Long-Spanning Shelves
A deep hardwood edge on plywood shelves creates a stiffer beam effect. August 14, 2007
Question
I need to make a bookcase unit 12" deep x 48" wide. I know the shelves should be thicker to not bow. I plan on doubling up the 3/4 vc plywood to give me 1 1/2" thick. I also will apply a cherry 1x2 glued to the edge. Will this be enough? I think it will be, but better to ask!
Forum Responses
(Cabinetmaking Forum)
From contributor M:
That should work fine (depending on the weight of what's going on the shelves). I have done about that size before, and I use a shelf pin on the cabinet back at the center of the shelf to help a little further.
Glue the two plies together, and glue the solid wood strip to the front to strengthen up the shelves further. The wood front will add tremendous strength to the shelving. Do you know what's going on the shelves? Knowing the weight will also help give a feel of whether the shelves will sag long term.
From contributor S:
That's a hell of a strong shelf, but 48" is a hell of a long span. If it were to be filled with solid books like in a library, for instance, it would still bow. I suggest you find out for sure what the customer plans to put on it. You might even be overbuilding it.
By the way, the strongest shelf I ever built was made of softwood 2x4s edge glued together and then veneered over. In a span of 4', you could literally walk on it and it wouldn't bow.
From contributor J:
Doubling the thickness of the shelf is overkill. What you need to keep the shelf from bowing is some good, old fashioned joinery. Put a 3/8" rabbet on the edge of the plywood and a 3/8" dado in that 1x2. When connected (dado/rabbet joint), it forms a right angle, keeping the bow from happening. That's why, in steel building construction, they use angle iron and not flat steel. If you stand on the flat end of a 2x4, over a distance, it is going to bow. If you stand on it on edge, it won't bow. The joinery will strengthen both edges.
From contributor O:
All you need is a 1 1/2" x 3/4" hardwood edge on your shelf. Pre-crown the material or select for a slight crown. We used to make a lot of computer furniture where the shelves are 44" wide and 16" deep. My computer cabinet has one right here that only has a slight sag. It has a monitor and cpu and a large printer on it. The quality of the plywood has a lot to do with defection. States makes a fir core 9 ply hardwood ply that would easily do the job. Then again, some Chinese plywood with plenty of poplar ply cores may or may not work, depends which way the plywood is curling up. One local brand uses a white fir 7 ply core. That would sag for sure, and it's not cheap.
From contributor F:
I think that although a dado will add actual glue surface, it won't be any stiffer than if the edging was just glued and clamped to the front of the shelf. Either way you are building an "L" which is stiffer than either member alone.
From contributor B:
I don't think you need two pieces of plywood, just a stiffer front and back on the shelves. Worked well for me.
From the original questioner:
Does everyone think that just adding 1x2 cherry to the front and back will be strong enough? It will save time and material just using 3/4. I just don't want an issue down the road. I'd rather spend more now to make sure. It is 12"x48".
From contributor F:
It depends on what gets put on the shelves. Try gluing a 1x2 to one edge of one shelf blank and test it for stiffness. If you think it's too limber, rip 3/4" off the back edge and glue on a second 1x2. Test again. Still too limber? Fit a second piece of plywood on the bottom between the two edgings and laminate.
From contributor B:
Yes. However, if you want to make the edge 3/4 x 1 3/4, it is the depth of the chord that gives its strength. Apply the edge to the front and back of the shelf. Try it!
From contributor O:
Listen to contributor B. I've been there and done that time and time again myself. Just make one first - you'll like it. Now, you could make a torsion box construction. Get out your old Fine Woodworking mags, go way back and there's an article in one of them showing you how to make one. That will work for sure, but maybe not any better than what has been suggested.
