How long can a shelf be before it sags?

1/32″ (0.03″) per running foot, or 3/32″ (0.09″) for a 3′ wide bookshelf or table, is typically visible. 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. So a good target for initial allowable sag is 0.02″ per foot or less.

branchingoutwood.com - How to Avoid Sagging Shelving - Branching Out Wood

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How Much Sag is Too Much

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. 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. So a good target for initial allowable sag is 0.02″ per foot or less.

Don’t Worry About Small Shelves

Many cabinet designs call for the flexibility of adjustable shelves. 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. This is a consequence of many factors in that $50 hand-me-down bookcase. But with a bit more planning and design consideration (and better materials), sagging can be wholly avoided. However, for “small” shelves, sag is not really a problem and one can just ignore considerations of sag for typical loads. 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. And a typical shelf load to plan for is 20-40# per foot - libraries often plan for 35#. A 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. 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. Deeper or shorter shelves? Or anticipating less weight? Sag definitely won’t be a problem for those small adjustable shelves. Narrower shelves aren’t as strong, however. But fortunately, as you get narrower, it’s less likely that you’ll get to 35# per foot. A 6” deep adjustable shelf will probably not get loaded more than about 20# per foot - made of any of those materials, a 30” span would still be fine. Of course, if you’re loading your shelves like that coffee table above, all bets are off!

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