Straightening Saggy Shelves
Nearly every bookcase has a shelf that’s sagging under the weight of too many books. If not remedied the shelf will either fail, spilling books all over the floor, or become permanently bowed. If you can’t reduce the number of books, try this: Cut a vertical divider out of ¾-in. solid wood to fit beneath the sagging shelf. Make the divider just tall enough to prop up the shelf to its original, straight position. If there's shelf below the divider, install a second divider directly under the first one. Otherwise the shelf below will begin to sag.