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.

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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

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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

Span Limits for Plywood Shelving

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Over three feet, and you're looking at potential sagging issues. Pros discuss span limits and shelf stiffening options. November 11,

Question
I have a customer that would like a large built-in entertainment center/bookcase. The two end bookcases will be approximately 45 inches wide by 14 inches deep. Am I going to run into problems with the shelves bowing in the future? I will be using 3/4 birch plywood and either hemlock or maple face frame. What would be the best solution - to divide the shelf in half and put a dadoed vertical divider between each shelf to give the middle some support? What is the cutoff for width of a shelf before it sags in the middle?

Forum Responses
(Cabinetmaking Forum)
No question the shelves will bow at that span. I wouldn't hesitate to put in a middle partition and cut the shelves in half. If the customer wants them that long, then you need to build the shelves thicker by laminating two layers. I don't normally build shelves longer than 36" and at that, I build them thicker to carry heavier loads.

My limit for 3/4" plywood shelves is 32", and I think that is pushing it for heavier objects. The other option is to have a stile in the middle of the span and put shelf supports on the stile and the backs of the cabinet, effectively cutting the shelf in two. With the center support, it will be like having shorter shelves without the center plywood divider.

Any shelf over 34", we laminate 2 layers of 3/4 ply and use 1 1/2" shelf nosing. We do it quite often.

I would use solid hard maple for the shelves, and put a 1/4 plywood back on, gluing and stapling through the back. Bow them up slightly before fastening, and they will be fine.

Aw, gwan! These shelves are adjustable, 48" wide and 12" deep. They're 3/4" ply with 3/4"x1 3/4" oak stiffeners (or whatever they're called) front and back. They have a heavy load on them, have been loaded for over 20 years and don't sag.

My limit for 3/4" plywood shelves is 32", and I think that is pushing it for heavier objects. The other option is to have a stile in the middle of the span and put shelf supports on the stile and the backs of the cabinet, effectively cutting the shelf in two. With the center support, it will be like having shorter shelves without the center plywood divider.Any shelf over 34", we laminate 2 layers of 3/4 ply and use 1 1/2" shelf nosing. We do it quite often.I would use solid hard maple for the shelves, and put a 1/4 plywood back on, gluing and stapling through the back. Bow them up slightly before fastening, and they will be fine.Aw, gwan! These shelves are adjustable, 48" wide and 12" deep. They're 3/4" ply with 3/4"x1 3/4" oak stiffeners (or whatever they're called) front and back. They have a heavy load on them, have been loaded for over 20 years and don't sag.

Talk with the clients and see what they have in mind. Usually, in the context of an entertainment center, the bookshelves don't have anything but pictures and small keepsakes on them. If that's the case, don't worry about them sagging. Shelves that big in a library are trouble. Shelves that big in a living room built-in probably aren't going to be really overloaded.

Even 32" shelves can sag without stiffeners. I'd go to 24" or double them up or use stiffeners.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Talk with the clients and see what they have in mind. Usually, in the context of an entertainment center, the bookshelves don't have anything but pictures and small keepsakes on them. If that's the case, don't worry about them sagging. Shelves that big in a library are trouble. Shelves that big in a living room built-in probably aren't going to be really overloaded.Even 32" shelves can sag without stiffeners. I'd go to 24" or double them up or use stiffeners.

Comment from contributor T:
I've also used the front and back solid edge (7/8" x 1-1/2") on adjustable shelves around 12 x 48 with no sag under a load of books. Another little thing that may help is to look for any bow in the shelf when adding the edge (pretty typical in veneer-core ply), and position the convex side up.

Comment from contributor L:
One aspect of reducing shelf deflection that hasn't been discussed yet is end fixation. By routing a 3/8" deep slot in the side panel, deflection is reduced to 1/3 of the deflection when the shelf ends are free to rotate (pinned shelves). You'll want to use a plywood bit so that you get a really snug fit. The sloppy fit obtained from a standard 3/4" bit won't fixate the ends. The depth needs to be at least 50% of the shelf thickness in order to prevent end rotation.

Comment from contributor J:
I've built utility shelves with spans up to 6 feet by, in effect flitching them. I screw a steel angle iron along the front edge of the shelf and get little or no deflection. It doesn't look like much, but it works well. I've never tried it with finish work. Still, judicious use of trim might well make a handsome job.

One aspect of reducing shelf deflection that hasn't been discussed yet is end fixation. By routing a 3/8" deep slot in the side panel, deflection is reduced to 1/3 of the deflection when the shelf ends are free to rotate (pinned shelves). You'll want to use a plywood bit so that you get a really snug fit. The sloppy fit obtained from a standard 3/4" bit won't fixate the ends. The depth needs to be at least 50% of the shelf thickness in order to prevent end rotation.I've built utility shelves with spans up to 6 feet by, in effect flitching them. I screw a steel angle iron along the front edge of the shelf and get little or no deflection. It doesn't look like much, but it works well. I've never tried it with finish work. Still, judicious use of trim might well make a handsome job.

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