Your bathtub drain has an overflow drain that is designed to allow water back down into the drain line after the drain stopper. This acts as a safety feature in the event that the tub valve begins to leak from the spout while the drain is in the closed position.
This feature is what slows the tub from overflowing if you were filling the tub and forgot to return in time and shut off the faucet. There are slots in the bottom of the overflow cover plate that will limit the volume of water that can be drained through the overflow drain.
Unfortunately, the design cannot handle the maximum flow rate while plugged so this safety feature is not really practical for draining the full volume of water that can come from the tub spout. It will, however, help buy some time and is effective against a slow pour from the spout (say you had a leaking tub valve and the water was dribbling out while you were gone and the drain was in the closed position). It will eventually pour over the side of the tub if the valve is left in the on position.
Another important element of the overflow is to provide drain clearing snake cable accessibility for clearing the tub drain. However, attempting to remove the clog with a snake cable through this opening will only be effective if the stoppage is local and blocking the drain line before the wastewater reaches the branch drain or mainline sewer. The whole drain system of the tub is known as the “Waste and Overflow”.
Determining Your Stoppage
If you have a bathtub that is holding standing water, first check that the drain is in the open position. If you don’t see the stopper in the bottom of the drain or you have a push/pull or twist style stopper that threads into the tub drain, check it to make sure that it is in the open position. If you don’t have that style of drain stopper, then use the toggle on the overflow plate and listen for the sound of water draining. If you hear it, you may have had it in the closed position, and it is draining fine now.
If you don’t hear it or notice a difference after a few seconds, try using a plunger to see if water comes up through the overflow. If the blockage is in the tub shoe, water will not come out the overflow. But if the blockage is anywhere after the tub shoe feeds into the overflow pipe, you should notice splashing noises and water pumping out.
Now, that you have determined that water is in the tub drain line itself, you should determine whether or not the clog is just a tub stoppage or a possible mainline sewer stoppage.
Carefully remove the tank lid to the toilet and place it on a soft surface on the ground where nobody will trip over it. Now put your hand gently into the tank, slowly lift the flapper and release water into the toilet. Does water come up into the tub? If the toilet appears to be functioning correctly, let it fill up one more time and then flush it thoroughly as you would normally.
Does water come up in the tub when you flush the toilet? If so, you have a mainline sewer stoppage. If not, you have a local tub stoppage.