Over three feet, and you're looking at potential sagging issues. Pros discuss span limits and shelf stiffening options. November 11, 2005
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.
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).
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.
