Supporting Wide Shelves

Article - September 15, 2010

This article was inspired by a question from Steve. He writes:

“I am building a large bookcase and don’t want to add any more support than necessary. Everything will be made from 3/4″ plywood. My plan is to mortise the shelves into the sides of the cabinet and screw the shelves to the back panel. Will that be enough support for a 76″ long shelf?? Will I have problems with sagging??”

The wider a shelf is, the more likely it is to sag. This can make an incredibly attractive bookcase look absolutely dreadful. Screwing the shelves into the back is a great start and will provide a significant amount of support along the length of the shelf. Another similar option would be to install a thicker back panel and rout a dado for the entire back of the shelf to sit in. If that’s not an option you could simply glue in a ledger strip under each shelf.

Here’s another cool option: attaching strips of wood to the shelf itself. A common implementation of this technique involves trimming out the front of the shelf with a 1 – 1 1/2″ wide piece of solid stock. This trim piece will give the shelf a lot of extra support and also give you enough material to rout a decorative profile, which is a nice bonus.

Whenever I have to decide how much support to give a shelf, I start by checking The Sagulator. This is a great online resource for calculating how much deflection to expect using different materials under different loads. Simply fill out the online form and the program tells you how much sag to expect. You just need to decide how much sag is acceptable. The author of the website gives a very helpful tip: “The eye will notice a deflection of 1/32″ per running foot.” You can even factor in the effect of a solid piece of trim at the front of the shelf.

With a little planning and the help of a handy online calculator, sagging shelves will be a thing of the past!

I’m curious what handy tips you guys have for supporting really wide shelves.

Discussion

Greg November 27, 2006 7:11 am

I would suggest to Steve that he make his shelving unit into three sections if visual appeal is important in his project.
I built an entire hallway of floor to ceiling shelves and having them roughly two feet wide looked great. If he does face frame them then having two vertical stiles for support and dividing the shelves into thirds visually would do the trick for strength and aesthetics.

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Howard Rosenberg November 30, 2006 4:43 pm

I’ve made bookcases in different widths with no supports between the gables.

The widest one I’ve made has 86″ wide shelves and exhibits zero sag.

I make my stuff entirely out of MDF and never use screws or nails.
As you probably know, MDF has no grain and can’t support its own weight.

The trick is to use housed joints – such as dadoes and grooves.
The dadoes hold the board in place and prevent sag.
They don’t need to be all that deep – 1/8″ deep is fine – but with 3/4″ plywood, you can easily go 1/4″ deep.

I route dadoes and grooves into the gables and back for the shelves to fit into.
You’re best making a jig of some sort that will let you route to the exact thickness you’re dealing with – there are many variances in 3/4″ plywood.

Seeing you don’t want to introduce supports between the gables this is the only way to diminish/eliminate sag.

BUT – you’re gonna need another refinement.

The shelves might sag front-to-rear.

You’ll need to introduce a a face-frame on the shelves themselves to maximize rigidity.

This can be a pain using plywood but you can do loooong mitres on an accurately setup TS.
(It’s obviously more forgiving using MDF and painting it)
But just set the blade for an ACCURATE 45deg and just glue the bastards together.

I’ve done this on about 20 different bookcases without nails.
Works every time.

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Chad September 15, 2010 8:16 pm

I’ve used Marc’s technique with good results. A variation of that is to rabbet the front trim piece so the shelf is supported on the bottom edge as well as the front.

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- thewoodwhisperer September 15, 2010 8:20 pm
  
  ooh I like that even better!
  
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  - David Young September 16, 2010 8:48 am
    
    This is the technique I’ve used when building shelves to hold audio equipment, though, I typically use two face laminated pieces of 3/4 stock to create the rabbit. that way, I get good surface area on the underside of the shelf, as well as a good thick face on the front that won’t be too thin, should I decide I want to route or roundover the top / bottom edges.
    
    I’ve had very good luck creating 48″ wide shelves that hold a considerable amount of weight with 0 sag and no support between the gables.
    
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- Glenn Folley September 16, 2010 8:28 am
  
  I have used the same procedure as Chad with great results. It also makes a great finish for the edge of the plywood.
  
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medfloat September 15, 2010 10:23 pm

I haven’t made a long shelf yet but when I do I plan on using a dado under the shelf for mini titanium I-beems. That should do the trick!! Actually I would use the rabbeted trim piece glued to the front edge of the shelf.

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Jonathan September 16, 2010 4:46 am

Shelves on the ground! Shelves on the ground! Looking like a FOOL with your shelves on the ground!

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Buxton September 16, 2010 7:50 am

Thanks for the Sagulator link, that’s gonna be a huge time-saver.

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Heath September 16, 2010 8:03 am

For exceptionally long shelves, what about using a piece of angle iron or aluminum? If polished or painted, it could become part of the design. Or it could be covered with some wood trim.

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Charlie Moore September 16, 2010 9:02 am

Love the sagulator, fun and useful, Thanks

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James September 16, 2010 9:40 am

As I was waiting on the post to load, I was pulling up the sagulator link to share – it’s a fantastic resource that I’ve been referring to for years.

James

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Mike Mader September 16, 2010 9:52 am

Good advice! Plus using the front rail can cover the side profile if you are using plywood for the shelves. It can also allow you to put a decorative face on the shelf using a router as well.

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Steve September 16, 2010 4:21 pm

For extreme rigidity, use steel. A piece of stainless steel angle could be attached to the front of the shelf in place of the hard wood trim strip. It would be an interesting contrast of wood and steel.

If the steel is not to be seen, you could trim out the front of the shelf with a solid wood strip and use a piece of steel. Basically, create a small rabbet on the back side of the trim strip and attach a steel bar with screws every 6 inches.

If your shelf needs to be thin, you could create a series of kerfs and grooves that run the length of the shelf. You would then insert 1/2×1/2×1/8 steel angle iron in the grooves and fasten with screws. A 60″ 1/2×1/2 steel angle is about $4

Just a few thoughts.
Steve

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- John B. October 14, 2013 11:28 am
  
  Steve: I am building shelves and would like to face them with either stainless steel or nickel plated metal to increase rigidity and also as you say provide a nice contrast. Where would be a good source for this material? Any ideas?
  
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- Bob October 13, 2016 11:11 am
  
  Steve, sounds like an interesting idea. I can’t quite visualize the grooves and kerfs arrangement. Would it be possible to elaborate, a picture would be even better. In any case thanks for your original suggestion. Bob
  
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Larry Clinton September 16, 2010 10:37 pm

I like using the wider piece of solid wood on the front to add stiffness to long shelves. I usually use a 1-1/2″ strip on a 3/4″ plywood shelf. You could also do the same on the back if you want, maybe a little narrower than the front one.
A thicker back with a dado would also help a lot.
What about using a partial divider. I’ve used triangular pieces that don’t come all the way to the front of the shelf with good results. For example, on an 11 inch wide shelf bring the angled support out to 6 or 8 inches and down about the same. Screw those in from the back and down through the shelf. You don’t lose much space and they’re barely noticeable. You can carve out a curve in the face instead of a straight triangle if you want.
Just a couple thoughts..
Good luck.

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Bill Neild September 17, 2010 12:18 pm

I built a large open cabinet out of 3/4 ply and put 1 1/2 wide inch pine on the front and back of the shelf. The shelves are solid as a rock. They would be even stronger if I used oak and/or made them more like 2 inches wide. Also, consider running a strip across the center of the shelf parallel to the front and back. The ultimate upgrade – torsion box shelves!

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Chadster September 18, 2010 6:43 pm

I built 46″ long shelves using construction grade 3/4 plywood with 1/4 pine plywood laminated on both sides using titebond. I also used a ledger strip on the fronts but they don’t hang over the bottom. The shelves were only held by the sides since I wanted them to be adjustable. Did this 10 years ago and have had a lot of weight in books on them. Hardly any sag. I honestly didn’t they would hold up that well but my Dad said it would suffice so I went with it. Never question the old man.

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jersey mike September 20, 2010 5:47 am

The first picture makes me think of a rope bridge bookshelf. My sister bought one from an art festival in Manhattan. I think the woman selling it just stole the idea from
http://www.instructables.com/i.....Bookshelf/

One person’s problem is another’s art. (Sometimes)

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mathom7 October 16, 2010 2:19 pm

I’ve actually used the support in the back before. i wanted to maintain the slender look on the front so just used a strip of hardwood the same thickness as the 3/4 ply and supported the back with support biscuited on. It has the same result, but, for some reason looks a little more elegant to my eye.

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robert lee October 29, 2010 6:05 pm

the Sagulator website is a really neat resource, thanks for the tip. As far as shelving goes, I’ve had luck using the strips in back of the cabinet to give more support for longer runs.

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Thom October 7, 2018 9:06 pm

New to woodworking and am building closet furniture (clothing racks) with black walnut. Vertical 87″ with 3 horizontals 6′ long and 14″ deep. Sagulator says I shouldn’t need anti sag strips at all with 10 lb load and while clothing will hang on metal rod attached to vertical I can’t predict if stuff will be placed on horizontals. With that in mind I’d like to employ some anti sag methodology. Would it be as effective to place a strip on the underside in the center of the board as opposed to edges? And since I don’t have any dado blades would 3/8″ dowels supply nearly as strong or stronger joint?

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David Doria September 17, 2019 7:42 pm

I just tried to “un-bow” some 5′ shelves with severe sag (about an inch) by adding edging as described here (I also threw 8 5×30 dominos in each). I did 3 with poplar and 1 with pine (just what I had laying around). 2 of the poplar ones worked like a charm (i.e. when I took them out of clamps the sag was gone(ish), but the third as well as the pine one didn’t seem to work. I wonder if using oak or something more stiff would be a useful thing to do next time?

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Tenderloin January 25, 2020 12:23 pm

If you don’t have the tools and space to make rabbets and dados and dovetails and kerfs and grooves (like me), it seems you could achieve a similar result by attaching a piece of quarter round, door stop, or other trim to the back side of the shelf ledger to support the lower edge of the shelf.

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Supporting Wide Shelves

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