Frameless Shelf Spans
Cabinetmakers discuss the span limits of 3/4-inch shelf material, and share tips on mid-span supports and a stiffening valance. December 28, 2005
Question
I'm replacing a 42" wide upper framed cabinet with a new frameless cabinet design. It is 12" deep. It has two doors that will be full overlay. Can I really span 42" with 3/4" melamine and not have the base shelf of other shelves sag? I could use a 3/4" back to provide more support, but is that enough?
Forum Responses
(Cabinetmaking Forum)
From contributor A:
Use two 21", and you will have no call back for saggy shelves.
From contributor B:
42" is too long a span for ordinary shelves. Either put a center panel in it, or use two 21" cabinets. No matter what you use, the lady of the house will put a huge pile of heavy plates right in the center, so you better be prepared for it.
From the original questioner:
Is 36" too long? What about is the max span? I don't want a callback for saggy shelves or saggy bottom.
From contributor C:
For 3/4" thick man made materials I wouldn't span much more than 32", for natural 3/4" wood you can get away with about 42 inches. It also depends on the depth of the cabinet. The deeper the cabinet the more length you can have, to a certain point of course. I would stick with the 32/42 rule.
From contributor D:
I agree with all answers so far. The only thing I would do is follow your suggestion to use minimum of 1/2" or 3/4" back and drill it to match and use shelf supports on the back also. In fact, I would use two rows on the back so the real span would be divided by 3 or 4, i.e. 42" is reduced to 13" maximum.
From the original questioner:
I would drill the 3/4" back for the movable shelves, but what about the bottom shelf - it is just attached to sides and back. Is that okay to span 42”?
From contributor D:
That bottom is going nowhere. It is fixed on all sides. Also, you haven't mentioned a center stile, but if you have one then it is really only 21" because the center stile supports it even more. That said, if you could get the customer to approve a center partition, which you hold back so doors close over it, that would solve all your issues.
From the original questioner:
Customer doesn't want center stile. So the bottom of the upper cabinet (which is the bottom shelf) is 42 x 12 connected on 3 sides (actually 2 sides and a back). Will that hold a stack of plates in the middle without sagging? If so, then I can run 3 columns of shelf pins for the moveable ones. If 42" is too wide for the bottom and I put a stile in, would just pocket screws to top and bottom be the right way?
From contributor B:
I should have mentioned before that you should have a valance glued to the front edge of the bottom panel to provide the strength you need, and provide a space under the cabinet for task lighting. I usually use about 2-1/4" x 1" thick stock for this valance. This will easily hold that dreaded stack of plates. You could also use a smaller valance on the shelves, which in combination with the aforementioned pins set in the back panel, will handle any reasonable load she puts on them. I have done this many times in this situation, and have never had a problem. Any shelf larger than 42", in my opinion, needs a torsion box construction with slotted standards.
From contributor D:
I like contributor B’s valance. Raise the floor the height of that valance 1-1/4" so you have a recess for lighting, and use biscuits and Roo Glue for joints into sides, floor etc. Do the same for shelves on front edge. 1-1/4” will give you a 1/2" lip on the bottom and hide shelf pins, plus it looks good. Run edge full and shelf into it. Use L-shaped biscuits and Roo, and don’t forget to edgeband the edges of that 1-1/4” piece.
