Here’s a complete, in-depth review of , covering what they are, how they work, pros and cons, installation, and maintenance. Complete Review: Overflow Drain for Sinks 1. What Is an Overflow Drain? An overflow drain is a secondary drainage hole (or series of holes) located near the top rim of a sink basin, just below the counter level. It connects internally to the main drain pipe, preventing water from spilling over the edge if the sink is left running with the stopper closed or drain clogged.