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Shelving Spans for Bookcases
If you're designing a cabinet or a bookcase, what's the greatest length (span) a shelf can be without an objectionable sag? There are four factors to consider: 1) how the load is distributed, 2) the expected load, 3) the shelf material, 4) the method of reinforcement.

LOAD DISTRIBUTION. For the tests we conducted to create our recommendations (see below), we wanted to determine the worst possible situation for the distribution of load. So we use six bricks (42 pounds) and placed them right in the center of the shelf. However, in a normal situation, the weight would probably be distributed over the entire shelf.

EXPECTED LOAD. Another factor used to determine maximum span is the total expected load -- the longer the shelf, the more books (and weight) it has to hold. A running foot of average sized books weighs about 20 pounds. So a three-foot shelf filled with average sized books would have to support 60 pounds. Records albums (does anyone use these anymore) and encyclopedias would weight more, paperback books less.

SHELF MATERIAL. The third factor used to determine maximum span is the type of material used -- particle board, plywood, solid wood. Each has a different stiffness.

REINFORCEMENT. Finally, if you want to increase spans, you can add reinforcement to reduce the amount of sag.

GUIDELINES. Taking the four factors into consideration, the chart shows some general guidelines for the maximum span for shelves to avoid objectionable sag. Note: The most practical approach is to use 4/4 stock or plywood with reinforcement. This will produce shelves with minimum sag and the best visual appearance.


Shelf Material Maximum Span
3/4" Particle Board 24"
3/4" Plywood 30"
4/4 (13/16") Solid Stock 36"
6/4 (1-5/16") Solid Stock 60"

3/4" Plywood Reinforced with:
1-1/4" wide face strip on edge 36"
1-1/4" wide face strip on side 32"
Aluminum strip underneath 36"
Molding strips underneath 36"

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