Leak Under the Basin or Kitchen Sink
Waste, Bottle Trap & Pop-Up (2026)

First: supply side or waste side?

Everything under a sink is either bringing water in or taking it away. The quickest way to tell them apart is timing. Dry the cabinet floor and every pipe with tissue, leave the tap off for an hour and look again. If there is water, the leak is on the supply side, which is under pressure all the time: the angle valve, the flexible hose or the tap connection. Our angle valve and flexible hose guide covers that side.

If it stays dry until you run the tap or empty a full basin, the leak is on the waste side, which only carries water when it drains. That is what this guide is about.

The parts of a basin or sink waste

• Waste outlet (plug and strainer) — the fitting in the plughole, clamped to the basin or sink with a sealing washer or putty underneath.
• Pop-up waste — a push-button or lever plug on many basins, with its own seals.
• Overflow — the slot near the basin rim, which drains through a channel into the waste; its seal can leak when the basin is filled high.
• Trap — a bottle trap (compact, often under wall-hung basins) or a P-trap or S-trap, which holds water to block drain smells.
• Slip joints — the plastic nuts and washers that join the trap to the waste and the outlet pipe.
• Wall or floor connection — where the trap pipe enters the drain, often through a rubber seal or a pushed-in joint.

ClickBina's basin types guide notes that a bottle trap is compact and neat under a wall-hung basin, while a P-trap or S-trap is the common choice inside a cabinet.

The tissue test: finding the wet joint

1. Dry every joint under the sink with tissue or a dry cloth.
2. Put the plug in, fill the basin or sink part way, and check the plug outlet from below.
3. Fill it higher, up to the overflow, and check again: a leak now points to the overflow seal.
4. Pull the plug and let it drain, running a hand or tissue along the trap, each nut and the wall or floor joint.
5. Watch where the first drop forms, not where water collects; water runs along pipes before it drips.

Put a tray or container under the trap while you test, and do not unscrew the trap without a bucket under it: it holds water even when the sink is empty.

Where it leaks and the usual fix

Pop-up wastes and overflows

Pop-up wastes are neat, but they have more seals and moving parts than a simple plug. Hair and soap build up around the mechanism, and people often lever them out to clean them and do not re-seat the seal. A pop-up that no longer closes fully is a nuisance; one that drips from its body underneath needs resealing or replacing. Basins without an overflow need a waste that cannot be closed by accident, or a tap left running will flood the counter.

Kitchen sinks have their own version: a basket strainer that clamps through the sink with a large nut and a rubber seal. When that nut works loose, water runs down the outside of the waste pipe and looks like a trap leak.

Bottle traps and P-traps

Most trap leaks happen after the trap has been opened to clear a blockage or retrieve a ring. The washer slips out of place, or the plastic nut is overtightened and cracks, or it is cross-threaded. Hand-tight plus a little is usually enough for plastic slip nuts. Old traps also go brittle and split. A trap that keeps blocking is a drainage question rather than a leak; ClickBina's clogged kitchen sink guide explains the causes.

A basin trap that smells but does not leak has often simply dried out in a rarely used bathroom; running the tap refills it.

Check for hidden damage

A slow drip under a sink can run for months inside a cabinet before anyone notices. When you find one, also check:

• The cabinet base and side panels for swelling, soft board or peeling laminate; see our kitchen cabinet water damage guide.
• The floor or wall behind the cabinet for staining or loose tiles.
• The ceiling of the unit below, in a condo or upstairs bathroom.
• Black spots or a musty smell, which point to mould.

What a fix usually costs

These are ClickBina's published ranges (indicative 2026, Klang Valley).

Leak detection is only worth paying for when the source is hidden, for example water appearing on a floor or ceiling with no visible drip under the sink. See our leak detection guide for what that visit involves.

DIY or call someone?

Re-seating a trap washer or tightening a slip nut is a reasonable DIY job. Call for help when the waste outlet itself needs removing and resealing, when the connection into the wall or floor has failed, when pipes are concealed, or when there is damage to the cabinet, floor or the unit below. A tap, a trap or a flexible hose is ordinary maintenance; the pipes inside the wall and floor are a bigger job.

How ClickBina can help

WhatsApp us your job details for a quote: whether it is a bathroom basin or kitchen sink, where the drip forms and whether it drips all the time or only when draining. Leak detection is RM300 – RM800 per visit, charged separately. ClickBina is an experienced KL & Selangor contractor for whole-house refurbishment and trade coordination. We reply within the hour and the price we agree is a flat price with no hidden charges. A site visit is RM150, waived if you go ahead.

Sources & methodology

• ClickBina angle valve and flexible hose guide — supply-side leaks and the sink trap / hose replacement price.
• ClickBina bathroom basin types guide — bottle traps, P-traps and overflows.
• ClickBina silicone resealing, clogged kitchen sink and leak detection guides — prices for related jobs.

How we estimated prices: every RM figure on this page is quoted unchanged from the ClickBina guides named above; they are indicative Klang Valley ranges for 2026.