Shelf Design Guidelines
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|
|3/4″ 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)|
|Hard Maple (Sugar)||1.83|
|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
Standard Dimensions of Bookcases and Shelves
|Height:||30″ – 84″|
|Highest shelf:||72″ – 78″|
|Depth:||8″ – 24″|
|Width:||24″ – 48″|
|Minimum shelf spacing:||7″ – 8″|
|Maximum shelf spacing:||13″ – 15″|
|Height of unit:||30″ – 42″|
|Height of base above floor:||36″ – 54″|
|Depth:||6″ – 16″|
|Width:||24″ – 36″|
|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|