What Is Longest Span for Plywood Shelves?

Basics

• Several factors affect the allowable span for shelves before they sag, including the shelving material, the thickness and depth of the shelf, and the load they’ll carry. A thin plastic shelf holding heavy books can have a shorter span than a solid-wood version carrying only papers. Some sample maximum spans for a 10-inch wide deep shelf of books are 24 inches for three-quarter-inch particleboard, 30 inches for three-quarter-inch plywood, 36 inches for thirteen-sixteenth solid stock, and 60 inches for 1 5/16 solid stock.

Reinforcements

• One way to increase the span of plywood shelves is to add reinforcements made out of wood or other materials. For example, adding a 1.25-inch wide face strip on the side of a three-quarter-inch plywood shelf holding books increases the span to 32 inches. Putting that same strip on the long edge increases the width to three feet. Other methods to increase the width to 36 inches include putting an aluminum strip or a molding strip under the shelf. Adding reinforcing cleats or using screws to attach the shelf to the case back will also allow longer spans.

Sag

• Sag or deflection is when a shelf starts to bend downward in the middle because its span is too long for the weight of the material it carries. Some ways of managing this problem include reducing the shelf span by one-fifth to double its stiffness. Doubling the thickness reduces sag by an eighth. And doubling the depth cuts sag in half. Conversely, increasing the span by a fourth doubles the sag. Doubling the shelf span increases the sag by eight times.

Standards

• Though estimating plywood shelf spans works in most cases, you can get more precise calculations from the shelf-deflection table published in Architectural Woodwork Quality Standards, put out by the Architectural Woodwork Institute in Centreville, Virginia. This document lists the loads that allow 8-inch and 12-inch wide shelves to sag by a quarter-inch when spanning lengths of 30, 38, 42 and 48 inches. It also offers techniques for beefing shelves to increase their spans.