My mat started to warp for some reason in the middle at an angle. I did the frozen cookie sheet method. Dh had a piece of 4x4 plywood and we put the mat on it and a piece of fabric then the cookie sheets on top of it and had a couple large old dictionaries and placed it under our bed. That way everything would be flat. I didn't want the cookie sheets in direct contact. was afraid what would happen because of the heat and cold. Worked fine. That was last year.