Quick Answer:

A leak that appears only when the sink is filled to the overflow usually means the overflow passage, its gasket, or the connection between the overflow and drain is letting water escape under head pressure. It can be a loose or damaged overflow plate, a failed gasket, a cracked sink body, or a problem with the pop-up/tailpiece seal.

Why This Happens

The overflow is a shallow channel that links the basin rim to the drain. It sits above normal water level, so it only carries water when the sink gets high. At that higher level the pressure on joints and seals is greater than when the sink is partially full. Any small gap, worn gasket, loose screws, or crack that stays dry at low levels can start to leak when the water reaches that height. If you have noticed that Water comes out of overflow hole, that flow can travel down behind the sink or into the cabinet and show up as an intermittent leak.

Step-by-Step What to Do

1. Prepare and protect the area

• Clear under-sink storage so you can see the cabinet bottom and the sink underside.
• Place a dry towel or a shallow pan under the drain and overflow area to catch any water that appears.

2. Visual check of the overflow plate and screws

• Look at the overflow opening and faceplate for loose screws, missing gaskets, or visible cracks.
• Tighten screws gently; don’t overtighten plastic parts.

3. The one-minute fill test (must-do)

• Fill the sink to just below the overflow and hold for one minute to see if water appears under the cabinet. This confirms an overflow-related leak versus a lower drain seal leak.
• Watch the towel or pan under the sink for new dampness. If water shows up only after the sink reaches that level, the overflow system is likely the source.

4. Inspect the overflow passage from the top and underside

• If the overflow has a removable faceplate, remove it and inspect the gasket and the inside of the passage for cracks, corrosion, or debris.
• From under the sink, look at where the overflow tube meets the drain or shell; some sinks have a flexible pipe or molded channel that can split or detach.

5. Check the pop-up and tailpiece seals

• Some sinks route overflow water through the pop-up assembly. Inspect the pop-up seal and the tailpiece nut for loose fittings or degraded washers.

6. Clean and retest

• Remove any hair or debris in the overflow passage and reinstall the faceplate with a new gasket if needed. Repeat the one-minute fill test.
• If the leak stops, monitor for a few days to be sure the repair holds under normal use.

7. Replace parts or tighten connections

• If tightening and cleaning don’t help, replace the overflow gasket or faceplate. For molded overflow channels or cracks in the sink body you may need a new sink or professional repair.

What Not to Do

• Do not leave a full sink unattended while diagnosing an overflow-only leak.
• Do not force plastic screws or fittings—overtightening can crack parts.
• Do not use a permanent adhesive or patch inside the overflow passage while water is present; it won’t seal properly and can cause blockages.

When to Call a Professional

• Visible cracks in the sink body near the overflow or basin rim.
• Leak continues after you replace the overflow gasket and tighten connections.
• Water is coming from a place you cannot access or the leak is causing damage to cabinetry or finishes.
• If the overflow system is integrated into a decorative or antique sink and you want a careful, lasting repair.

Safety Notes

• Wear gloves and safety glasses when working under the sink. Old gaskets or debris can be sharp or contaminated.
• Turn off water only if you plan to work on supply connections; most overflow checks don’t require shutting water off.
• Keep electrical devices and outlets away from standing water under the sink.
• If you smell sewage or suspect a sanitary line issue, stop and call a plumber rather than attempting complex fixes.

Common Homeowner Questions

Overflow water smells like sewer
A sewer smell usually means a trapped line (P-trap) problem or a dry trap elsewhere. Check the drain trap and vents; if the smell is tied to overflow use, call a plumber.

Why does the sink only leak when it’s full?
Because higher water level raises pressure on overflow seals and joints; a gap or damaged gasket that isn’t exposed at normal levels will leak when the sink is full.

Can I fix this myself and how long will it take?
Simple fixes (tighten screws, replace gasket, clean debris) can take 30–60 minutes. Cracks or complex assemblies may require a pro and take longer.