Bookcase Shelves That Do Not Sag

Books are heavy and most shelving materials, especially low-cost engineered wood materials like particle board, will slowly sag under their weight. Here's a simple method for making cheap particle board shelves that look good and keep a straight horizontal span despite long-term heavy loading.

I've also discussed a simple design for an open-frame free-standing bookcase using these shelves. Over the past 20 years, I've built many bookcases and workbenches this way, and none has any visible sag. Of course, your mileage may vary....

The short wood fibers (sawdust, wood chips, sawmill shavings) used in making particle board (AKA chipboard, fiberboard, pressed board) make it notoriously fond of sagging. Our trick is use of a long and relatively large dado -- a rectangular slot that allows a rear 2X4 support to prevent the particle board shelf from deforming.

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Step 1: Materials and Tools

Although you can make the shelves any of a wide variety of sizes, for this Instructable, we'll focus on making a roughly 4' long shelf suitable for use in a bookcase. Lumber generally comes in lengths near 8' or longer, but there is no need to drag such long lumber home to cut it. Wherever you buy the lumber, have them cut the lumber to length; an 8' piece becomes two nearly 4' pieces. The parts you'll need per shelf are:

A roughly 4' long slab of 3/4" thick raw particle board (shelving blank) 10"-12" deep
The same length of 2X4 lumber (which is typically really 1 1/2" by 3 1/2"); hand pick a nice piece of construction-grade material
Wood glue
Flat black latex wall paint (easiest to find around Halloween)
Wood stain and/or polyurethane in the color of your choice

The tools you will need (and should use with appropriate care) are: