How do I prevent bookcase shelves from sagging?

Shelf sag is caused by overloading or insufficient shelf thickness. To prevent it: stay within the manufacturer's load rating, use four shelf pins per shelf (not two), choose shelves made from 3/4 in. solid material rather than hollow-core, and for spans over 36 in. use a center shelf support. Center supports can reduce maximum span effectively by half. OfficeFurniture2go.com carries heavy-duty shelving to resist sag — call 1-800-460-0858.