I have an old ugly concrete patio that has settled and slopes toward the house. I would like to have a flagstone patio with a slight slope away from the house. What is the best way to do that? Should the old patio be removed? Or could the slope be corrected by putting sand (or something else?) on top of the concrete and then have the stones laid on top of the sand? What are the pros and cons of each and which would be less costly? Thank you!