Empty the cupboard, dry everything, then test one thing at a time: fill and drain the sink for the waste, run the taps for supply drips. Most leaks under a kitchen sink are a loose trap joint or a failing flexi hose, both cheap fixes. A swollen base unit or a leak with everything off means a bigger problem.
Leak Under the Kitchen Sink: Finding and Fixing It
The cupboard under the kitchen sink is the busiest square metre of plumbing in the house. Hot and cold supplies, a waste trap, usually a washing machine or dishwasher connection, sometimes the main stopcock, all packed behind a bin and a basket of cleaning sprays. So when the shelf paper goes dark and the cupboard starts smelling of damp, there are half a dozen candidates within reach of each other.
That crowding is actually good news. Everything is accessible, nothing is buried, and with a torch and twenty minutes you can usually trace a leak under the kitchen sink yourself. This guide works through the tests in order, the fixes that genuinely hold, and the handful of signs that mean the water is coming from somewhere less convenient.
What this guide covers
Find it first: dry everything, then test one thing at a time
Water travels along pipes and shelf edges before it drips, so the wet spot is often not below the fault. Diagnosis starts with a completely dry cupboard.
Step 1: Empty and dry
Clear the cupboard, dry every pipe and fitting with a towel, and lay clean kitchen roll across the base. Kitchen roll is the cheapest leak detector there is: it shows a single drip instantly and marks where it landed.
Step 2: Test the waste
Put the plug in, fill the sink well past halfway, then pull the plug and watch the trap and waste pipe with a torch while it drains. A joint that only leaks under a full flow will show itself now and stay dry the rest of the day, which is exactly why it went unnoticed.
Step 3: Test the supplies
With the waste dry, run the hot and cold taps for a minute each and feel along the flexi hoses, tap tails and isolation valves with dry fingers or a strip of kitchen roll. Then wiggle the tap body: some leaks only weep when the tap is pushed sideways in use.
Step 4: Test the appliances
Run the washing machine or dishwasher through a fill and a drain while you watch its valve and hose connections. Appliance leaks often appear only at specific points in the cycle, so a quick glance mid-wash misses them.
If the kitchen roll stays dry through all four tests but keeps getting wet overnight, note when it happens. Timing is diagnostic, and it becomes important in the last section of this guide.
Where a leak under the kitchen sink usually starts
| Component | How it fails | Difficulty |
|---|---|---|
| Trap and waste joints | Compression rings loosen with vibration and temperature swings; seals harden | Easy: hand-tighten, re-seat washers |
| Flexi hoses | Rubber liner ages inside the braid, then splits suddenly | Easy to replace, serious if ignored |
| Isolation and appliance valves | Weep at the spindle or the compression joint | Moderate: some need the water off |
| Tap base and tap tails | Seals wear, monobloc taps loosen on the sink | Moderate: awkward access |
| Sink rim sealant | Splash water tracks under a failed bead into the cupboard | Easy: strip and re-seal |
Note where the sealant sits on the list. A surprising number of “pipe leaks” under kitchen sinks are splash water finding its way under a tired sealant bead every time someone does the dishes. If the dampness only appears after washing up and the pipes test dry, look at the rim, not the plumbing.
Flexi hoses: the quiet burst risk
Those braided silver hoses connecting the mains to your taps deserve their own section, because they fail differently from everything else in the cupboard. The stainless braid hides a rubber tube inside. The rubber ages, kinks and weakens invisibly while the outside looks perfect, and when it lets go it does not weep politely. It bursts, at mains pressure, often overnight.
Escape of water is one of the most common home insurance claims in the UK, with insurers paying out £1.8 million every day according to the Association of British Insurers, and failed flexible hoses are a regular cause we meet.
Check yours: rust spots on the braid, bulges, kinks or corrosion at the crimped ends all mean replace it now. They cost a few pounds. Fitting them without kinks and without overtightening matters more than the brand.
Washing machine and dishwasher connections
Appliance connections fail at three points: the small isolation valve on the pipework, the hose seal at that valve, and the standpipe where the drain hose hooks into the waste. The first two show as slow weeps that corrode the valve green or track along the hose. The third shows as a splash-and-dry cycle that leaves tide marks but is never wet when you look.
Two habits prevent most of it. Use the valve occasionally (a valve that has not been turned in years seizes, and a seized valve cannot help you in an emergency), and leave slack in the hoses when the machine goes back so nothing sits under tension. While you are down there, confirm you know where the main stopcock is; if the answer is a shrug, our guide to finding your stopcock is five minutes well spent before you ever need it in anger.
Fixes that actually hold
- Trap jointsHand-tight plus a nudge is correct for plastic compression fittings. Gorilla-tightening distorts the sealing washer and creates the next leak. If a joint keeps weeping, open it, check the washer sits flat and the mating faces are clean, and re-make it.
- Worn trap washersReplace rather than rescue. A pack of assorted trap washers costs little and lives happily in the cupboard it will one day save.
- Flexi hosesWater off, old hose out, new hose in with a smooth curve rather than a tight bend. Check the sealing washers are present and do not overtighten the nuts.
- Weeping valve spindlesA quarter-turn of the gland nut often stops a spindle weep. If the valve body itself is corroding, replace the valve.
- Sink rim sealantStrip the old bead completely, dry and de-grease, and re-run it. Sealant over old sealant fails early.
What we would skip: self-amalgamating tape and putty-style wraps on pressurised joints. They have their place as a get-you-through-the-night measure, but they hide the evidence and postpone the real fix. If a repair needs tape to hold, the fitting needs replacing.
Signs it is a bigger problem than the cupboard
Sometimes the cupboard is just where the water surfaces. Take these seriously:
- The base unit or kickboard is swellingChipboard swells from sustained wetting, not the odd splash. By the time the panel blows and the laminate lifts, water has been arriving for weeks.
- The floor in front of the sink feels soft or liftedWater has moved beyond the unit and into the floor structure. Our article on telling if you have a leak under the floor covers what happens next.
- Everything tests dry but water keeps appearingSuspect the pipes inside the wall or floor behind the unit, or in a flat, a leak from the property above tracking down the pipe boxing.
- You hear water with everything offIn most Scottish homes there is no meter to confirm it (most households pay through council tax, so a leak never shows on a bill), but a hiss at the stopcock with every outlet closed is a reliable warning. Our guide to checking for a leak without a meter walks through the isolation test.
A quiet leak behind a kitchen unit does its damage on credit, and the bill arrives as a replaced floor. If any of these signs are present, call us on 07700 152 467 before pulling the kitchen apart on a hunch.
When to stop guessing and get it located
DIY diagnosis under a sink is genuinely worth doing: the components are visible, the tests are safe, and most fixes cost pennies. Where DIY stops is the point the water source is not in the cupboard at all. Removing base units to search for a leak is destructive, expensive and frequently wrong, because water travels before it shows.
Professional detection turns that search into a measurement. Moisture mapping establishes how far the water has spread and in which direction the readings climb, thermal imaging shows anomalies behind panels without removing them, and acoustic equipment picks up a pressurised escape inside the wall or floor. The outcome is a marked location and a report you can hand to a plumber or an insurer, rather than a demolished kitchen. For an honest breakdown of which jobs suit which approach, see our guide to DIY leak detection vs calling a professional.
Frequently asked questions
Why does my sink only leak sometimes?
Because different components are stressed at different times. A waste joint leaks only under a full drain-down, a tap seal only when the tap runs, an appliance valve only mid-cycle, and a failed sealant bead only when someone splashes. That is why testing one component at a time finds leaks that random checking misses.
How do I know if the leak is the waste or the supply?
Dry everything, then separate the tests. Fill and drain the sink without running the taps for long: wetness means the waste side. Later, run the taps into a jug so little goes down the drain: wetness now means the supply side. Kitchen roll under each fitting shows exactly which joint failed.
Should I worry about a small drip under the kitchen sink?
Fix it, yes. A drip into a cupboard ruins the base unit slowly and feeds mould, and some small drips are early warnings of components about to fail properly, flexi hoses especially. The repair is usually ten minutes and a washer. The consequences of ignoring it are measured in flooring.
How often should flexi hoses be replaced?
Inspect them yearly and replace at any sign of rust spots, bulging, kinks or corrosion on the end fittings, or whenever a tap or appliance is changed. The rubber inside ages even when the braid looks perfect, so treat an old hose of unknown history as due for replacement rather than proven.
Does home insurance cover a leak under the kitchen sink?
Sudden escapes of water, like a burst flexi hose soaking the kitchen, are generally covered. Slow drips you could reasonably have noticed often are not, because policies exclude gradual damage. Fix small leaks promptly and photograph anything that has already caused damage before repairing it.
Related reading
- How to Find a Water Leak: 10 Ways to Track It Down
- Water Leak Under the Floor? How to Tell Without Lifting It
- DIY Leak Detection vs Calling a Professional: An Honest Guide
MCR Leak Detection provides professional leak detection across Scotland for homes and businesses.
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