Yes the waste is above the floorboards that travels across the middle of where the shower tray is going to be, along the skirting board and through the wall and down through the house as we are on a septic tank. Also we have an extension to our house so the waste for the sinks and toilets pass through the middle of the new and old part , under the garage and into the septic tank which is located under our drive.
Bit hard to explain, but quite hard to re-route under the floorboards as the waste to the main bathroom meets up with the wastes in the en suite next door, and both travel through the middle of the new and old part of the house.
I've built a solid frame and have ply'd on top and am going to lay a bed of rapid set and mount it onto that hopefully that's right?