Understanding Sag-less Spans

Shelves that don't sag are the goal for any kind of shelving. Each material has a different span limit, the maximum distance it can span between supports without sagging or breaking under a load.

According to architect calculations, books represent an average load of 25 pounds per cubic foot. At right 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).

Cut a rabbet into a piece of solid molding and attach it as a rail that conceals the plywood edge and adds support.

Simply attach a 1x2 molding to the shelf's front edge. This method also hides the unsightly edge.