Bathroom Sink Drains with an Overflow

 An Overflow Drain is a type of sink drain that works in conjunction with a sink that has an overflow port.  This is useful if you have risk of the sink not draining, filling up, and flooding the bathroom. For example, if you close the drain and fill a sink, an overflow sink will fill to the drain ports and flow down a channel to the overflow drain.  

If you have seen a bathroom sink with holes in the upper wall of the sink, that is an over flow. Conversely, a sink that only has a drain hole at the bottom, is a non-overflow sink. 

Choosing the correct drain is important. If you choose an overflow drain without an overflow sink, you can have water damage

If you have an overflow sink, and use a non overflow drain, you are risking having water damage.