# Unsupported span for laminated sheets of plywood?



## jaykal001 (Nov 27, 2013)

Hi All,
I'm curious if anyone has any feedback regarding what sort of unsupported span you could get from plywood (I'm ok, if you say to use good cabinet grade 11 or 13 ply), specifically if you laminate 2 or maybe even 3 sheets together. I'm specifically asking for a computer desk that I've been working on a design for. I know there are other options beyond that large span, but I wanted to pursue it, as I like the look of it. Additionally, I'm thinking about doing something funky with the exposed plywood edges, so I'd rather not put a frame around it.

Right now, my to span from left to right is 57.5 ", between the legs. Front is unsupported, but distance from the front to back support is 23.5".
I've attached a few pics to make life easy as well. Drawing assumes 3 sheets of laminated ply, for the top, FYI - 2.25" thick.

Thanks!
Jay


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## woodnthings (Jan 24, 2009)

*What causes bending?*

You could probably just use 2 sheets of 3/4" and it won't bend much in that span. If you were to use 3 sheets it may warp less, but will be heavy. If you think about a hollow core door with 2 thin skins, and a light weight core inside, they are pretty strong for their weight. That is actually a thin "torsion box" method of construction. Instead of 3 sheets of ply, you can use 2 sheets and separate them with a light weight grid.










How does a torsion box work? The forces on the top skin are in compression, the forces on the bottom skin are in tension. By keeping the skins separated, the "whole" is stronger than it's parts.
The more you separate the skins ... to a point, the stronger it will be.

End of physics lesson. :wink2:


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## jaykal001 (Nov 27, 2013)

woodnthings said:


> You could probably just use 2 sheets of 3/4" and it won't bend much in that span. If you were to use 3 sheets it may warp less, but will be heavy. If you think about a hollow core door with 2 thin skins, and a light weight core inside, they are pretty strong for their weight. That is actually a thin "torsion box" method of construction. Instead of 3 sheets of ply, you can use 2 sheets and separate them with a light weight grid.
> 
> 
> 
> ...


Thanks for the feedback, but that would mean then that I can't go with the exposed plywood that I was thinking about as well. 

I've gone back and forth about whether or not I want a frame to support less plywood, a torsion style box, maybe a separate, 1" tub steel support embedded in the top, etc. I've also considered edge banding, but I think the exposed edges have a different modern feel to it, and I don't always love the edge banding look.

Also, I'm not sure the weight of the piece bothers me, it's only going to get moved a few times over my lifetime, and the box and legs will all be separate assemblies. Net result, the top will weigh roughly what 1.5 sheets of 3/4 4 x 8 plywood does. It's heavy, sure, but it's not 300lbs or anything.


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## GeorgeC (Jul 30, 2008)

First you write about "unsupported" and then you show pictures that show it supported. Which is it?

George


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## Steve Neul (Sep 2, 2011)

When you laminate the sheets together be sure to use a resin glue. Wood glue will dry around the edges and stay wet in the middle for a very long time where resin glue will function like epoxy. Once mixed it will harden anywhere. Still the top isn't exempt from sagging. With that span and thickness of the top it will take a long time but will eventually sag. If you could do it, it would be helpful to screw a piece of 1 1/2" angle iron back under the front edge.


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## jaykal001 (Nov 27, 2013)

GeorgeC said:


> First you write about "unsupported" and then you show pictures that show it supported. Which is it?
> 
> George


Thanks for the note on terminology and properly laying out the details. 

The front of the desk is supported at the sides and back, but not the front, as shown. My concern is the potential for sag at the front, unsupported edge, where people would be using the keyboard, writing, etc.


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## jaykal001 (Nov 27, 2013)

Steve Neul said:


> When you laminate the sheets together be sure to use a resin glue. Wood glue will dry around the edges and stay wet in the middle for a very long time where resin glue will function like epoxy. Once mixed it will harden anywhere. Still the top isn't exempt from sagging. With that span and thickness of the top it will take a long time but will eventually sag. If you could do it, it would be helpful to screw a piece of 1 1/2" angle iron back under the front edge.


I had slightly thought about the angle iron, wondering if I could make a cut on the underside of the top about the thickness of the saw keeg and inserting the angle into the top at the one edge. It would let me put the angle iron place, fastened to the bottom, and would prevent visibility of it as well.

Does a contact cement + screws work as a better "glue" option?


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## m.n.j.chell (May 12, 2016)

jaykal001 said:


> ... that would mean then that I can't go with the exposed plywood that I was thinking about as well.


Sure you can. Just cut out unnecessary portions of the center piece.

You could even use 1/2" on the top and bottom, and 3/4" in the middle, to reduce the overall weight a little more.


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## woodnthings (Jan 24, 2009)

*Or you can ....*

Instead of cutting out pieces from a whole sheet, you can rip the pieces out of a sheet in 3" widths and make a grid of rectangles, keeping in mind the grid is there to separate the two other panels, not to add any significant strength. That's the "theory" behind a torsion box.

My "theory" on this question is because the span is roughly 4 ft and the width is roughly 2 ft AND it is supported along the rear by a frame member, a two ply laminate will most likely sag just a tiny bit over time. There will be no constant heavy load on it since it's just a desk, and need only support a computer/laptop/monitor and occasional arm resting on it, not very much weight.

I would be inclined to use the ripped ply approach because it will add another 3/4" thickness for "looks" and definitely add strength.
It will still allow for any edge treatment you have in mind. :smile3:


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## jaykal001 (Nov 27, 2013)

woodnthings said:


> Instead of cutting out pieces from a whole sheet, you can rip the pieces out of a sheet in 3" widths and make a grid of rectangles, keeping in mind the grid is there to separate the two other panels, not to add any significant strength. That's the "theory" behind a torsion box.
> 
> My "theory" on this question is because the span is roughly 4 ft and the width is roughly 2 ft AND it is supported along the rear by a frame member, a two ply laminate will most likely sag just a tiny bit over time. There will be no constant heavy load on it since it's just a desk, and need only support a computer/laptop/monitor and occasional arm resting on it, not very much weight.
> 
> ...


Plus there's the added bonus, that I'd need one less full sheet of ply 
Agreed about the weight/monitors, they'll be at the back support, or potentially even beyond if I use a sort of mounting arm with them.


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## FrankC (Aug 24, 2012)

Your answer may be here:
http://www.woodbin.com/calcs/sagulator/


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## woodnthings (Jan 24, 2009)

*this issue here is ....*

It's not "unsupported" or free span, since the back edge will be supported by the frame in the rear. The "sag" will be non-linear or my guess, elliptical in shape, from zero in the rear to maximum in front center. To me "sag" is personal and one man's sag is another's design feature... or who cares anyway?


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## Steve Neul (Sep 2, 2011)

jaykal001 said:


> I had slightly thought about the angle iron, wondering if I could make a cut on the underside of the top about the thickness of the saw keeg and inserting the angle into the top at the one edge. It would let me put the angle iron place, fastened to the bottom, and would prevent visibility of it as well.
> 
> Does a contact cement + screws work as a better "glue" option?


I don't care much for contact cement. I've used a lot of it making store fixtures but store fixtures are abused so much they are only intended to last a couple years. Something you want to keep I would use most any other adhesive than contact cement. If you do choose to use contact cement use the Weldwood flammable type contact. If you have the means of spraying contact cement works better sprayed. It's more fun too. Visually it looks like you are spraying rubber bands. 

If you are going to use wood glue and are able to put screws in it from the underside that would help. Wood glue once you seal the air off dries very slow but it will dry eventually and the screws will hold the plywood together until that happens.


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## m.n.j.chell (May 12, 2016)

Contact cement has it's place ... applying a top layer of Formica to a plywood desk or table top, for example. 
But, I agree on both of Steve's points above ... 
1) It's not for "construction" purposes.
2) If you DO use contact cement, use the "DAP" brand "weldwood" cement.


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## jaykal001 (Nov 27, 2013)

Steve Neul said:


> When you laminate the sheets together be sure to use a resin glue. Wood glue will dry around the edges and stay wet in the middle for a very long time where resin glue will function like epoxy. Once mixed it will harden anywhere. Still the top isn't exempt from sagging. With that span and thickness of the top it will take a long time but will eventually sag. If you could do it, it would be helpful to screw a piece of 1 1/2" angle iron back under the front edge.


Thanks for the tip!


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## jaykal001 (Nov 27, 2013)

Thanks for the ideas all.
From the sounds of it, based on the advice in the group, it seems like my interpretation of unsupported is is not as much of a problem as if there were no back support on it, and maybe making a mountain out of a mole-hill.

At this point I've been tweaking the design a bit, and I think i'm going to go with a slightly thinner top, and go with 2 laminated sheets of 3/4 birch ply. If I get to the point where sagging becomes and issue, I'll suck it up and get a chunk of angle iron as previously suggested, or a 1.5" steel tube, etc.

Jay


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## Steve Neul (Sep 2, 2011)

jaykal001 said:


> Thanks for the ideas all.
> From the sounds of it, based on the advice in the group, it seems like my interpretation of unsupported is is not as much of a problem as if there were no back support on it, and maybe making a mountain out of a mole-hill.
> 
> At this point I've been tweaking the design a bit, and I think i'm going to go with a slightly thinner top, and go with 2 laminated sheets of 3/4 birch ply. If I get to the point where sagging becomes and issue, I'll suck it up and get a chunk of angle iron as previously suggested, or a 1.5" steel tube, etc.
> ...


Keep a close eye on it. With steel that small it's easier to keep straight than to straighten. Very much of a bow on a top like that would just bend the steel.


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## FrankC (Aug 24, 2012)

jaykal001 said:


> Thanks for the ideas all.
> From the sounds of it, based on the advice in the group, it seems like my interpretation of unsupported is is not as much of a problem as if there were no back support on it, and maybe making a mountain out of a mole-hill.
> 
> At this point I've been tweaking the design a bit, and I think i'm going to go with a slightly thinner top, and go with 2 laminated sheets of 3/4 birch ply. If I get to the point where sagging becomes and issue, I'll suck it up and get a chunk of angle iron as previously suggested, or a 1.5" steel tube, etc.
> ...


According to the Sagukator calculations with 30 lbs sitting in the center of a 60" long shelf made of 1.5" thick ply only supported at the ends you will get .004" of sag per foot, chances are your top will already be curved that much one way or the other so mount it with the crown up and you will be golden.


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## shoot summ (Feb 21, 2014)

Ate lunch at Chipotle today, the store I ate in had table tops made from 2 sheets of 3/4" laminated, with steel on top, very sturdy.

I can't see how 2 sheets, laminated properly, for your application wouldn't be sufficient.


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