The shower pan itself sits on a layer of mortar, which in turn sits on a layer of concrete. The mortar serves as both a waterproofing agent and as a bonding agent between the pan and the underlying layers. The concrete provides a stable base for the entire system.

There are a few things that can go under a shower pan. One option is to install a layer of gravel first, then set the shower pan on top of that. This will help drainage and keep the shower pan in place. Another option is to install a layer of mortar or concrete before setting the shower pan in place. This will create a more solid foundation and also help with drainage. Whichever option you choose, make sure the subfloor is level and stable before installing anything.