The simple answer is "It's not required by building codes." However, there is some reasoning behind this. Press play on this video and start reading.
The average kitchen sink will hold eight gallons on each side, and most have a lip around the edge forcing water that reaches above the first side to flow into the second. The fastest flowing faucets you can currently buy have a flow rate of around two gallons per minute, so it would take at least eight minutes to overflow a kitchen sink. If you have a more common aerated faucet that flows at 1.5 GPM, this will take 11 minutes. Most bathroom faucets also flow at 1.5 GPM, but a bathroom sink is a lot smaller. If you are an average reader, the sink in the video should be overflowing right about now.
That only took about 80 seconds. As you can see, you are far more likely to have a bathroom sink overflow than a kitchen sink. Bath tubs also have an overflow, but most tub faucets flow around 4 GPM, creating a much bigger problem if they overflow.
There are kitchen sinks on the market that do have a built-in overflow mechanism, and since this is internal to the sink, installing one is relatively simple.
Posted 4468 day ago