We are in the process of installing an Ikea kitchen and have just had the quartz countertops installed professionally. We have done the cabinet installation ourselves and felt we got them all pretty level prior to the countertops going on. We have noticed a small sag in the island countertop as when I placed a 6 foot level on it there appears to be a gap of approximately 3mm between the level and the countertop except for about 70mm from each end where it curves up. When you look at the countertop it appears flat. There is a small gap (2mm) between the top of the middle cabinet side panels (24 inch wide cabinet) and the counter bottom surface. This middle cabinet has a 30 inch wide cabinet on one side and a 12 inch cabinet on the other side and the counter appears to sit down on the side panels and front and rear supports of these outer cabinets. There are cabinets at the back that make an island 4.5 feet by 7.5 (1 corner is cut off) The countertop installer advised me that the quartz can sag if not properly supported. My question is: should the 30mm thick quartz countertop be able to support its own weight over a 30 inch span without sagging? Hope my description is clear.