To get a little perspective on sagging shelves, I have a customer that asked me to fix some shelves they had. The shelves were made out of 5/8" particleboard made 24" wide and 58" long. They had support under both ends and the back but not the front. Still it was only sagging 2"and been there for decades. All I did to fix them was buy some crooked 2x4's and screw them flat underneath the shelves.