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.