Shelves that don't sag are the goal for any kind of shelving. And whether or not that goal is met depends on the materials used and the items placed on each shelf. Each material has a different span limit—or the maximum distance it can span between supports without sagging or breaking under a load. We'll walk you through sag limits for different types of materials, plus offer a few additional shelf support options.
According to architect calculations, books represent an average load of 25 pounds per cubic foot. Listed above are the no-sag span limits under load for the most commonly used shelving materials.
Solid hardwood has the best no-sag rating; however, some species are stiffer than others. Birch, maple, and oak are the stiffest, followed by ash, cherry, and walnut.
You can increase the stiffness of a shelf by sinking screws into it through the solid back of the case. Or, as shown below, add more strength and maintain adjustability by attaching a cleat or molding to the front of the shelf. Aprons can also be added under the shelf.
Increase a wooden shelf's span limit by fastening wood-matched 1x2 aprons underneath (top) or simply by using two plywood pieces (bottom).These supports make the shelf more sturdy without much effort.
Railing is another easy way to extend the span limit of your shelf. To add railing, cut a rabbet into a piece of solid molding. Then attach it as a rail that conceals the plywood edge and adds support.
Molding does double duty when supporting shelves. It adds strength, but also hides the unsightly edge of the shelf. To add molding, simply attach a 1x2 piece of molding to the shelf's front edge.