Architectural Woodworking Forum -- Span Limits and Materials for Library Shelving
Furniture and cabinet makers discuss the theory and practice of bookshelf sag control. February 16, 2006
Question
I am working on a bid for library casework. The units will be prefinished maple or red oak. There will be 4 units, 36” wide, 24" deep and 96” high. I will build back-to-back units 12" deep and 8’ high. The customer wants them to be nice but affordable. I would like some input for the shelf material to carry the weight of the books. Frameless is an option.
Forum Responses
(Architectural Woodworking Forum)
From contributor A:
By all means consider using solid wood for the shelves. By the time you're done putting a nosing over a piece of plywood, you can have the solid shelves ready to go, and they won't sag under the weight of the books, plus your customer will be pleased.
From contributor B:
How thick would you make the solid wood shelves for Larry's 36" wide shelves? Or, for some 24" wide shelves I am designing?
From contributor C:
Solid wood shelves are out if they really want affordable. Solid wood or plywood will sag with 3 feet of books on them. Are the shelves adjustable? If so, you'll need adjustable holes in the back of the cabinets along with the sides. This will help carry some weight. You can either drill the back if you've got the thickness or use strips. Whatever you end up using, make your shelves so there isn't a top or bottom side - in other words, make them reversible. That way, when they sag they can simply be flipped over. I know some woodworkers will cringe hearing that, but this is a real life solution. In a year, if you get a call saying "my shelves are sagging", it's an easy fix. Or you can explain this to your client as a feature - tell him it's just like dusting, and should be done from time to time.
From contributor D:
I warn them up front that the shelves will deflect and will need to be flipped occasionally. A hardwood edge will help, but not prevent it.
From contributor E:
Remember that plywood doesn't have to stop at 3/4" thickness. I have used up to 1-1/2" for wide spans or where visual considerations demanded it. If you can't get veneered ply that thick from your supplier you can have it made or laminate two thinner pieces but use something besides contact cement.
From contributor F:
Has the customer approved back-to-back units? I interpreted the 24" depth as giving a single, see-through unit for free-standing use. If so, it'll place a fairly high demand on the base and crown to control any wracking load. Will he permit a fixed shelf in the middle? If not, I'd be concerned that a frameless structure with all adjustable shelves would bow.
From contributor G:
A minimum of 1" thickness should be used for the solid shelving material. My personal design preferences based on the scale of the units that were provided would be to use 3/4" plywood material with a fold over edge to give a 1-1/2" shelf apron. I would use 1/2 x 1/2 channel steel behind the front apron and never worry about the shelves again.
From contributor H:
I built two bookcases 42" wide for my home 5 years ago. I used 1" thick oak for the shelves and they are full of books. So far they have not bowed at all.
From contributor I:
I have also found that with a 36" span, a 3/4" solid oak shelf won't sag under the weight of books for a very long time - probably years. If you are gluing up the boards yourself it's not hard to get close to 7/8" if you're starting with pretty straight wood to begin with.
From contributor J:
There is a book at Barnes and Noble on building shelves and I think it says that the maximum span for 3/4" plywood is 3 feet and solid hardwood it is 4 feet. I had a clear engineering chart for this problem
From contributor G:
It's really the depth of the hardwood shelf that mostly effects warping. For example, an 11" deep hardwood shelf 4 feet long will be more stable than the 12" deep equivalent.
