How to Tell Where a Leak Under the Sink Is Actually Coming From
A leaking pipe under the sink is one of those problems that looks simple until you actually open the cabinet doors. Before you tighten anything or reach for the sealant, the first job is working out exactly where the water is coming from — because a drip that looks like it's from the waste pipe can just as easily be tracking down from the tap connections above it. Start by drying the whole cabinet floor and the underside of the pipework thoroughly with a cloth or kitchen roll, then lay a few sheets of dry paper towel underneath the different connection points. Run the tap on its own first, checking after a minute or two whether any of the paper has picked up moisture, and note exactly which joint it's nearest to. Next, run the waste independently — pull the plug and let a full bowl drain through — to see whether the leak only appears when water is moving through the trap rather than the supply. If there's a dishwasher or washing machine using the same under-sink space, run that separately too, since its hose connections are often mistaken for a leaking pipe under the sink when the sink's own pipework is fine. This process of elimination tells you whether you're dealing with a supply-side leak (tap, valve, or feed pipe) or a waste-side leak (trap, waste pipe, or overflow) — and that distinction determines which fix you need next.
The Most Common Causes of a Leaking Pipe Under the Sink
Most under-sink leaks come down to a handful of usual suspects. Worn tap washers or O-rings are one of the most frequent causes of a drip at the tap itself, and they're generally an easy, low-cost fix. Loosened compression fittings are another common culprit — the joints connecting sections of pipe can work loose over time through thermal expansion and contraction, or simple vibration from everyday use, and often just need re-tightening or a new olive. Perished seals in the U-bend or waste trap are especially common in older installations, where the rubber washers harden and crack with age, causing water to seep from the joint rather than from the pipe itself. Flexible tap connector hoses ("flexi hoses") can fail too, particularly where they've been installed with a tight bend or have been kinked against the cabinet wall. And in older properties, corroded or perished copper pipe under sink units, or ageing metal pipe under sink connections, can develop pinhole leaks or weep at the joints as the material deteriorates.
One thing worth ruling out before you assume you've got a genuine leak: condensation. Cold water pipes and the underside of the sink bowl can sweat in warm, humid kitchens or bathrooms, and that dampness is very easily mistaken for a leaking pipe under the sink when there's actually nothing wrong with the plumbing at all. If the "leak" only appears in warm weather or after a hot shower nearby, condensation is worth checking first.
Simple Fixes You Can Try Yourself
The fixes below are for leaks that are clearly isolated to a single joint or connector, with dry, undamaged cabinetry around them and no signs of the water having been there for any length of time. If the cabinet base is swollen, stained, or smells musty, skip ahead — these DIY steps aren't the right starting point for that.
Tightening or Replacing a Loose Connection
Follow these steps to tighten or replace a loose compression connection:
- Turn off the water supply at the isolation valve, or the stopcock if there isn't one, before touching anything.
- Place a bowl or towel underneath the joint to catch residual water.
- Use an adjustable spanner to gently tighten the compression nut a quarter turn at a time — over-tightening can crack the fitting or damage the olive, so go slowly and check for drips after each turn.
- If tightening doesn't stop it, undo the nut fully, slide off the old olive, fit a new WRAS-approved olive (available cheaply from any plumbing merchant), and reassemble.
- For a flexi hose connector, check the rubber washer inside the fitting first — a cracked or perished washer is often the actual problem rather than the hose itself — and replace the whole connector if it's more than a few years old or shows any bulging in the braid.
- Turn the water back on gradually and check the joint under pressure before walking away.
Repairing the U-Bend or Waste Trap
Follow these steps to repair or replace the U-bend or waste trap:
- With the bowl empty, place a bucket underneath the trap.
- Unscrew the two large nuts connecting it to the waste pipe and the sink outlet — these usually turn by hand or with light help from pliers.
- Slide the trap free and tip out any standing water.
- Inspect the rubber washers at each end — if they're flattened, cracked, or misshapen, replace them (they're inexpensive and sold to fit most standard trap sizes).
- Clean any built-up grime from the threads before reassembling, then hand-tighten the nuts.
- Run water through and check for seepage at both joints. If the trap itself is cracked or the plastic has become brittle, replace the whole unit rather than trying to patch it.
Signs It's Not a Simple Fix
Some signs mean it's time to stop tightening things and get a professional opinion instead. Watch for water appearing in the cabinet even when nothing has been run recently, a swollen or damp cabinet base, a musty smell or visible mould, or water tracking to an area with no pipework anywhere nearby — all of these suggest the water is travelling from somewhere you can't see, not from the joint you can. A leak that returns shortly after you've fixed it, a water bill that's crept up with no obvious explanation, or any leak near electrics such as an under-sink boiler or waste disposal unit are also reasons to stop DIY work. Depending on your buildings insurance policy, trace and access cover may apply to the cost of locating a hidden leak like this — though that's a big enough topic to deserve its own separate post.
Kitchen or Bathroom: Does the Leak Location Change Anything?
It can. A kitchen sink leaking underneath is very often traced to a dishwasher or waste disposal connection sharing the same cabinet space, rather than the sink's own pipework — so if you've got either appliance fitted, check their hoses before assuming the kitchen sink pipe itself is leaking underneath. A leaking pipe under the sink in a bathroom, by contrast, is more often a cold water supply issue — a loose isolation valve, a worn washer on the cold feed, or a failing flexi connector — since bathroom sinks typically don't have the same appliance connections crowding the cabinet. Either way, a bathroom sink leaking underneath or a cold water pipe leaking under a sink is worth checking at the valve first, since that's often where these leaks start in both rooms.
When to Call a Professional Instead of Continuing to DIY
If you've tightened, resealed, or replaced parts and the leak keeps coming back — or you simply can't find where it's actually starting — that's the point to bring in a specialist rather than keep guessing. At Clear Flow Leak Detection, our DBS-checked engineers use non-invasive detection methods, including thermal imaging and acoustic equipment, to trace leaks that have travelled beyond the visible pipe, minimising guesswork and unnecessary disruption from ripping out cabinetry. We work on a No Find No Fee basis and provide a written report with an itemised invoice, which is the kind of documentation insurers commonly request for a trace and access or escape of water claim. We cover all 32 London boroughs plus Surrey and Kent, and we're Checkatrade and WaterSafe accredited. If you'd rather have it confirmed properly than keep tightening the same joint, call us on 0203 886 2316.
FAQs
Can I Fix a Leaking Pipe Under the Sink Myself?
Yes, if the leak is at an accessible joint, washer, or connector and there's no surrounding damage — these are straightforward DIY jobs. No, if there's hidden dampness, mould, a leak that keeps returning, or you can't pinpoint exactly where the water is coming from — in those cases, a professional diagnosis is the safer route.
How Do I Seal the Pipe Under My Sink?
PTFE tape on the threads, or a new olive on a compression fitting, is the right fix for a leak at a threaded or compression joint. Sealant is the wrong tool here — it doesn't hold under water pressure and can mask a leak rather than fix it. If the leak isn't at an obvious joint, or a resealed connection starts leaking again, that points to a problem sealant won't solve, and it's worth getting a proper diagnosis instead.
How Much Does It Cost to Fix a Leaking Pipe Under the Sink?
It depends on the cause. A worn washer, new olive, or replacement trap washer typically costs only a few pounds in parts and can often be fixed within an hour. If the leak turns out to be hidden or keeps recurring, professional leak detection adds to the cost but avoids paying for guesswork or unnecessary repairs. Contact Clear Flow for a free, no-obligation estimate.
Why Is My Pipe Still Leaking Even After I've Tightened Everything?
If a joint keeps leaking after tightening or a new olive, the water may actually be coming from somewhere else — a supply pipe, an adjoining appliance connection, or even condensation dripping onto the same spot. It can also mean the pipe or fitting itself is corroded or cracked rather than just loose, in which case tightening will never fully stop it. At that point, a proper diagnosis is the only way to be sure you're fixing the right thing.
Can a Leaking Pipe Under the Sink Cause Mould or Damage to My Cabinet?
Yes. Even a slow, small leak left unaddressed can gradually swell chipboard cabinet bases, encourage mould growth, and damage flooring underneath — often before it's obvious from the outside. If you notice a musty smell, staining, or a soft cabinet base, that's a sign the leak has been running longer than a quick DIY fix should take to find.
How Do I Turn Off the Water Supply to Fix a Leaking Pipe Under the Sink?
Most sinks have a small isolation valve on the supply pipe itself, usually just before the tap connection — turn it a quarter turn until it's perpendicular to the pipe to shut it off. If there's no isolation valve fitted, you'll need to turn off the property's main stopcock instead, usually found under the kitchen sink, in a utility cupboard, or near where the supply enters the property. If you're ever unsure, a plumber registered with a body such as the Chartered Institute of Plumbing and Heating Engineering can talk you through it safely.