In addition to the material used to build the shelf, there are a couple of other factors you should consider when designing a shelf for a bookcase: the load you expect to put on the shelf, and how that load is distributed on the shelf. Once you know both of these factors, you can determine how great a span the shelf should be able to stretch across without sagging.

EXPECTED LOAD. A running foot of average-size books weighs about 20 pounds. So a three-foot shelf filled with average-size books would have to support about 60 pounds.

LOAD DISTRIBUTION. For the sag tests explained on the opposite page, I wanted to determine the "worst possible situation" for the distribution of the load on the shelf. So I used six bricks (42 pounds), and placed them right in the center of the shelf. However, in a normal situation the weight would be distributed over the entire shelf.

GUIDELINES. Taking all three factors (shelf material, load, and distribution) into consideration, the chart at right shows some general guidelines for the maximum span for shelves to avoid an objectionable sag.

As you can see from the chart, if you have to span more than a couple of feet, the best material to use when building a shelf is solid stock.

maximum span

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