Am building a walk-in shower- using just sand as my pre-layer over plywood sub-floor to create proper slope. Next layer is rubber matting water proof layer and the top layer will be a mortar mix. The concern is that the sand will shift- What do you think?

There are dozens of good Youtube videos on membrane under mortar showers. If you want help finding some, we can. I'm positive that you should NOT use just sand for the slope. The slope of mortar isn't that steep, especially if the drain is in the center. There's also a lot to know about the corners and the walls and how they meet the floor, and so on.

Thank you- have watched the YouTube videos and will switch to a mortar based pre-slope and then the membrane etc.

Sand is not a good idea at all. Membrane goes over the ply and 6 inches up on the sides. Your base is then all mortar which creates the slope.

Sand will shift and you will have a cracked floor in no time. Build a wet mortar bed or use a preformed base if the shower is standard size.

Thanks appreciate your interest