Sink Leaking? How to Find the Leak and What to Fix

Key Takeaways

• A sink leaking underneath is usually a loose bottle trap, a worn flexible hose or a failed waste pipe joint, not a burst pipe.
• Find the side first. A supply-side leak drips all the time and needs the water turned off. A waste-side leak only drips when the sink is used.
• Dry everything and use a tissue. Run water in stages and press a dry tissue on each joint to find the one that’s wet.
• PUB lets a handyman replace a sink, basin, bottle trap, tap or flexible hose. Work on the pipes in the wall needs a PUB Licensed Plumber.
• A sink or basin leak repair costs S$70 in Singapore and takes about 60 minutes with Urban Company.

The Swollen Cabinet in Tampines

It’s Sunday morning in your 4-room flat in Tampines. You reach under the kitchen sink for a new sponge and the cabinet floor is wet, with the laminate starting to swell. Nothing looks broken. A towel, a box of tissues and 10 minutes will tell you where the water is coming from.

Why Is My Sink Leaking?

A sink leaks underneath because a joint has loosened, a seal has worn, or a part has cracked with age. The usual causes, most likely first:

1. The bottle trap. HDB’s home care guide notes that the common wash trap for the wash basin and kitchen sink in HDB flats is the bottle trap. Its lower section unscrews for cleaning, and it often leaks after being opened and put back slightly crooked.
2. The flexible hose between the tap and the supply pipe. These braided hoses wear, corrode at the ends or get kinked by things stored in the cabinet.
3. The waste pipe joint where the trap’s outlet meets the pipe in the wall.
4. Sealant around the sink or basin. When it cracks or lifts, water from the counter runs underneath.
5. The tap base or sink outlet, where a worn seal lets water track below the counter.

How Do You Find Where the Water Is Coming From?

Dry everything, then run water in stages and see which joint gets wet first. Finding water under kitchen sink cabinets tells you little on its own, because water travels along pipes before it drips.

1. Empty the cabinet and dry every pipe, joint and surface with a towel.
2. Turn the tap on and off without using the sink, by running it into a bucket. Press a dry tissue on the hose ends and valve. A wet tissue here means a supply-side leak.
3. Fill the sink, then pull the plug. A full sink pushes water through the outlet, trap and waste pipe at once. Check each joint with a fresh tissue.
4. Splash the counter around the sink edge and the tap base, then look underneath for drips.

PUB’s tips on finding leaks suggest looking for water stains, mould or peeling material under the sink area, and checking your water meter. If the dial keeps moving with every tap off, a supply leak is likely. That test can also point to a toilet cistern leaking into the bowl, which has its own guide on why a toilet cistern keeps running.

Symptom, Likely Cause and What to Do

What you notice	Likely cause	What to do
Drips all the time, even with the tap off	Flexible hose or valve (supply side)	Close the ball valve, replace the hose or call someone
Drips only when the sink drains	Bottle trap or waste pipe joint	Reseat and hand-tighten the trap
Leak started after clearing a choke	Trap not seated straight, washer displaced	Reassemble it carefully
Water appears after splashing the counter	Cracked or lifted sealant	Reseal around the sink
Wet only at the back, under the tap	Seal under the tap body	Professional, or replace the tap
Water at the wall pipe, or damp patch on the wall	Waste or supply joint at or in the wall	PUB Licensed Plumber

Why Is the Bottle Trap Leaking?

A bottle trap leaking usually means its lower section isn’t screwed back straight, or the rubber washer inside has slipped or worn out. It happens most often right after someone has opened the trap to clear a choke.

Put a basin underneath, unscrew the lower section anticlockwise, check the washer is sitting flat in its groove, and screw it back clockwise by hand until it’s snug. Don’t use a wrench. Overtightening plastic threads can crack them, and a cracked trap leaks more. Pouring boiling water down the sink can also soften a plastic trap, so let the kettle cool first.

If the trap is cracked, or still drips after reseating, it needs replacing. If you’re opening it because the sink won’t drain, here’s how to clear a choked kitchen sink safely.

Is a Leaking Pipe Under the Sink Urgent?

A leaking pipe under the sink is urgent if it’s on the supply side, and can usually wait a day if it’s on the waste side. The supply side is under mains pressure 24 hours a day, so it keeps dripping while you sleep. The waste side only carries water when the sink is in use.

For a supply-side leak, close the ball valve below the basin or, for the whole flat, the stopcock next to the water meter. HDB’s guide on pipes says to shut the stopcock at the PUB water sub-meter if there is a leakage and to engage a licensed plumber for repairs. For a waste-side leak, stop using the sink and keep a container underneath.

There’s a second reason not to wait: water under a sink can reach the floor slab. In an HDB block, flat owners are responsible for their floor and ceiling slabs, and untreated ceiling leaks can damage electrical items and cause mould. If your neighbour below reports a damp ceiling, the cost of fixing it is usually shared.

What About a Basin Leaking in the Bathroom?

A basin leaking in the bathroom has the same causes as the kitchen sink, but often goes unnoticed because the floor is wet anyway and water drains to the floor trap. Look for a damp cabinet, stained grout at the base of the wall or a musty smell.

Black mould on failed sealant means water has been getting in for a while. Our guide on how to clean bathroom tiles and grout covers surface mould. If the smell is coming from the drain rather than the cabinet, read why a bathroom floor trap smells.

What Can You Safely Fix Yourself?

You can reseat a bottle trap, tighten a hand-tight joint and reseal around a sink, as long as the leak is in the parts you can see. PUB draws the line in law. Its list of simple plumbing works that handymen without a licence can do includes replacing any tap, any flexible hose connected to a tap, and any sink or washbasin including the bottle trap below it.

Everything else on the water supply and sanitary pipes is regulated work for a PUB Licensed Plumber, and since 1 April 2018 it has been an offence to engage an unlicensed plumber for it. In practice, that means:

• Fair DIY: reseating the trap, replacing a washer, hand-tightening a loose nut, cutting out and redoing sealant.
• Book a professional: a leak at or inside the wall, a corroded valve, a damp patch spreading on the wall, or anything where you’d need to cut or rejoin a pipe.

Always close the ball valve before undoing anything on the supply side.

Why Do Quotes for a Sink Leak Differ So Much?

Two quotes for the same leak differ because providers price the visit, the diagnosis and the parts differently. Before you agree to anything, ask:

• Is it a fixed price for the repair, or a visit fee plus a quote on inspection?
• Are parts included, such as a new trap or hose, or charged on top?
• What if the sink or tap needs replacing? That’s a different, larger job.

Our guide to why plumber quotes vary so much explains how each pricing model works.

When Should You Call a Plumber?

Call one when the leak is on the supply side and won’t stop, when it’s at or inside the wall, when reseating the trap hasn’t fixed it, or when your neighbour below reports a damp ceiling.

With Urban Company, a sink or basin leak repair is $70 and takes about 60 minutes. You book in the app, and the professional’s profile and rating are visible before they arrive. For every current plumbing service, see the full plumbing price list and book a plumber in Singapore.

Frequently Asked Questions