Plywood Shelf Deflection

      Thoughts on plywood shelf thickness, ways of reinforcing shelves, and othe ideas. February 26, 2007

Question
Is 1 inch VC plywood a sufficient thickness for library shelving with the greatest span being 34 in. W X 14 in. D? The customer does not want any battens. I have to re-work a job where the last millworker used 3/4 in. thick throughout and all the ones 30 in. or greater are severely bowed. These are all going to be painted.

Forum Responses
(Cabinetmaking Forum)
From contributor D:
You could use inch and a half plywood and it will still sag over time with the weight of books on it. Take the 3/4 shelves and flip them over - they're pre-tensioned.



From contributor A:
One inch VC plywood may or may not sag over time. Several variables: if it's baltic birch, it probably won't. If it's full of voids or soft cores, it probably will sag (at least a little) if it's really loaded down with large books.

An interesting solution I've seen but not used is to cut one or two 1/8" W x 1/2"(+) grooves along the length, and epoxy or polyurethane 1/8" x 1/2" steel into the grooves. Only the ends (6"-12") need to be glued. I've seen this used in 3/4" ply to span greater than 36" and in 3/4" ply shelves used to carry heavy loads without front and/or rear battens. The downside is you either have these embedded strips visible from below, or you have to inset them and fill and finish over them.



From contributor T:
I've been beat up for this idea before, but it does work. Start with two pieces of 3/4 inch plywood and plane them to 1/2 inch thick. They will immediately take the shape of a banana. The deflection comes from the tension that results from an imbalanced panel.

If you glue these two pieces back together with the tensions opposing each other you will have a one inch shelf you can do chin ups on without deflection. This is viable process in my shop because we use Tersa knives in our planer. There is always a set of beat up blades lying around so it's not so hard on the knives. It is a bit hard on the ears, but it does work.



From Gene Wengert, forum technical advisor:
Are the shelves supported in the rear? Could you put a strip of solid wood on the front edge (and back edge if unsupported)? The amount of deflection controls the design rather than the total strength. Formulas exist for various designs. They were published in Cabinet Maker magazine. I believe that that APA also has them available.



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