone will not provide a lasting result.

Water from the sink overflow

Many kitchen sinks have an overflow channel that carries excess water from the upper part of the bowl into the waste. If water appears after the sink has been filled high, the overflow connection or hose may be loose, blocked or split.

Test this carefully by putting a small amount of water into the overflow opening, rather than allowing the whole sink to overflow. The repair may involve resealing or replacing a connector behind the bowl, which can be awkward in a confined kitchen cupboard.

Drips from tap pipes or flexible hoses

Water appearing when the taps are on, even if the sink is not draining, points towards the supply side. Look at the flexible tap tails, isolation valves and connections to the pipework. A slow drip can travel along a pipe before falling, so the wettest point is not always where the fault begins.

Do not ignore a damaged flexible hose. These hoses are under mains pressure and can fail more seriously than a waste-pipe leak. Turn off the water supply to that connection and arrange a repair as soon as possible.

Common causes of a leak under the sink

Kitchen sink pipework moves more than people realise. Cleaning around the cupboard, pulling out a bin, storing heavy bottles or knocking a pipe can gradually loosen a connection. Grease and food debris can also contribute to blockages, causing water to back up and expose weaknesses in older joints.

Age is another factor. Rubber washers harden, plastic traps become brittle and metal fittings can corrode. A new tap installation can also reveal an existing issue if the pipework has been moved or was not correctly supported. In rented properties, repeated short-term fixes can leave a collection of mismatched fittings that are more prone to leaking.

The right repair depends on the condition of the whole section of pipework. Replacing one washer may be sensible on a relatively new, sound installation. If several joints are worn or the trap is cracked, replacing the affected waste assembly is often better value than returning to the same leak a few weeks later.

What you can safely check yourself

A basic inspection is reasonable if the leak is minor and you can clearly see the pipework. With the water off where necessary, check for loose fittings, damaged washers, cracks and signs of old water staining. Make sure pipes are supported and not being forced out of line by items stored in the cupboard.

Avoid using sealant as a quick fix on the outside of a leaking pipe joint. It rarely addresses the cause and can make a proper repair more difficult. Likewise, do not use excessive force on plastic nuts. They can split or distort, turning a small drip into a larger leak.

If you decide to remove a trap for cleaning, place a bucket underneath first. The trap will contain water, and older fittings may not reseal properly once disturbed. Take a photograph before dismantling anything so you can see how the parts were arranged.

When to call a plumber

Call a plumber if the source is unclear, water is still escaping after a basic check, the cabinet base is becoming saturated or the leak involves a pressurised supply pipe. Professional help is also advisable where pipework needs replacing, the sink has to be removed or the waste pipe is blocked further along the run.

Urgent attendance is sensible if water is reaching electrical sockets, appliances, laminate flooring or a neighbouring property. For flats, a leak can quickly affect the home below, even where the visible puddle seems small. Landlords and letting agents should act promptly too, as moisture inside a kitchen unit can cause hidden damage before a tenant notices the full extent of the problem.

An experienced plumber can trace the source, replace failed parts and check that the sink, waste and water connections are properly secured. Simply Plumbing provides domestic plumbing repairs across Dublin and County Kildare, with clear upfront pricing and emergency support when a leak cannot wait.

Protect the cupboard after the repair

Once the leak is fixed, leave the cupboard open until the area is fully dry. Remove any damp shelf liner or absorbent material, as these can hold moisture against the cabinet base. Check the unit again over the next few days, particularly after washing dishes or emptying a full sink.

If chipboard has swollen, it may not return completely to its original shape, but drying it quickly can prevent further deterioration. Watch for lingering musty smells, black spots or softened sections around the back of the cabinet. These may indicate water has reached an area that needs further attention.

A kitchen sink should drain cleanly and stay dry beneath the bowl. If it does not, dealing with the leak early is usually the simplest, least disruptive and most cost-effective choice.