Planning For Shelving Spans

Like most structural or engineering issues related to wood, shelving spans are subject more to general guidelines rather than precise formulas. Why? Because wood originates as a natural living tissue of trees rather than a uniform manufactured material tailored to the whims or needs of woodworkers. Engineered wood products such as plywood are more consistent, but many variables still affect the outcome.

Short of calculating a formula with a slide rule (does anyone remember slide rules?) or a computer, most woodworkers have to “ballpark” their dimensions, and there’s nothing wrong with that. If you want a simple rule-of-thumb that’s easy to remember, think of the common scale used in architectural drawings: ¼ inch equals 1 foot. Borrowed for this purpose, this ratio means a solid hardwood shelf loaded with books can, under the right conditions, span 1 foot of horizontal distance for every ¼ inch of its thickness. For example, a ¾-inch thick oak board typically can span up to 3 feet without significant deflection.

Are there exceptions? Of course, but this guideline provides a reasonable margin of safety. Just keep two key principles in mind: First, wood consists of longitudinal fibers and is strongest in the direction parallel to those fibers (“with the grain”). Second, cutting or breaking those original fiber bonds and reconstituting them with adhesive (such as when producing particleboard) alters and often diminishes the strength of the material.

With these caveats in mind, we can deal with the variables. For instance, dense hardwoods such as oak or maple will support more weight than will a softwood such as pine. Plywood has a cross-grain construction with fibers running in two directions, so its rigidity is compromised slightly. Composites such as particleboard or medium-density fiberboard (MDF) have random fiber orientation, making them more susceptible to bowing and bending.

That said, there are several simple techniques for reinforcing shelves to make them stronger. The most common is to attach a strip of wider solid wood (say 1½-inch) to the front edge of a ¾-inch thick shelf. This creates a rib to stiffen the shelf. If you don’t mind having a lip on both faces, centering the strip on the shelf edge will allow you to flip adjustable shelves over occasionally to compensate for any bowing.

If your bookshelf project has a back panel, as most do, you can drive screws through that panel into the rear edges of the shelves to anchor them at the correct height. This works best on shallow depth shelves (less than 9 inches), because if they are too wide the front edge may sag even while the rear edge is fastened tight. You can also attach metal support brackets to the front face of the back panel and to the underside of the shelf without interfering with the placement of books.