hi room need some help notsure on something ive put down a new hob in the bathroom and i am about to waterproof one thing is do i waterproof 1st then use sand and cement to level the floor to the waste or put down the sand and cement and water proof on top of that and 1 more thing my house is on peers should i use a cooper tray or just water proof straight on the slab cheers and whats teh best diy waterproof on the market cheers