Fixed with front and rear support – ends secured with clamps, front edging, rear beam support. Fixed with rear support – ends secured with clamps, rear edge supported by wooden beam underneath. Fixed with front support – ends secured with clamps, 1-1/2″ edging screwed to underside of front edge of shelf. Fixed – ends of shelf clamped to support ledges. #Small shelf for books free
Free floating – ends of shelf not screwed or otherwise secured to support ledges. The following shelf stiffening and support techniques were tested. 16″ deep shelves are less common although they are ideal for oversized books, newspaper storage, and other odd-sized reading materials. A 12″ shelf is perhaps the most common size for bookcases since it can accommodate magazines and the majority of textbooks. An 8″ deep shelf would be appropriate for paperbacks and small hardcover books. Three different shelf depths were used – 8″, 12″, and 16″ – representing narrow, standard, and extra wide bookshelves. If your favorite wood is not represented in these sag tables, you have a decent shot of extrapolating its sag performance if you can locate a species with a comparable stiffness rating. 
As an example, you can see that Sugar Maple is approximately the same stiffness as Northern Red Oak. To get an idea of how these materials stack up to other materials in terms of stiffness (and hence sag), take a look at this table of wood strength properties, in particular the Static Bending, Modulus of Elasticity column. These materials which range in stiffness from low to high, are: Six types of shelving materials were used (all 3/4″ thick) including sheet stock, softwood, and hardwoods. The measurements were collected with a shop-built shelf deflectometer. I was also curious to see how the real-life values would compare to the theoretical values computed by the Sagulator.
The objective was to collect real-life sag values for shelves that are representative of common bookshelf construction in terms of materials, shelf dimensions, and support/stiffening mechanisms. The links above reference tables of shelf sag measurements for shelves that range in width from 30″ to 54″ in 6″ increments.
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