Why Do Pipes Under the Kitchen Sink Start Leaking?

What are the Most Common Causes of Leaking Pipes Under the Kitchen Sink?

The vast majority of leaks under the kitchen sink originate from one of four or five locations. Since these parts have different constructions, the solution will also differ in each case – therefore, the first step is always to accurately determine where the water is coming from, rather than immediately unscrewing anything.

• The siphon nut is loose or the rubber seal is worn – the most common cause, water usually drips slowly.
• The tap base is leaking on top of the sink – water runs down the pipe, appearing as if it's coming from the pipe, although the actual source is above.
• The flexible water hose (flexi) is cracked or chafed – often older than five years or installed at the wrong angle.
• The dishwasher drain hose has come loose or is blocked and overflowing.
• The drain flange, or the joint at the bottom of the sink, is corroded, or the putty has dried and cracked.
• An old metal compression fitting is rusted, or the nut's thread is worn.

How to Pinpoint the Exact Location of a Water Leak?

Since several pipes and hoses are usually clustered in the cabinet, water often travels along surfaces, making it seem as if the leak is coming from the wrong place. The most accurate way to identify the leak's source is to dry the entire cabinet interior and then observe where the first wet spot appears.

Why Does the Siphon Nut or Seal Start Leaking?

Siphon connections have plastic nuts that tend to loosen over time – especially if heavy items have been pushed under the sink or the cabinet has been moved during cleaning. Rubber seals also harden and lose elasticity typically within 5–8 years, creating a small gap through which water begins to drip.

This is one of the easiest problems to solve: often, simply tightening the nut by hand is enough. If tightening doesn't help, the seal is likely worn and needs replacing. More detailed siphon maintenance and cleaning require a longer discussion, as it also includes preventing drain blockages, not just repairing leaks.

• Try tightening the nut by hand or with an appropriate spanner, but avoid excessive force – plastic can crack.
• Check if the rubber seal is displaced or has a hard, cracked surface.
• Replace the seal if it's stiff, cracked, or has lost its round shape.

How to Determine if the Leak is Coming from the Tap, Not the Pipes?

A leak from the tap is characterised by water appearing in the cabinet along the back pipe – it runs from top to bottom and eventually collects at the bottom of the cabinet, giving the impression that the leak is from the pipe. However, the actual cause is often a worn O-ring around the tap body or dried, cracked putty between the sink and the tap.

To check this, thoroughly dry the area around the tap on the sink, let the water run, and observe if moisture appears first around the tap base on the sink, rather than inside the cabinet. If so, the seal should be replaced or the putty reapplied – simply drying items from under the pipes won't solve it.

Why Do Flexible Water Hoses (Flexi) Start Leaking?

Flexible connection hoses that connect the wall valve to the tap are convenient to install, but their lifespan is limited – typically 5–10 years, depending on quality and water pressure. The inner lining of the hose can crack, the metal braiding can rust, or the end connections can come loose if there's higher pressure or vibration in the pipework.

Since the hose is often located on the back wall of the cabinet, where light is poor and space is confined, an initial leak often goes unnoticed until it becomes larger. It's advisable to visually inspect the hoses a couple of times a year – look for rust spots, cracks, or white lime deposits around the connections.

• Rust spots or a white lime layer on the hose indicate prolonged moisture.
• A hose that is too tight or installed at a sharp angle will wear out faster.
• It's worth replacing old rubber hoses with stainless steel braided hoses.

Why Does the Sink Drain Flange Start Leaking?

The drain opening connection at the bottom of the sink, also called a flange, is secured with putty or a sealing ring directly to the metal or granite of the sink. Over time, this material dries, shrinks, and loses its sealing property – especially if hot water is frequently poured into the sink or aggressive cleaning agents are used.

The symptom of this leak is that water appears in the cabinet precisely under the central part of the sink, not between the pipes. To repair, the flange must be disassembled, old material removed, and a new seal or putty installed – this job requires temporary removal of the sink and, in many cases, is best entrusted to a plumber.

How Does Corrosion of Old Metal Fittings Contribute to Leaks?

In older buildings where the pipework is partly original, old metal compression fittings may be found under the kitchen sink. The threaded nut inside them wears over the years, and rust particles begin to weaken the joint until a small but constant drip occurs.

Such fittings are usually not salvageable – tightening helps temporarily, but due to the rust layer, the leak will return sooner or later. If you notice corroded fittings, it's safer to replace them with new plastic or stainless fittings in a timely manner, rather than waiting until they rust through completely.

Can Cleaning Products and Drain Unblockers Promote Leaks?

Chemical drain unblockers are often used in the kitchen when a drain starts to clear slowly. These products typically contain strong alkalis or acids that dissolve grease and organic matter, but prolonged or overly frequent use can also damage the rubber and plastic seals of siphons and joints. A seal exposed to aggressive chemicals loses elasticity faster than under normal use and starts leaking earlier.

If you need help unblocking a drain, it's advisable to prefer mechanical methods – such as a pipe brush or plunger – and reserve strong chemical products for rare exceptions. If you do use a chemical cleaner, flush the pipework with plenty of water afterwards to reduce residual contact with the seals.

Do Seasonal Factors, Such as Cold Weather, Affect the Risk of Kitchen Sink Pipe Leaks?

Although pipes under the kitchen sink are generally located inside a warm room and the risk of freezing is low, seasonality indirectly affects leaks. For example, during holidays, when more cooking is done in the kitchen and dishes are washed continuously for longer periods, the drainage system is put under more strain than usual – this can reveal a previously weakened joint or seal problem that might not have emerged otherwise in the near future.

It's also worth considering that if an apartment is left vacant for an extended period, for instance, during a summer holiday trip, an initial small leak might go unnoticed in time. In such a case, it's particularly useful to check the cabinet's contents before leaving and, if necessary, close the wall valves if water usage is not required during that time.

• Before an extended absence from home, check the pipes and hoses under the cabinet.
• Consider closing the wall valves if the kitchen sink will not be used for a long period.
• Under higher load, such as during holidays, pay closer attention to whether moisture has appeared in the cabinet.

What Does a Plumber Usually Check and Do for a Kitchen Sink Leak?

If you decide to call a specialist, it's useful to know the typical inspection process – this helps to better understand why some solutions take longer than others. A plumber usually starts with what you can also do yourself: drying the cabinet and observing water flow in different modes to precisely identify the leak's location.

Once the source is clear, an assessment is made whether replacing a seal or hose is sufficient, or if more extensive intervention is needed, such as replacing the flange or updating part of the pipework. If necessary, the rest of the pipework near the sink is also checked, as one weakened fitting can indicate that neighbouring connections are from the same period and material and will soon require attention.

Finally, the repaired area is tested with water flow for a sufficiently long time to ensure that the leak has truly been eliminated, not just temporarily reduced.

What Can You Check Yourself in the Kitchen Cabinet Without a Plumber?

Before calling a plumber, it's worth performing a simple visual inspection – this will later help the specialist identify the problem faster if you can accurately describe where the leak is coming from.

• Empty the cabinet and thoroughly dry all surfaces and pipes.
• Visually inspect all nuts, hoses, and joints for rust marks, lime deposits, or moisture.
• Tighten loose nuts by hand – do not apply excessive force to plastic parts.
• Let water run in different modes (cold, hot, drain) and observe where the first drip appears.
• Check the dishwasher drain hose connection if the machine is connected close to the cabinet.
• Place a dry paper towel at the bottom of the cabinet for a few days for a leak test and check it regularly.

What to Do as a Temporary Solution Until Help Arrives?

If a leak is identified but cannot be repaired immediately, the first step should be to limit damage, rather than attempting to definitively solve the problem without the right tools and spare parts.

• Close the wall valves (angle valves) under the sink if the leak is in the water pipe, not the drain.
• Do not use the sink if the leak is in the drain or siphon – this prevents adding water to an already wet area.
• Place a bucket or wide bowl at the bottom of the cabinet to collect water, preventing it from reaching the cabinet walls and under the floor.
• Remove moisture-sensitive items from the cabinet (cardboard, textiles, cleaning product packaging) as soon as possible.
• Ventilate the cabinet by leaving the door open to reduce the risk of mould growth.
• Waterproof tape or leak sealant can be used as a temporary solution, but this does not replace a proper repair.

What Happens if a Leaking Pipe Base is Left Unaddressed for Too Long?

Most kitchen cabinets are made of MDF board, which swells, crumbles, and loses its load-bearing capacity when exposed to moisture – this process can initially be almost imperceptible and only worsen over weeks or months. If the cabinet base is already softened or darkened, it's often necessary to replace part of the cabinet, as swollen wood material cannot be reversed.

Long-term moisture also promotes mould growth and unpleasant odours in the cabinet, which in turn spreads throughout the kitchen. If the problem progresses further and water begins to seep into the floor, one must also consider the risk of damage to the floor and underlying rooms – especially in apartment buildings where water can reach the ceilings of lower floors.

What to Do if a Leak Reaches Downstairs Neighbours?

In an apartment building in Tallinn and elsewhere in Harjumaa, one of the most serious consequences is when water leaking from under the kitchen sink passes through the floor to the ceiling of the floor below. In such a case, it's crucial to act quickly: shut off the water supply, inform the neighbours and, if necessary, the apartment association, and document the damage with photos – this is required for processing an insurance claim and for planning future repairs.

If the leak has been prolonged and recurring, it's worth considering a full inspection of the entire pipe system, not just repairing one spot – this helps prevent a similar situation from recurring in the near future.

When Should You Call a Plumber for a Kitchen Sink Leak?

Some leaks can be resolved with household tools, but in certain situations, it's safer and faster to have a professional inspect it – especially if there's significant water loss or damage has already spread.

• The leak recurs in the same spot, despite tightening the nut or replacing the seal.
• Water is constantly flowing, not just when the tap is in use.
• You suspect the problem is with the sink's flange itself or inside a pipe in the wall, not a visible part.
• The cabinet or floor is already moisture-damaged and needs assessment before repair.
• The leak has reached downstairs neighbours or the apartment association has been informed.
• The pipework is old and corroded, meaning replacing one fitting might trigger another problem in an adjacent area.

How to Prevent Leaks in Kitchen Sink Pipes?

Most leaks under the kitchen sink develop slowly and are preventable if the cabinet's contents are occasionally inspected and older parts are replaced before they start leaking completely.

• Check the cabinet's contents and pipes at least a couple of times a year, especially before a longer absence from home.
• Replace flexible hoses every 5–8 years, even if there's no visible leak.
• Do not store heavy items in the cabinet that could press on or displace pipes and joints.
• Use high-quality sealing material between the sink and tap that can withstand hot water and cleaning agents.
• To prevent blockages, avoid pouring grease and oil down the drain – this also helps protect siphon seals.
• Consider scheduled replacement of old metal pipes and fittings if the pipework is decades old.