Sink leaking, not the usual suspects, please help!

last month

last modified: last month

My porcelain bath sink has overflow drain holes near the top of the bowl.

I installed a drain/stopper that has two open slots (red arrow below) just below flange that fits into the bottom of the sink bowl.

I do not know if the drain/stopper was installed with one of its slots lined up with the rectangular shaped hole that is molded into the sink just below the hole for the drain. (red arrow below).

I assumed the slots in the drain/stopper would let overflow water flow in and down the drain pipe. Maybe it would, BUT...when a significant amount of water goes down the drain hole in the bowl, a LOT of it leaks out of the sink, from two ("weep?") 1/2" holes at the back and bottom of the sink. (one shown with red arrow below)

It seems like water flows out from the slots in the drain/stopper, into the cavity of the sink, and out through the two holes.

If I replace the drain/stopper with one that does not have the slots, my guess is the water won't leak out. But what happens if there is an overflow?

I could plug up the holes easily enough, for example with epoxy putty. But then water will collect inside the sink cavity, yes?

Is the sink defective? Or what have I done wrong here?

Edit: It turns out that the two leaking "weep" holes are 2 inches higher than where the drain/stopper slot meets the sink bowl overflow "passageway". Which means that if I plug the two holes any water that flows into their area should (in theory!) drain down the pipes.

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Comments (4)

Sigrid
last month
You need a plumber. Or a book with some good pictures.
Jake The Wonderdog
last month
last modified: last month
You have a defective sink.

Don't plug those holes, you will end up with stagnant water in there and it will be nasty. This is a casting problem with the sink.
bk_backyard thanked Jake The Wonderdog
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bk_backyard
Original Author
last month
Thank you @Jake The Wonderdog !
PRO
Rosedale Plumbing
24 days ago
’ve run into this exact head-scratcher more times than I can count, especially with some of the "big box" porcelain sinks we see around the GTA.

While Jake The Wonderdog is usually on the money, before you condemn the sink as defective, there’s one "pro" trick we check first in the field that might save you from a full replacement.

1. The "Internal Bypass" Leak

The way these porcelain sinks are cast, there is a hollow "jacket" or cavity between the bowl you see and the underside of the sink. Your overflow water travels through that hollow space.

What's happening: When you pour a lot of water down, it’s not just going down the pipe; it’s backing up slightly and entering those overflow slots in the drain tailpiece.

The Culprit: If the factory didn't glaze the internal "tunnel" where the overflow meets the drain hole, the water can seep into the hollow cavity of the porcelain itself. Those 1/2" holes at the back are actually safety vents designed to let moisture escape so the sink doesn't crack from internal pressure or trap stagnant water.

2. The "Double Gasket" Fix

Sometimes, the leak isn't a defective casting, but a seating issue.

The Test: Check the large rubber gasket (the mack washer) that sits underneath the sink, held up by the big nut. If that gasket isn't perfectly centered or if the bottom of your sink is slightly irregular/un-glazed, water can escape the "overflow" area of the drain and leak into the sink cavity.

The Pro Move: I often apply a thin, clean bead of 100% silicone (or high-quality plumbers' putty, if the manufacturer allows it) around the threads of the drain where that bottom gasket meets the sink. This ensures that any water entering those overflow slots must go down the pipe and can't wander into the hollow part of the sink.

3. Whatever you do, Don't Plug the Weep Holes!

I have to double down on what Jake said - do not use epoxy or putty on those 1/2" holes. If you plug them and the leak continues, you’ll essentially be filling your sink's "body" with a gallon of stagnant, soapy water. It will eventually smell like a swamp and could even lead to the porcelain failing.

The "Bucket Test"

To know for sure if it’s a casting defect:

Dry the underside of the sink completely.

Plug the drain with the stopper.

Fill the sink and let it run into the overflow holes at the top.

If water immediately pours out of those 1/2" weep holes, the internal "tunnel" is definitely cracked or wasn't sealed at the factory. In that case, the sink is indeed toast.