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Desk sag prevention

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MattChow

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Chaps, I am in the process of making a desk using a solid oak worktop that is 2100 x 620 x 40mm with 40mm staves.

Ideally I just want legs at each end with no central support.

I am drawn to fitting either some Wickes finest 38 x 63 CLS on edge or some 50 x 30 steel box flat as a rib down the middle underneath using slotted holes to allow for the differential expansion. Is this likely to be overkill? I do like to over engineer.
 

MikeG.

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Avoid the steel if you can. Steel and oak don't go together well, chemically.

How are you holding the legs on? Aprons usually deal with any potential sagging issues.
 

MattChow

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Its just for a laptop, printer and paperwork. No real weight.

Interesting about the steal to oak reaction. I was just going to screw my powder coated hair pin legs to each corner.... Presumably this is a bad plan?

Ergonomically I can't have an apron on the front edge as my knees wont fit if i do.
 

fatbob

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MattChow2":3se4535j said:
I do like to over engineer.
Two lengths of stainless steel angle, say 20mm x 20mm x 2OOOmm, route grooves in the underside of the desk and epoxy and screw (stainless screws) them in place.
You can use stainless screws for the legs.
You could even inset the flat part of the angle to hide it completely.
 

Yojevol

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Peter Sefton":3h3tgk11 said:
Check out the Sagulator below, you should get less than .5mm sag, so no need for any extra construction. It sounds like a solid Oak kitchen worktop? If in doubt put it on two saw horses to test. Personally I would happily stand on it to paint the ceiling :D

https://www.woodbin.com/calcs/sagulator/

Cheers Peter
Who would be in charge of the horses?
 

MattChow

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Awesome. Thanks chaps. Love the sagulator, most helpful!

It is indeed kitchen worktop, I had to have a quick lie down after carrying it upstairs on my own. Its a heavy beast!
 

Fitzroy

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I made a smaller desk with those hairpin legs, top is 110cm x 52cm and 19mm thick.

7327FD3E-242D-47D7-B06E-83F932E1ACC6.jpeg


I found along the length the sag was not an issue but front to back the desk wanted to sag and the legs splay, so I added a couple of reinforcements.
5F5688EF-58D1-44C0-8EFF-E87D1D49DB9B.jpeg


The back to front sag surprised me and if you tested your worktop on a couple of saw horses you’d not see it, if it were a problem for your desk. Unlikely due to your thickness but just my experience.

Fitz.
 

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profchris

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Fitzroy":wfzgcnjx said:
I made a smaller desk with those hairpin legs, top is 110cm x 52cm and 19mm thick.

I found along the length the sag was not an issue but front to back the desk wanted to sag and the legs splay, so I added a couple of reinforcements.

The back to front sag surprised me and if you tested your worktop on a couple of saw horses you’d not see it, if it were a problem for your desk. Unlikely due to your thickness but just my experience.

Fitz.
The stiffness of a board along the grain is several times its stiffness across the grain. You don't notice this in thick boards much, but thinned to musical instrument thickness the difference is quite dramatic - under gentle finger pressure the board will deflect maybe 3x as much cross grain as along the grain.

So I'd expect even quite a thick board, unsupported, to gradually sag under its own weight if the cross-grain span was long enough.
 

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