Shower Drain

I have a fiberglass stall shower. When the drain plugs up and its at the slow drain stage it leaks down stairs. I unplug it and than it works fine and does not leak. Are these designed to do this. Perhaps an over flow escape route for the water or is something wrong with it like a broken seal etc. Thank you.

Somewhere there is a leak that only becomes noticable when the drain is clogged. How high does the water go before the leaking starts?

A picture of the bottom of this shower would help quite a bit. The leak could be from a seam between the bottom and the side. Most likely, the drain needs to be either tightened or re-sealed and tightened. The extra water pressure from the standing water is causing this borderline seal to fail.

ok I will post pictures of it from the underside and from inside the shower. As to answer your question in regards to how high the water gets. Well the base fills about an inch and than slowly drains. The outside rim of the drain would be exposed to water. Thank you.