This article was inspired by a question from Steve. He writes:
“I am building a large bookcase and don’t want to add any more support than necessary. Everything will be made from 3/4″ plywood. My plan is to mortise the shelves into the sides of the cabinet and screw the shelves to the back panel. Will that be enough support for a 76″ long shelf?? Will I have problems with sagging??”
The wider a shelf is, the more likely it is to sag. This can make an incredibly attractive bookcase look absolutely dreadful. Screwing the shelves into the back is a great start and will provide a significant amount of support along the length of the shelf. Another similar option would be to install a thicker back panel and rout a dado for the entire back of the shelf to sit in. If that’s not an option you could simply glue in a ledger strip under each shelf.
Here’s another cool option: attaching strips of wood to the shelf itself. A common implementation of this technique involves trimming out the front of the shelf with a 1 – 1 1/2″ wide piece of solid stock. This trim piece will give the shelf a lot of extra support and also give you enough material to rout a decorative profile, which is a nice bonus.
Whenever I have to decide how much support to give a shelf, I start by checking The Sagulator. This is a great online resource for calculating how much deflection to expect using different materials under different loads. Simply fill out the online form and the program tells you how much sag to expect. You just need to decide how much sag is acceptable. The author of the website gives a very helpful tip: “The eye will notice a deflection of 1/32″ per running foot.” You can even factor in the effect of a solid piece of trim at the front of the shelf.
With a little planning and the help of a handy online calculator, sagging shelves will be a thing of the past!
I’m curious what handy tips you guys have for supporting really wide shelves.
Building Bookshelves That Will Not Sag
Bookshelves sag because they were incorrectly designed, poorly built, improperly installed, or the wrong material was used. Building sturdy bookshelves isn't rocket science; almost anyone can do it. Follow a few recommendations to build bookshelves that will not sag no matter how heavy the reading material.
Best Material for Built-In Shelves
The most common cause for bookshelf sag is the use of wrong materials. The culprit is almost always particleboard, says Popular Woodworking. Even if high-density particleboard is used it can sag. There is one exception to this rule, however; medium-density-laminate – which has a particleboard core – can be used effectively. This type of composite material has a slick, plastic laminate on both sides that gives it sufficient strength. It's affordable and is used for utility-style bookcases and shelves.
The most common building material used for bookcases is hardwood plywood. It can be cut with any table saw and is available in almost any species of hardwood. Other options include hardwood solids. Single pieces of hardwoods that are wide enough for a bookshelf are sometimes available, but this application is not recommended due to the propensity of single pieces to split or crack. Look for laminated panels that have numerous pieces of hardwood glued together to give it more strength. This also makes the shelf less likely to warp or crack.
Thick Wood Bookshelves
Use 3/4-inch thick materials for shelves and bookcase structure. If you're using hardwood solids, it's OK to boost the thickness to 1 1/4 inch for almost unlimited support. This type of shelf looks more like a mantel and is expensive, but adds the opulent look of craftsmanship. You might see this type of bookshelf in mansions or museums. Bookshelf width is typically no more than 12 inches. If the space is limited, the shelf can be reduced to 9 inches.
Single bookcase units with two sides, top and bottom should be no wider than 36 inches without a center support. Single bookcase shelves installed on a wall should have supports no further apart than 32 inches due to 16-inch stud spacing commonly used on homes. Screws for fastening shelves and multiple shelf units to the wall must always screw directly into studs. Bookcase shelves using 3/4-inch plywood or hardwood solids with braces spaced no farther apart than 36 inches will not sag.
Fasteners, Joints and Backs
Bookcase shelves can sag if the fasteners or joints fail. For single, rectangular free-standing bookcases with two sides and a top, it's best to dado the shelves into the vertical jambs on either side. Dadoes are small channels cut on a table saw to receive the ends of the shelves on both sides. It's necessary to cut them into the sides before assembly. It's acceptable to skip the dado process if you fasten the shelves securely with nails or screws. Finish nails work fine, but screws work better.
If you're in doubt about the integrity of the shelves on any bookcase, add 3/4-inch by 3/4-inch screw cleats or blocks to the bottom of the shelves, suggests Better Homes & Gardens. Screw them to the sides of the bookcase under the shelf. Screw the shelf to the cleats from the top. Another way to add strength to bookcases is by adding a back. For economy and efficiency, use either 1/4-inch hardboard or 1/4-inch hardwood plywood for the back. Apply glue around the perimeter of the bookcase unit and the back of the shelves. Place the back on, and staple it to the backs of the shelves and around the perimeter using a staple gun and 1-inch staples.
Face Frames for Strength
Bookcase shelves that include face frames are the strongest. Build any bookcase using 3/4-inch plywood or hardwood solids. Stay with the correct spacing, and the bookcase shelves will not sag, but if you attempt to slide or move the bookcase, it can weaken joints on the frame. Over time, the bookcase may become slanted or shelves pull loose from the side.
This Achilles heel can be remedied by including a hardwood face frame. Glue, clamp and nail 3-inch hardwood pieces to the sides, top and bottom around the perimeter of the bookcase. Flush the edges of the hardwood to the outside of the frame to form an enclosure around the bookcase. You don't need to add hardwood to the shelves, but if you choose to, it's fine. Use 1-1/4-inch hardwood to stiffen the shelf.
Using Corbel Supports
If you've got shelves that already sag or plan on installing single bookshelves on a wall, use corbel supports to prevent sag. Corbel supports are commonly installed on single bookshelves at 32-inch intervals. They consist of single pieces of either solid hardwood or hardwood plywood cut into 10-inch squares for 12-inch shelves or 8-inch squares for 9-inch shelves. Cut the front of the square into a sloping curve diagonally from the top corner in front to the bottom corner in back.
Leave the back of the corbel at 90 degrees. One side fits flat against the wall; one side supports the shelf on top. Screw the corbels to studs in the wall through the bottom of the curve, spaced every 32 inches. Place the shelf on top. Screw the shelf to the top of the corbel. If desired, you can purchase fancy corbels. Some are made using carved hardwood, plastic, vinyl, or even imitation stone to make the shelf resemble something you might see in a Gothic cathedral. Use properly placed corbels, and your bookcase shelving will not sag.
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