I am installing a 3'x3' shower in the basement on a slab-on-grade. I have "blocked out" a 15" x 15" x 15" recess in the slab (which has been poured). It has the 2" PVC drain pipe coming out of the side of the hole to which the trap will be attached when the exact location of the shower is determined. It will probably be a prefabricated pan (copper or plastic) on which I will install ceramic tile. A two level drain fitting with weeps has been selected to attach to the pan liner which I think will take care of the drainage issue.

My question is: what supports the pan as it spans over the 15" hole? Fill it with concrete after the exact position is determined? I thought the idea of a blockout was to provide access to the trap if there would be any problem in future without having to jackhammer up a slap ( which has radiant heat embedded in it). Spanning with plywood seems to put a critical but vulnerable component in an inaccessible location. I could fill it with sand but since the vapor barrier is below the concrete and above the underslab extruded styrofoam insulation, any leaking water will stay above the vapor barrier. Maybe that's not a problem. Would sand do the trick or settle, and/or what else should I use?

Sand will not be a good option here since it can shift around as the pan deflects from weight. Fill the hole with concrete.

If you were going to build a shower the 15" dap out should have never gone down. On a slab we position the shower trap and raiser and the cement guys make the pour. Here's a option for you. Fill the hole with sand up to the last inch or so and then cement over the dap out. Good luck, Tom

Thanks, it answered my basic question.

Excellent. I was thinking the same. Pouring in place originally leaves no positioning flexibility.