![]() Narrower shelves aren’t as strong, however. Deeper or shorter shelves? Or anticipating less weight? Sag definitely won’t be a problem for those small adjustable shelves. Make it of poplar - the stiffest of those three materials - and you can make a narrower adjustable shelf, down to about 8”, still without any problems. And a typical shelf load to plan for is 20-40# per foot - libraries often plan for 35#.Ī 30” wide adjustable shelf that is 10” deep, made of any of these three (plywood, poplar, pine) materials, will have an acceptable sag for a load of up to 35# per foot, or about 90# for the entire shelf. ![]() What’s “small”? The most frequently used quality materials for custom-built wood shelves are 3/4” thick plywood, poplar, or pine definitely avoid the melamine-laminated composite shelving and MDF. However, for “small” shelves, sag is not really a problem and one can just ignore considerations of sag for typical loads. But with a bit more planning and design consideration (and better materials), sagging can be wholly avoided. This is a consequence of many factors in that $50 hand-me-down bookcase. Unfortunately, however, adjustable shelves are more prone to sagging than fixed shelves, as anyone who has loaded a cheap Ikea bookshelf with all of their college textbooks can attest. Many cabinet designs call for the flexibility of adjustable shelves. So a good target for initial allowable sag is 0.02″ per foot or less. ![]() And over many years, the sag of a shelf tends to increase by as much as 50% as it is stressed repeatedly in the same direction. You will probably notice deflection far before it gets to the sorry state of that coffee table! 1/32″ (0.03″) per running foot, or 3/32″ (0.09″) for a 3′ wide bookshelf or table, is typically visible.
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