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Discussion Starter · #1 ·
I know this is a basic question, but hey, I’m still learning! I’m building a simple desk for our office room. It will be L shaped. I’m planning on using 3/4” plywood for the top. For my husband’s work area, he has a heavy Lift Desk setup that will go on the desk top. I want to make sure I build this in a way that the desk top won’t warp/sway under this over time. I’ve considered using two layers of the plywood and/or adding support pieces under the desk top (something like pictured here, which I did on a bench). What would give the desk top the most strength? I’m also assuming the harder the wood the better. ? But I am working with a budget, so some of the harder wood plywood might be out of my price range.
Thanks for any insight!
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Did you make the bench in the pic? If so, you should be able to gauge how much strength the support piece gives which, I would guess, is a lot.

I would use the 3/4 ply and then support around the perimeter with 1x or 2x. Think how little a 2x4 or even a 1x4 flexes, on edge. Then add supports between them front to back every 12 inches or so.

To cut costs, I would use scraps for the supports as well. Even if I needed to glue scraps together to get a piece the length needed.
 

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What makes a piece of wood strong?
If you have a 1 X 4 that's 4 ft long and place it's ends on two saw horses it will bend in the middle if you press down hard enough.
A 2 X 4 X 4 ft will be twice as strong, because there's a larger area of material and thickness is twice as much.
What if we took the same 1 X 4 and put it on edge and pressed it down? Not much deflection would happen because the resistance is 4 times as much.
So, to resist bending or deflection we just need to add ribs to a flat panel like a desk top surface. You can add them around the outside or both inside and outside.
Most tables have aprons that are just ribs that run around the outside, sometimes flush with the edge, other times about 4" in from the outside.
Finally, to make the strongest panel with the least weight and least material, we make a "sandwich of two panels with ribs in between, called a "torsion box".
The panels or skins can be much thinner, so it's lighter. I made a orsion box for my outfeed and assembly table attached to my table saws:

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Dimensions would help. The skirt shown in the picture would give ample support up to a 5' span. A longer span may need a center leg or a piece of steel angle iron fastened behind the skirt to prevent the top from sagging. It would also make a difference if the desk were fastened to the wall. .You could make a longer span without a center leg if the desk is mounted to the wall as the wall would prevent the top from sagging.
 

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Your instincts are correct plywood sags, and badly. We’ve all seen the bookshelves with long sagging shelves.

Those are called aprons, and they are an age old way to build a table. Will this be mounted to the wall or free standing? Apron to leg attachment is the weakest part of a table.

I would start with a frame with a crossmember ever 16” or so. 2x4’s will work. The apron can be covered with ply to hide them, but if you’re careful you can find them without knots or blemishes. I suggest you double up the thickness just if the overhang by adding a 1” wide strip to the bottom, creating the illusion of a thicker top, and add solid wood edge band 👍

Since it’s L shaped I suggest you to a wall
 

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Before we get to insufficient vs. overkill, what are the actual dimensions of the top going to be?
And as @DrRobert stated or queried, is this going to be against a wall or two walls (corner) or free standing?
Any cabinets for drawers for this desk top to rest on?
Without this info, any info from the forum members will be a guess
 

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Discussion Starter · #8 ·
Did you make the bench in the pic? If so, you should be able to gauge how much strength the support piece gives which, I would guess, is a lot.

I would use the 3/4 ply and then support around the perimeter with 1x or 2x. Think how little a 2x4 or even a 1x4 flexes, on edge. Then add supports between them front to back every 12 inches or so.

To cut costs, I would use scraps for the supports as well. Even if I needed to glue scraps together to get a piece the length needed.
Thanks!
 

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Discussion Starter · #9 ·
What makes a piece of wood strong?
If you have a 1 X 4 that's 4 ft long and place it's ends on two saw horses it will bend in the middle if you press down hard enough.
A 2 X 4 X 4 ft will be twice as strong, because there's a larger area of material and thickness is twice as much.
What if we took the same 1 X 4 and put it on edge and pressed it down? Not much deflection would happen because the resistance is 4 times as much.
So, to resist bending or deflection we just need to add ribs to a flat panel like a desk top surface. You can add them around the outside or both inside and outside.
Most tables have aprons that are just ribs that run around the outside, sometimes flush with the edge, other times about 4" in from the outside.
Finally, to make the strongest panel with the least weight and least material, we make a "sandwich of two panels with ribs in between, called a "torsion box".
The panels or skins can be much thinner, so it's lighter. I made a orsion box for my outfeed and assembly table attached to my table saws:

View attachment 447143
I really appreciate your detailed response. Very helpful! Thank you.
 

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Discussion Starter · #10 ·
Dimensions would help. The skirt shown in the picture would give ample support up to a 5' span. A longer span may need a center leg or a piece of steel angle iron fastened behind the skirt to prevent the top from sagging. It would also make a difference if the desk were fastened to the wall. .You could make a longer span without a center leg if the desk is mounted to the wall as the wall would prevent the top from sagging.
Thanks. There won’t be a span of more than 4’ without a support of some kind.
 

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Discussion Starter · #11 ·
Your instincts are correct plywood sags, and badly. We’ve all seen the bookshelves with long sagging shelves.

Those are called aprons, and they are an age old way to build a table. Will this be mounted to the wall or free standing? Apron to leg attachment is the weakest part of a table.

I would start with a frame with a crossmember ever 16” or so. 2x4’s will work. The apron can be covered with ply to hide them, but if you’re careful you can find them without knots or blemishes. I suggest you double up the thickness just if the overhang by adding a 1” wide strip to the bottom, creating the illusion of a thicker top, and add solid wood edge band 👍

Since it’s L shaped I suggest you to a wall
Thanks!
 
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