A customer of mine wanted to know if we could install a flagstone patio directly over her existing concrete patio. We've never done flagstone over concrete before, and am wondering if its a matter of just using a thin set for installation? My concern is what prep needs to be done to the existing concrete so the thin set will adhere well. If this is not the preferred method, can anyone offer any suggestions? Patio is approximately 15x25. Existing concrete is 4" (5bag) with wire mesh. Thanks in advance.