Lessons/Guides – How to Build Shelves That Won’t Wilt Under Load

Everyone can use a few more shelves. Indeed, in many homes, an available shelf can be as difficult to find as the TV remote. So if you build a set of bookshelves, they’ll probably be filled as soon as the finish dries.

Bookshelves can work in any room. You can make them free standing or built-in. They can be big or small. And they can take any form, from simple screwed-together and painted plywood for use in a utility room, to sophisticated formal library shelves made from beautiful hardwoods.

Shelves should look good

A successful bookshelf design must achieve a balance between appearance and function. A shelf with the perfect look might not be adequately strong. That often means making changes as you work out the design.

A good approach is to start by writing out a wish list that summarizes your ideal shelf design. The list should include the shelf depth, determined by the width of the books or other cargo going on the shelf. Next, choose a shelf length (book-case width). Then, choose a shelf thickness – 3⁄4-in. stock is readily available, but let your eye make the final determination. After that, decide if you want the shelves to be fixed, adjustable, or some of each. Finally, choose a joint or mounting system that offers the look you want.

The design process is just beginning once you’ve worked out your bookshelf design “brief.” Now you must determine if your initial choices will be strong enough. If not, you’ll have to make some design changes. But before we get to that, it helps to understand how a shelf reacts to load.

Sag is the main enemy

As the load on a shelf increases, the weight eventually reaches a point where the shelf bends, or sags. The same factors that affect appearance also affect shelf sag: the thickness, width, and length of the shelf; the wood species used; and the method used to mount the shelf.

As a general rule, our eyes won’t notice sag if it’s less than 1⁄ 32 in. (0.031 in.) per foot. With time, even if the contents don’t change, a shelf’s initial sag could increase by 50% or more as the wood relaxes. Wood engineers call this “creep.” To be safe, design shelves to limit any initial sag to no more than 0.02 in. per foot under a load of full-size books (see chart,
below right).


In extreme cases (loading a bookcase with your anvil collection, for example), shelves can deflect so much that the wood actually fails. This is not a common worry. More common, especially on long shelves, is that sag causes the effective length of the shelf to become shorter, causing it to slip off the shelf supports. Or, too much weight on a long shelf can cause some adjustable shelf supports to crush the wood fibers in the case sides. As a result, the supports tilt downward.

Fixed vs. adjustable

The method used to mount a shelf affects how much it will bend under a load. All else equal, a fixed shelf will bend less than an adjustable shelf. That’s because on a well-secured fixed shelf, the ends resist both tilting and being pulled inward by the sag (see pp. 26-29 for fixed- and adjustable-shelf options).
Be aware that fixed shelves aren’t immune to failure. With enough weight (per-haps adding your spouse’s anvil collection to your own on the same shelf) and its consequential sag, even fixed shelves can fail at the ends. When that happens, the shelf curves and effectively shortens, the ends pull free, and everything can head south in a hurry.

Design shelves for maximum load

You don’t need to guess at how much a shelf is going to sag. The chart below provides a quick way to determine if a shelf will be sag-free. If the chart doesn’t work for your shelf, you can use the Sagulator, an online program that makes it easy to determine sag. Both the chart and the Sagulator assume unfixed shelf ends. Fixed ends sag less.

The chart is easy to use. It provides the maximum shelf-weight limits (in pounds per foot) and works for most designs. You need to know the thickness of the shelf ( ¾ in. or 1 in.) and its length (24 in., 30 in., 36 in., or 42 in.).

If the expected load exceeds the weight limit shown in the chart, you’ll have to make compromises. To do that, use the Sagulator.

An answer of more than 0.02 in. per foot of shelf means you need to put less load on the shelf; use a stronger wood; make the shelf thicker, wider, or shorter; or add wide edging. With the Sagulator, you can adjust those values and calculate a new sag number.

Material makes a difference

Some shelf materials resist sag better than others. Red oak is one of the better ones, eastern white pine less so. MDF makes a weaker shelf.

Fixed shelves sag less and strengthen the case

Fixed shelves attach to the sides of a case with either wood joinery, hardware, or a combination of the two. Unlike adjustable shelves, fixed shelves help strengthen the entire case. And because they are attached to the case sides, fixed shelves sag less.

– DADO
Strength: Good
Appearance: Good (excellent if using a stopped dado—one that’s not exposed at the front—or if covered by a face frame)
A dado joint houses the ends of the shelf in a long notch, providing some mechanical strength. But a dado joint connects mostly porous end-grain surfaces, and adding glue increases the strength only nominally. The attachment strength of a shelf can be improved by combining Confirmat screws (right) with either a dado joint or a rabbeted dado joint. The screws pull the ends of the shelves into the dado, while the dado shoulder (or horizontal surface) adds strength against shear.

– RABBETED DADO
Strength: Good
Appearance: Good (excellent if using a stopped dado or if covered by a face frame)
A minor variation on the dado joint is to rabbet the ends of the shelf to fit into a narrower dado. The main advantage is the ability to fit the joint more easily, especially if the shelf thicknesses are inconsistent. This joint is useful when working with hardwood plywood, which typically measures less than ¾ in. thick. In this case, a dado cut by making a single pass with a ¾-in.-dia. straight router bit ends up too wide. However, with a rabbeted dado, you cut a narrow dado first, then cut the rabbet for a perfect fit.

SPECIAL SCREWS ADD STRENGTH

I’m not a fan of screwing shelves in place with the typical tapered wood screw. They rarely hold up long-term. That said, I have found a specialized screw that works much better. Called
a Confirmat screw, it has a thick body with sharp, deep threads. It’s mainly used with particleboard, melamine, and MDF, but it also holds well in solid wood. When used in a dado or a rabbeted dado, the joint strength is excellent. Confirmat screws require a pilot hole and a shank hole. A special bit is available that does the drilling in one step.

– BISCUITS
Strength: Fair
Appearance: Excellent
It’s easy to fix a shelf in place using biscuits. And, because the biscuits are hidden when assembled, there is no joinery, support parts, or hardware to distract the eye. Use at least two biscuits on each end of the shelf. Add a third if there’s room. The jig shown at right is a good one to use here because it helps to align the biscuits horizontally from one shelf side to the other.

– SLIDING DOVETAIL
Strength: Excellent
Appearance: Very good (excellent if stopped or covered by a face frame)
A sliding dovetail adds considerable mechanical strength, but sliding a 10-in.-long dovetail into a tight-fitting slot before the glue sets up is a challenge. Using a fairly slow-setting epoxy glue will help considerably. Epoxy is a slippery glue that helps get this type of joint together without excessive expansion and stress.

– SCREWED CLEATS
Strength: Very good
Appearance: Fair
Screwed cleats let you add shelves without too much fuss, but they come up a little short in the appearance department. With the exception of the hole closest to the front, all of the holes in the shelf should be slotted to accommodate wood expansion. For the same reason, if you wish to glue this joint, bear in mind that you should glue only the front inch or so.

JIG ALIGNS BISCUITS ACROSS SHELF

This plywood jig helps to align biscuits accurately across the bookshelves. The plywood cleat keeps the end of the fence square to the side. Mark centerlines for the slots on the end of the fence. After cutting each set of shelf slots, cut the fence to a shorter length. Toss the jig when done.

Adjustable shelves add versatility

Adjustable shelves make it easy to change the spacing as needs change. But there is a structural cost: These shelves do nothing to hold the cabinet sides together. So, on taller bookcases, it’s a good idea to have one fixed shelf to help anchor the case sides.

– SHELF PINS
Strength: Good (very good with sleeves)
Appearance: Good
Shelf pins come in a wide variety of shapes, sizes, materials, and finishes. My favorites are the machined solid brass ones from Lee Valley. I also like the very small round pins by Häfele for smaller cases. Shelf pins also come with special clips for securing the shelves or for holding glass shelves. Sleeves are a great way to recover from poorly drilled holes. Stamped sleeves (short tubes with a flared and rounded-over end) tend to look like shoelace eyelets when installed in a cabinet. Solid brass machined sleeves look better, even though they accentuate the row of holes in the case sides somewhat. Some sleeves are threaded for specially threaded shelf pins.

– HIDDEN WIRES
Strength: Good
Appearance: Very good
These bent-wire supports fit into holes drilled in the case sides. A stopped kerf cut in the ends of the shelf slips over the support, hiding the hardware. Structurally, this means the end of the shelf is thinner. This affects the shelf’s shear strength, but will have little effect on preventing sag.

– WOODEN STANDARDS
Strength: Very good
Appearance: Very good
Wooden shelf standards have been around in various styles for generations. They are easy to make and add an interesting look to almost any bookcase. The style shown in the top-left photo (I call it zig-zag) is one of the more common forms. Another style (I call it half-moon) is shown in the lower left photo. To make a pair, you’ll need a piece of stock that’s at least double the width of each standard. Scribe a lengthwise centerline along the stock, then lay out the shelf spacing by making evenly spaced marks along the centerline. Use a spade bit or a Forstner bit to drill a through-hole at each marked centerpoint. Finally, using a tablesaw, rip the stock down the middle. The net result is a pair of standards, each with a series of half-moon shapes. Make the cleats just loose enough to slip in and out with ease.

– METAL STANDARDS
Strength: Very good
Appearance: Fair
It’s hard to beat metal shelf standards for ease of installation. Just run a pair of grooves down each side of the case, and nail, staple, or screw the shelf standards into place. Shelf supports usually just hook into place, although one new version has brass support pins that
screw into threaded holes in the brass standards. In general, shelf standards seem out of place on finer furniture. But they are great for utilitarian pieces, and even in larger bookcases, where any support system will be pretty much invisible once the shelf is full of books.

ZIG-ZAG SUPPORT IN THREE STEPS

Start with stock wide enough to make four standards. Using the tablesaw, make a vertical cut at each shelf location (1). An auxiliary miter-gauge fence with a location pin in front (much like a finger-joint jig) makes it easy to position the stock for subsequent cuts. Follow with 45º cuts (2) after relocating the location pin. Remove the triangular waste piece, then clean the resulting flat with a chisel. Rip the stock to create four standards (3).