What Is a Bathtub Overflow Drain and How Does It Work?

It’s the end of a long week, so you start drawing yourself a bath. But you hear the kids arguing in the other room, and the dogs are pawing at the door. Luckily, the overflow drain in the bathtub saves the day. Thanks to this often-forgotten sink and bathtub part, your bathroom floor isn't flooded despite the distractions.

What Is an Overflow Drain?

An overflow drain is an extra opening toward the top of a bathtub or sink that collects excess water and diverts it to the plumbing pipes. It minimizes the chance of flooding, and you may have heard of this bathtub part referred to as a wastewater drain. In addition to draining excess water, the overflow drain can speed up draining when the sink or tub vessel is full of water or help maintain a minimum level of water.

Bathtub Overflow Drain

It’s easy to start bathwater for yourself or a family member, then start multitasking while the tub slowly fills up. The overflow drain is helpful in preventing the water from overflowing and flooding the bathroom floors. We’ll go into more detail below about how bathtub overflow drains work. 

Sink Overflow Drain

You’ve probably noticed that extra hole in the sink while you brush your teeth. But unless you frequently fill the sink with water, you may not have given it a second thought. When the water level meets the overflow drain, water begins flowing down that drain into a concealed tube (the overflow channel) inside the walls of the sink basin. Then, the water moves down into the main drainpipe.

The sink overflow drain is also helpful in speeding up the draining process. The additional opening lets air into the pipes, preventing the suction effect that can otherwise slow down drainage.

This drain can help minimize the risk of flooding by directing excess water, but it may not be able to reroute water as fast as the faucet pours new water into the sink vessel. Avoid leaving the sink running without supervision.

Why Is an Overflow Drain Important?

We all get distracted sometimes, but when you leave the sink running or take too long to check on the water filling the bathtub, the sink or tub could overflow with water. The overflow drain cuts down on that risk. If the water overflows, it can cause water damage and lead to mold in your cabinets and floors.

Types of Overflow Drains

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While sink overflow drains are largely the same, bathtubs have one of two types: a traditional overflow drain or an integral overflow drain. Many tubs in older homes have the traditional version. But if you don’t want exposed piping, you may want a tub with an integral overflow drain.

Traditional

A traditional overflow drain is a hole cut inside the tub that connects to the overflow pipe and directs excess water through the plumbing to the P trap. Then the water moves down into the main drainpipe. 

Integral 

This type of overflow drain is an opening along the tub’s interior wall that directs excess water between the walls of the tub (through a hidden channel) and down the bathtub drain, similar to a sink overflow drain. Then the water continues until it reaches the main drainpipe.

How Does an Overflow Drain Work?

The overflow drain works by allowing air into the drainpipe through the overflow channel. This function helps the basin drain faster, thereby preventing—you guessed it—overflows. The overflow drain is triggered when the basin fills with water past the drain overflow opening. Once that happens, water flows down the overflow drain into the overflow channel tube located inside the sink basin. From there, the water moves down into the main drainpipe. 

Does an Overflow Drain Prevent Overflow?

While an overflow drain can minimize the risk of water overflowing and flooding the bathroom or kitchen, it’s not guaranteed to work. If the overflow drain is clogged, it may not drain quickly enough to prevent overflowing. 

Ways to Prevent a Bathtub or Sink Overflow 

Most sink overflows happen due to a clogged drain, so the best way to prevent a bathtub or sink overflow is to keep your drains as clear as possible. Be sure to clear hair from bathtub and sink drains on a regular basis. You can either pull the hair out after each bathing session, or snake the drain to get hair and other debris out all at once. You can go a step further and buy a screened basket to catch hair before it makes its way into your pipes. 

Also, if you have small children, keep an eye out for small toys and cloth getting stuck in the bath drain. 

You can prevent sink overflows in the kitchen by always scraping every bit of food waste from your dishes before putting them in the sink. Also, never let grease, oil, or other fats enter your sink drain (we see you, bacon lovers).

Common Overflow Drain Problems

You may not realize just how handy an overflow drain is until you really need it. But if that time does come and the overflow drain has a broken gasket or is clogged, you may end up with a flooded bathroom and some expensive emergency repairs. Check for the following common problems with the overflow drains in your home to ensure they’re clean and working when needed.

Corrosion

In the bathroom, a lot of moisture and heat make contact with the sink or bathtub overflow drains and pipes, which can cause corrosion. The corrosion can also be a sign that the gasket or pipe is leaking.

Leaks

Over time, the overflow pipes may break down and leak. This can cause water damage behind or below the sink or tub area or on the floors, which can be costly to repair. Leaks may also occur when the overflow drain gasket cracks.

Clogs

Hair, toothpaste, and other debris that makes its way into a sink or bathtub can clog up the overflow drain with extensive use over time. Aside from preventing the overflow drain from working properly when needed, clogs can also lead to weaker water pressure. You can clear your overflow drain by snaking it or cleaning it with equal parts baking soda and vinegar and hot water.

If you suspect a problem with your overflow drain, hire a plumber to perform an inspection and make any necessary repairs. An overflow drain can prevent costly water damage, so you’ll want to make sure yours are in good working condition.