Shelf sag is the bane of bookshelf builders. Few things are worse than completing a wonderfully built set of bookshelves only to see the shelves sag under the weight of their contents.
Books weigh 20 to 25 pounds a running foot, and even more if they are unusually tall. Depending on how they are stored, CDs and DVDs can weigh much more (if, for example, your CDs or DVDs are stored not in individual slip cases but in binders with dozens in a book sized holder). This can add up to a heavy load in a hurry, and a load that many types of shelving cannot sustain without sagging if the span is too wide.
One way to stiffen up a shelf is with an edging strip. Attached along the front of the shelf, an edging strip can help stiffen the board to withstand the weight and defeat shelf sag.
The best approach - whether or not you use an edging strip - is to crunch some numbers before you start building to make sure the materials you are using are up to the task.
Thanks to a handy online tool, calculating what you need only takes seconds.
An online program called the Sagulator allows you to input the type of shelving material (including glass, particle board and dozens of different species of wood), the span of the shelf, and the thickness of the shelving. It also makes adjustments if an edging strip is used. The Sagulator then lets you know what sag to expect. All you need to do is adjust your materials until you get to a level of sag you can live with.
You want to brag, not sag, so calculate your material needs before you make your first cut.