1. For ease of shelf access, bookcases are generally no higher than 84 inches. Small bookcases are usually 30 to 42 inches high. The highest shelf in a bookcase should be as high as an average-sized person can comfortably reach - about 72 to 78 inches. If the shelves will be accessed from a sitting position, the highest shelf should be no more than 60 inches above the floor. The lowest shelf should be 3 to 4 inches from the floor level.
2. Shelf width or span is determined both by aesthetics and sag considerations. A fully loaded bookshelf carries a load of 20 to 25 pounds per running foot. The eye will notice a deflection of 1/32 inch per running foot so that a fully loaded (70-80 pounds) bookshelf that is 3 feet wide, should sag no more than 3/32 inches. For most shelves made from 3/4 inch stock, a practical span is 30 to 36 inches. 3. Shelf depth is determined by the size of the items to be stored, generally within the range of 6 to 24 inches. Shelf depth for a general purpose bookshelf is usually 10 to 12 inches. 4. Shelf spacing will generally range from 7 to 15 inches with 8 to 12 inches being common for bookshelves. To prevent the case from looking and being top-heavy, use a smaller shelf spacing near the top and a larger one near the bottom. 5. A well built shelf unit will exhibit minimal side-to-side racking. One of the most effective means of preventing racking is to add a 1/4 inch plywood back that is rabbeted and tacked to the shelves. Other methods include a diagonal back brace, corner blocks, and support cleats on the top and bottom of the unit. 6. In choosing a shelving material, factor in each material's stiffness and susceptibility to creep. Solid wood is stiffer than plywood which is stiffer than particleboard. Particleboard will creep more than other materials so that an unsightly sag could develop if an excessive shelf span is used. Maximum No-Sag Spans for Various Shelf Materials The following table depicts the maximum spans for 10 inch wide shelves supporting 20 pounds per square foot (typical loading for a bookshelf) without noticeable sagging. Shelf Material Maximum No-Sag Span 3/8" Glass 18" 3/4" Particleboard 26" 3/4" Plywood 32" 3/4" Plywood w/ one 1-1/2" wide support cleat 42" 3/4" Plywood w/ two 1-1/2" wide support cleats 48" 3/4" Yellow pine 36" 1" Yellow pine 48" 1-1/2" Yellow pine 64" 3/4" Red oak 44" 1" Red oak 52" 1-1/2" Red oak 78" Stiffness Ratings for Various Wood Species Here are stiffness ratings for several woods commonly used for shelves, ranked in order of decreasing stiffness. Shelf stiffness is measured by the modulus of elasticity (E). The higher the E value, the stiffer the material, and the less it will sag. Wood Species Modulus of Elasticity (E) Yellow Birch 2.01 Douglas Fir 1.95 Hard Maple (Sugar) 1.83 White Oak 1.78 White Ash 1.77 Shortleaf Pine 1.76 Black Walnut 1.68 Yellow Poplar 1.58 Black Cherry 1.49 Red Oak 1.49 White Fir 1.38 Ponderosa Pine 1.26 White Pine 1.24 Western Red Cedar 1.12 Techniques for Increasing Shelf Stiffness 1. Shorten the shelf span or use vertical support pieces. 2. Attach a 1 to 2 inch wide cleat to the front or back of the shelf. 3. Screw case back to back edge of shelves. 4. Use thicker material. 5. Change the shelving material. 6. Employ torsion box construction for the shelves. Shelf Space Requirements for Various Items Shelf Storage Item Shelf Spacing Shelf Depth Compact discs 6" 7" Stereo equipment 7" 18" Children's books 8" 8" Small paperbacks 8" 8" General reading 10" 10" Video cassettes 10" 7" Slide trays 10" 10" Magazines 12" 12" Photo albums 13" 12" Record albums 13" 14" Linens,clothes 14" 18" Large hardcover 15" 12" Standard Dimensions of Bookcases and Shelves Standing Bookcase Height: 30" - 84" Highest shelf: 72" - 78" Depth: 8" - 24" Width: 24" - 48" Minimum shelf spacing: 7" - 8" Maximum shelf spacing: 13" - 15" Hanging shelves Height of unit: 30" - 42" Height of base above floor: 36" - 54" Depth: 6" - 16" Width: 24" - 36" Stepback Cabinet Height of unit: 72" - 84" Height of base cabinet: 30" - 36" Height of shelves above base: 36" - 54" Base cabinet depth: 15" - 25" Shelf depth: 10" - 13" Width: 36" - 48" Methods of Attaching Fixed Shelves Butt joint Simple, minimal racking resistance Cleats, quarter rounds Simple, unattractive, shelf height reduced on ends Dado Fairly strong, relatively simple, must cover exposed ends Stopped dado Fairly strong, clean, not exposed, additional work to notch shelves Sliding dovetail Strongest joint, good racking resistance, close tolerances required Dowels Fairly strong, clean precise alignment, minimal racking resistance Biscuits Fairly strong, simple, clean minimal racking resistance Metal brackets Simple, unattractive, ok for utility shelving
