Understanding Sag-less Spans

This story covers what materials are the strongest for shelving and how you can add to a shelfs strength.

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.

Sturdy shelves: Aprons or additional pieces

Increase a wooden shelf's span limit by fastening wood-matched 1x2 aprons underneath (top) or simply by using two plywood pieces (bottom).

Sturdy shelves: Railing

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

Sturdy shelves: Molding

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


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