My new ensuite is being built over an existing little concrete balcony.Bth/shr is installed already in a hebel wall. I need to raise the floor of rest of room about 100mm and have a fall to a drain in the middle. Should I use concrete (25 bags I caculate) and if so what type and how do I make it stick on old concrete or should I use Fibrous boards.(But then how do u get the fall in the boards.