Reinforcing plywood used as desk top? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 13 Old 10-20-2016, 10:55 PM Thread Starter
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Reinforcing plywood used as desk top?

Building a desk, and as much as I would love to use a 2.5" slab of solid fir, budget is dictating fir plywood with a nice cap to give the illusion of thickness.

Desk is 1500mm wide and 760mm deep. I thought the 2.5" molding on the front and then 300mm plywood back would make the 3/4" top rigid. But it has significant flex, well hardly any, but enough that I can't fill the seam because it flexes a bit with the normal stress I expect it to see with leaning on it to look closer and assistance getting up. Everything is glued.

I am replacing a cheap ikea desk that didn't flex at all even with me standing on it. So that's a little frustrating.

I was thinking a 1x2 on edge ripped down so it's not lower than the front molding, but that only leave 1.75" and it's not very stiff even on edge.

Ideas?
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post #2 of 13 Old 10-21-2016, 07:52 AM
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Basically, 5 foot by 3 foot ... sorry, I needed to convert to standard.
That's a pretty good stretch. Does this table just have a leg at each corner?
If you can get to the under side of the top, I'd recommend a grid of, at least, 2"x2" (50mmX50mm) boards. If you're planning on standing on it often, I'd recommend 2x3s or even 2x4s.
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post #3 of 13 Old 10-21-2016, 08:51 AM
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Post a sketch/picture/plan of how your desk is/is to be constructed and then somebody can give you a good critique.

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post #4 of 13 Old 10-21-2016, 04:48 PM Thread Starter
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Quote:
Originally Posted by mikechell View Post
Basically, 5 foot by 3 foot ... sorry, I needed to convert to standard.
That's a pretty good stretch. Does this table just have a leg at each corner?
If you can get to the under side of the top, I'd recommend a grid of, at least, 2"x2" (50mmX50mm) boards. If you're planning on standing on it often, I'd recommend 2x3s or even 2x4s.
Biggest issue is I need to keep the overall thickness less than 2.5"
Realistically it'll never really be stood on. Just an observation that the cheap super lite ikea desk is significantly more rigid while weighing 1/3 the weight.

What if I used 1/2" plywood and put some 1.5" strips of 3/4" plywood on edge between the sheets?

Just feels cheap to me when a desk isn't rigid.


Sorry about the metric. I use both a lot. Mostly because well lumber and everything is imperial in Canada. But I like metric more.
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post #5 of 13 Old 10-21-2016, 05:19 PM
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1.5" strips of ply won't add much rigidness. It will still flex. More like a 2.5 to 3" piece.
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post #6 of 13 Old 10-21-2016, 06:07 PM
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I like metric, too. I use it on all the boats and motorcycles I work on ... even the American ones.

But when it gets into the larger numbers, I have a hard time "computing" metric.
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post #7 of 13 Old 10-21-2016, 06:48 PM
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You Canucks and your blasted commie commie metric system...


Now, I have to point it out now, but that was a joke. Anyway, on the table issue, you could get some added stiffness with strips of solid wood glued on the bottom, ive done it more than once. I wouldnt use plywood strips through, plywood flexes a little more than solid. It's pretty handy to remember that stiffness goes up exponentially with thickness, so even a little added thickness can result in a much more rigid piece

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post #8 of 13 Old 10-21-2016, 08:39 PM Thread Starter
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Quote:
Originally Posted by was2ndlast View Post
1.5" strips of ply won't add much rigidness. It will still flex. More like a 2.5 to 3" piece.
I was thinking by laminating the 'ribs' between two sheets of 1/2" it would help make it much more rigid.


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Originally Posted by mikechell View Post
I like metric, too. I use it on all the boats and motorcycles I work on ... even the American ones.

But when it gets into the larger numbers, I have a hard time "computing" metric.
1000mm in a meter. 1000m in 1km.

Im an electrician by trade, but like working with metal and wood as hobby, and makes a few bucks too.
But I almost exclusively work with metric prints, but when I do get imperial I wonder how things get built with any accuracy. I can hardly get guys that can add whole numbers correctly, I'd hate to see them with fractions. Lol.

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You Canucks and your blasted commie commie metric system...


Now, I have to point it out now, but that was a joke. Anyway, on the table issue, you could get some added stiffness with strips of solid wood glued on the bottom, ive done it more than once. I wouldnt use plywood strips through, plywood flexes a little more than solid. It's pretty handy to remember that stiffness goes up exponentially with thickness, so even a little added thickness can result in a much more rigid piece
Trudeau is a commie, lol.

Would even a softwood as the ribs help then?
My thinking was the plywood on the bottom too would make it like an I beam.
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post #9 of 13 Old 10-21-2016, 11:23 PM
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1000mm in a meter. 1000m in 1km.
I don't have a problem with the "math" of metric. I have a problem envisioning it. I've gotten used to working with millimeters, and can immediately "see" distances like "25mm" ... up to about "200mm". Once we start getting into larger numbers ... I start getting a little fuzzy.
I can immediately see 1000 yards (my maximum target shot distance) and a mile (driving down a straight stretch of highway, I can usually pick out a sign when it's a mile away).

But ... sighting in a kilometer ???
Envisioning 637mm ???

These I am unfamiliar with.
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post #10 of 13 Old 10-21-2016, 11:33 PM Thread Starter
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Quote:
Originally Posted by mikechell View Post
I don't have a problem with the "math" of metric. I have a problem envisioning it. I've gotten used to working with millimeters, and can immediately "see" distances like "25mm" ... up to about "200mm". Once we start getting into larger numbers ... I start getting a little fuzzy.
I can immediately see 1000 yards (my maximum target shot distance) and a mile (driving down a straight stretch of highway, I can usually pick out a sign when it's a mile away).

But ... sighting in a kilometer ???
Envisioning 637mm ???

These I am unfamiliar with.

Ah yes, that makes sense. I've used both for so long that I am pretty good with both, except miles.
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post #11 of 13 Old 10-22-2016, 01:31 AM
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Quote:
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Would even a softwood as the ribs help then?
My thinking was the plywood on the bottom too would make it like an I beam.
Better than plywood, less so than hardwood. Ibeam style construction would help immensely, though all you really need to do is increase the thickness in areas most likely to flex. How you do that doesnt matter, it just changes how much the thickness needs to increase. Plywood would work, solid would just work better. Heck, about the easiest way to stiffen everthing up would be a 1/4 steel bar screwed to the underside

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post #12 of 13 Old 10-22-2016, 08:36 AM
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look at a hollow core door ...

A hollow core door consists of 2 skins separated by a fiber/paper core. Why is it so stiff? It's about the physics.... again. Two things come into play, the depth of the core and the strength of the skins in tension.

When you put a distributed load on the door, which is supported at each end, the bottom skin is in tension. When the tensile limit of that material is reached, it will break and fail. So, to reinforce a 3/4" plywood desk top, just place a grid of narrow strips on the bottom surface, glue them down and add a layer of 1/4" plywood also glued to the bottom of the strips... you will end up with a "torsion box".
http://www.bayareawoodworkers.org/jun02/torsionbox1.pdf

If you just add strips to the bottom and don't put a skin on them it will not be as strong/stiff, meaning resistant to deflection. You can keep the overall thickness within your dimension by adding the thickness of the skins leaving the difference for the strips. In this case a 1/4" plywood skin on the bottom will work just fine, leaving 1 1/2" for the strips.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 10-22-2016 at 08:43 AM.
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post #13 of 13 Old 10-22-2016, 09:51 AM
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You could just buy a hollow core door, cut it down to size and attach it to your table top.
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