Why Is My Bathroom Sink Leaking Underneath? (Delhi, 2026)

Find the source first: dry everything, then run one thing at a time

This is free, it takes about fifteen minutes of mostly waiting, and it is what a good plumber does first anyway. Resist the urge to tighten nuts before you know which one is wet.

• 1. Empty the cabinet and dry every surface. Take everything out. Wipe the floor of the cabinet, the pipes, every nut and every joint until bone dry. You cannot read old water.
• 2. Lay dry paper down. Newspaper or tissue on the cabinet floor, and a strip wrapped loosely around each joint — the trap nut, the base of the trap, each supply connector where it meets the tap and where it meets the angle valve, and around the waste under the basin. Paper shows you a single drop, which your eye will not.
• 3. Wait, touching nothing. Give it fifteen minutes with the basin unused. If paper gets wet now, the leak is on the pressurised side — a supply connector or an angle valve — because that is the only water under pressure when nothing is running. This is also the most urgent case, since it leaks all day and all night.
• 4. Now run the tap for a minute, with the plug out. Water flows through the waste and the trap. New wet paper now points at the trap, its nut or the pipe joint into the wall.
• 5. Now fill the basin, hold it, then release it. Put the plug in, fill to the top, and leave it standing for a few minutes before pulling the plug. Wet paper while it is standing points at the waste flange seal where the waste passes through the bottom of the basin. Wet only on release, when the full basin dumps through at once, points back at the trap connection under load.
• 6. Take a torch to it. Get your head down and shine a torch upward at the underside of the basin and along the top of every pipe. Water runs downhill and drips off the lowest point, so the puddle is almost never below the actual leak. Trace the wet line back uphill to where it starts.

Whatever the paper tells you, note it down. “Wet only when the basin is full” is worth more to a plumber than a photograph of a puddle.

The bottle trap: its compression nut and its seals

If the water appears when you run the tap, start here. The trap is the curved or bottle-shaped fitting under the basin that holds a plug of water to stop drain smells rising, and it is the commonest source of an under-sink leak.

• The compression nut has worked loose. The plastic nut joining the trap to the waste pipe seals against a tapered washer inside. Vibration, someone knocking it while reaching for a bucket, or the pipework being under slight strain will let it back off. How it looks: a drip from the nut itself while water is draining, and nothing between uses.
• The washer inside is perished, missing or fitted the wrong way round. These washers are tapered and only seal in one orientation, and a great many leaks that appear right after someone cleaned out the trap are a washer put back reversed.
• The trap is under strain. If the trap does not line up naturally with the waste pipe going into the wall, the joint is being pulled sideways every time it is nudged and it will not stay sealed. This is why a leak sometimes returns a week after being tightened.
• The plastic has gone brittle. Years of hot water and cleaning products make thin plastic traps crack, usually at the thread or where the body meets the base. A hairline crack weeps only under flow, so it is easy to miss.
• Silt has packed the bottle trap. A bottle trap collects whatever settles out of the water, and in Delhi NCR it fills faster after a tanker delivery, after the supply returns following a cut, or in the days after an overhead tank has been cleaned. A packed trap makes water back up and pushes at joints that were sealing perfectly well before.

Is it DIY? Partly. Hand-tightening a compression nut a quarter turn is reasonable — put a bucket underneath first and test again. What is not reasonable is putting a wrench on a plastic nut: over-tightening splits the thread or crushes the washer, and turns a drip into a job. If a gentle tighten does not hold, take the joint apart properly, or get a plumber to. If the basin is also draining slowly, mention it — the same visit can deal with both, and a blocked trap under pressure is itself a reason joints start weeping.

The flexible supply connectors and their washers

If the paper gets wet with nobody using the basin, this is your leak — and it is the one to deal with quickly, because it is the only one that can turn into a flood.

Those two braided hoses running from the angle valves on the wall up to the underside of the tap are under mains or tank pressure every second of the day. When one fails, it does not wait for you to be at home.

• The washer inside the nut. Each hose seals with a small rubber washer at the tap end and at the valve end. They harden and flatten with age and start weeping — a slow, steady drip that does not vary with use.
• The braid itself corroding or fraying. Look along the steel braid for rust staining, a bulge, or strands standing proud of the surface. Any of those means the hose is near the end of its life. Replace it rather than watch it.
• A crust of scale at the joint. On hard NCR water, a white or greenish deposit building at a connector is not cosmetic — it is dried mineral left behind by water that has been seeping there for a while. Deposit at a joint is a leak that has been going on quietly.
• The angle valve itself. If the wetness starts at the valve on the wall rather than at the hose nut, the valve spindle or its seal is at fault. That is a plumber’s job, because the water has to be isolated further back to change it.

What to do right now: turn both angle valves clockwise to shut off the supply to that basin. That stops the leak dead while you arrange a plumber, and you still have every other tap in the house. If the valves are seized and will not turn — common on older fittings in this city — shut off at the main stopcock for the flat and treat it as urgent.

Costs for this kind of work sit in our plumber charges guide, and if the tap itself is also dripping or stiff, tap and mixer repair covers that side.

The basin waste flange: the leak that only shows when the basin is full

If water appears only while the basin is holding water, and stops once it has drained, the seal where the waste passes through the bottom of the basin is the suspect.

The waste fitting — the metal grating you see in the bottom of the basin — is a threaded body that passes down through the hole and is pulled tight from underneath by a large locknut, with a seal above and a washer below. That seal is the only thing stopping water that is sitting in the basin from running straight down the outside of the fitting.

• How to confirm it. The full-basin test in the first section is the whole diagnosis. Wet while standing full, dry when empty, and the drip appearing right at the underside of the basin rather than lower down.
• Why it fails. The sealant or washer under the flange dries out and shrinks over years; the locknut underneath backs off; or the fitting was over-tightened at installation and has distorted the seal. It commonly appears months after a basin has been refitted, a tap changed or a waste replaced.
• The overflow slot matters too. Many basins have an overflow channel moulded into them that connects to the waste. If that channel has cracked, water leaks when the level reaches it. A plumber checks this by filling only to just below the overflow and then above it.
• Is it DIY? Not really. Doing it properly means dropping the trap, releasing the locknut, lifting the waste out, cleaning off the old seal completely and re-bedding it. Skipping the cleaning-off step is why it leaks again, and it is easy to crack a ceramic basin by over-tightening from underneath.

Condensation mistaken for a leak - and when to call a plumber

Before you call anyone, rule out the case where nothing is leaking at all. In a Delhi monsoon, and in a bathroom that has just had a hot shower in it, humid air meets a pipe carrying cold water and water forms on the outside of that pipe. It runs down, drips off the lowest point, and puddles in exactly the same place a real leak would.

• How to tell it apart. Condensation is an even film of fine droplets along the whole length of a pipe, not a drip from a single joint. Dry the pipe completely: a leak reappears at one point, condensation reappears everywhere at once.
• It follows the weather and the shower. Worst in a humid monsoon week, worst right after someone has showered with the door closed, and largely absent in dry winter air. A leak does not care about any of that.
• What helps. Ventilation — run the exhaust fan, leave the door open after showering — and, if it is persistent, pipe insulation sleeving on the cold run. Our bathroom care tips cover the damp-management side.

Call a plumber when: paper gets wet with the basin unused, which means a pressurised connector; a braided hose is rusting, bulging or fraying; the leak is at the angle valve or where the waste pipe enters the wall; the waste flange is the source; a plastic trap is cracked; the cabinet base has swollen or the wall behind has darkened; or a gentle tighten did not hold and you do not want to take the joint apart. Anything that has started showing on the ceiling of the flat below is urgent rather than routine, and worth telling them about the same day.

What to tell them, so they bring the right parts: exactly when the water appears — unused, tap running, basin full, or on release; which joint the paper found; whether the basin is on a pedestal or in a vanity unit; whether the trap is plastic or metal and roughly how old; and whether the angle valves turn or are seized. That is a five-minute conversation that saves a second visit for a part.

And the habit worth keeping. Once every few months, open the cabinet, take everything out and shine a torch over every joint and along the cabinet floor. It takes a minute. A slow weep inside a closed vanity does all its damage invisibly — swollen board, lifted laminate, a stain creeping up the wall — and by the time it shows on the outside you are replacing the unit rather than a washer. Our leak detection habits page sets out the rest of the round, and blocked drains and low water pressure cover the neighbouring symptoms.