Sink Overflow

Water overflowing in kitchen sink
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The sink overflow is a built-in design feature included with most bathroom sinks, and it has two purposes. Preventing accidental overflow of the sink is actually its secondary purpose. The primary and most commonly used, purpose of a sink overflow is to help the sink basin drain faster.

How Sink Overflows Work

A standard sink overflow is a hidden channel running along the underside of the sink basin. You can't see it from the top of the sink and often can't see it from below. At the top of the overflow channel are the two holes near the rim of the basin. The bottom of the channel has an outlet hole that lets water flow into the drain just below the basin. The basin and the overflow use the same drain pipe, but the overflow is always open and isn't affected by the sink stopper.

Overflows Help Prevent Overflowing

If you close the drain of your bathroom sink and fill up the sink basin to the level of the overflow hole, the water will begin draining into the overflow and down the drain. This helps prevent the water from spilling over the top of the basin, but because sink overflow holes are usually small, they often can't prevent overflows for long. If the faucet is on full-blast, the overflow will merely delay an overflow for a short time.

It's a common misconception that sink overflows prevent overflowing water in the event that the sink drain is clogged. The overflow outlet on most sinks is very close to the drain opening in the basin, and it's unlikely that a clog would occur between the drain opening and the overflow outlet, stopping up the former but not the latter. Overflow drains on bathtubs are a bit different and have more distance between the drain opening and the overflow connection. Overflows on tubs can still work if a clog is close to the tub drain opening.

Overflows Improve Drainage

Sink overflows help the basin drain faster by allowing air into the drain pipe via the overflow channel. When there's enough water in the basin to completely cover the drain opening, the drain can't get air through the water, creating a suction effect that slows the flow of water going down the drain. But with the overflow feeding air to the drain pipe, there's much less suction effect, and the water goes down much faster. By contrast, sinks that don't have overflows commonly send up bubbles as they drain and typically drain more slowly than sinks with overflows.

Cleaning a Sink Overflow

To ensure that a sink overflow can serve its purposes, it needs to stay clean and clear. Gunk can build up near the bottom of the overflow channel, restricting the flow of water and air through the channel. And since an overflow usually doesn't get flushed out unless the sink is overflowing, the channel doesn't get a regular cleaning. There are a few ways to manually clean the overflow and keep the sink draining properly:

  • Zip tie: Long plastic zip ties work very well because they bend around the curve of the sink but are rigid enough to dislodge any buildup. Insert the pointy end of the zip tie into the overflow opening and slowly work it down as far as possible. Move the zip tie up and down and side to side to loosen and push out any blockage. Be careful not to drop the zip tie into the overflow. A longer zip tie is easier to hold on to than a shorter tie.
  • Rubber hose: Fill up the sink with water slightly higher than the overflow opening. Let some of the water run down the overflow. Put one end of the rubber hose against the overflow opening and blow several puffs of air through the other end. Drain the sink and blow air through the hose again. The air should blow through clearly without resistance. Repeat as needed. A 7/8-inch dishwasher hose works well, but any sturdy hose can work as long as it fits over the overflow opening.
  • Liquid cleaner: Create a 50/50 mixture of chlorine bleach and plain water and pour the mixture down the overflow hole(s), using a container with a pour spout. Let the mixture sit for 10 minutes, then flush the overflow with plain water. Note: Do not use bleach if your home has a septic system; instead, use a commercial enzyme drain cleaner that is safe for septic systems. Mix the product as directed and pour it into the overflow hole(s). Let it sit overnight, then flush the overflow with water.