Because the plumbing was bad. We had to rip it up and will be fixing the drain under it to make it code. So now we need to replace the floor of the shower. It's a 3/4 size with a curved bench and sloping walls at the floor. I think we should fill the 4" gap under the floor to the cement slab, to make it stable and then tile it. Not sure how to handle the edges where the floor meets the sloped walls. Any other ideas?

It was just awful, the previous owner had put the shower enclosure over a basement drain but didn't plumb it. There was a drain in the bottom of the enclosure but it ran onto the cement floor under the enclosure and then eventually went down the basement drain. Think laundry room drain type of drain. Should have known better when we moved in she had a fan on the floor in front of the bathroom and said that she had spilled water. NOT. After living there a year or so noticed a lot of moisture in the carpet in the hallway outside the bathroom. Husband finally cut a square foot out of the floor of the enclosure and saw the problem.