My garden is on a slight slope and my grass is not level. I am raising all my grass (more on one side) so that it is level. What I need to know is how much top soil is needed on top of my existing turf for the new turf to grow? Taking up and relaying the current grass is a waste as its not fantastic but is weed free. It will be about 4'' higher on the highest side so would 4'' of top soil be enough, or do I need to remove all the existing grass before putting new down?