Desk build - need support to prevent sag?

[Mike Caputo]

472595
Hello. Would love your advice. I'm building this desk. The top is a maple slab 1 9/16" thick. The span between the legs is 58". Underneath I plan to add two shallow (5") drawers on the sides and a keyboard tray in the middle. I have 3 C channels in place across the grain - 2 outside the legs and one in the center - to control any twisting. My question is whether i need a lengthwise support to prevent sag over the next decades? the shelf unit is dominoed in with those 4 verticals. Might actually be adding a bit of stiffness to the slab.

Options: I can add a 3 or 4" board (vertically) below running the length but I would prefer not to as that will limit the depth of the drawers. Would prefer to be able to see the dovetailed drawers from the back of the desk.
I can remove the center cross grain C channel and install a long C channel running with the grain from leg set to leg set.
Or I can go with what i have here.

Thoughts? Thanks!

[Bob Riefer]

I'll let the more-experienced guys weigh in on the sag (although I don't think I would worry about it in this case personally), but wanted to jump in to say that I really like the design you have there, and would love to see additional pictures.

[Charles Coolidge]

Trying to restrict solid wood from moving and doing what it wants to do usually leads to the piece breaking apart or splitting. It's going to shrink and swell with the seasons/environmental conditions. Most solid wood furniture construction techniques I have seen allow for movement.

[Zachary Hoyt]

I think the C channels across the grain will work if the holes where the channels are screwed to the wood are elongated to 1/2" or so to allow for wood movement with humidity changes. Otherwise the top will split, in a year or two at the longest. I think the lengthwise strength should be fine. That's quite a bit of wood, and a desk would not have more than 100 or 200 pounds of stuff on it, I imagine.

[Paul F Franklin]

Seek out the sagulator over at woodbin, enter in your numbers and guess at the load you might put on it, and it will tell you about how much it will sag.

[Mike Caputo]

I'll let the more-experienced guys weigh in on the sag (although I don't think I would worry about it in this case personally), but wanted to jump in to say that I really like the design you have there, and would love to see additional pictures.

Thank you, Bob. Here are a few more pics of this work-in-progress.

[Mike Caputo]

I think the C channels across the grain will work if the holes where the channels are screwed to the wood are elongated to 1/2" or so to allow for wood movement with humidity changes. Otherwise the top will split, in a year or two at the longest. I think the lengthwise strength should be fine. That's quite a bit of wood, and a desk would not have more than 100 or 200 pounds of stuff on it, I imagine.

Yes, the C channel is slotted for that reason. The legs are attached with bolts into threaded inserts in the top. I oversized the holes in the legs by 1/16". They are only a foot apart. Might go back and add another 1/16" just to be sure. 3/8"bolts through 1/2" holes.

Thanks for your thoughts on the lengthwise strength as well.

[Mike Caputo]

The sagulator says 0.003" for 150 pounds of load and my dimensions. I arrived to the 150 based on the weight of the wood plus a little. It's a desk so it's just going to have computer stuff and paperwork. This sagulator seems meant for now not for 30 years from now though. I want this to be a multi-generation piece. Am i wrong on that? Is it a projection for how a shelf will sag over a long time? Thanks!

[Bryan Lisowski]

Since this is a desk and the weight will be minimal I wouldn’t worry about it sagging. I can’t view your design, but as long as the legs are slightly within the length of the top that should provide enough support. So you say the top is 58” long, so you should want 2-3” overhang from outside edge of top to the base.

[Mike Caputo]

Thanks Bryan. The top is actually 78" long. The legs are shaped like an Arkansas flat top letter A. They are 5" wide. So of that 78" it's 5" overhang, 5" leg width, 58" free span, 5" leg width, 5" overhang. The legs are bolted (4 bolts in a rectangle pattern) to the top which has threaded inserts in it.

[Malcolm McLeod]

Your upper 'shelf' - depending on how it is attached - combined with the top, forms a I-beam. This means its beam strength is probably 3-4 times what the sagulator assumes.

Relax. Enjoy. Nice work!

[Mike Caputo]

Your upper 'shelf' - depending on how it is attached - combined with the top, forms a I-beam. This means its beam strength is probably 3-4 times what the sagulator assumes.

Relax. Enjoy. Nice work!

Thanks Malcolm! It’s attached with dominos. 2 of the 10x50 per joint.

[Brian Tymchak]

The sagulator says 0.003" for 150 pounds of load and my dimensions. I arrived to the 150 based on the weight of the wood plus a little. It's a desk so it's just going to have computer stuff and paperwork. This sagulator seems meant for now not for 30 years from now though. I want this to be a multi-generation piece. Am i wrong on that? Is it a projection for how a shelf will sag over a long time? Thanks!

I vaguely recall reading a note several years ago about the assumption sagulator makes regarding time under load. But I don't recall the specifics other than I ignored it for the workbench top I was building, and I didn't find the note when I looked just now. Anyone else recall seeing that?

[Zachary Hoyt]

About the elongated holes, I'm glad you already did that. If I was making it I think I would want the holes maybe 1/4" longer, or I would think you could keep an eye on it and see if the screws get pushed up against one end or the other. If they do it may be a lot harder to unscrew them at that point, though.

[Lee Schierer]

This desk has an unsupported span of 53 inches and the top is only 3/4" thick.
472657

The wood is Hickory. The weight of the PC is supported by the top and the legs on the end by the wall. The desk top is still perfectly flat. I wouldn't be concerned about your 1-9/16' thick top sagging. Just make sure your C-channels allow the wood to move seasonally across the width with slotted holes in the c-channels. If the wood can't move your top could split.

[Mike Caputo]

This desk has an unsupported span of 53 inches and the top is only 3/4" thick.
472657

The wood is Hickory. The weight of the PC is supported by the top and the legs on the end by the wall. The desk top is still perfectly flat. I wouldn't be concerned about your 1-9/16' thick top sagging. Just make sure your C-channels allow the wood to move seasonally across the width with slotted holes in the c-channels. If the wood can't move your top could split.

Thank you Lee. That’s reassuring. Regarding the C channel, yes, it’s slotted. I’m going to check that i didn’t overtighten the bolts however to be sure it can slide in the slots. Thanks!

[Alan Rutherford]

The sagulator says 0.003" for 150 pounds of load and my dimensions. I arrived to the 150 based on the weight of the wood plus a little. It's a desk so it's just going to have computer stuff and paperwork. This sagulator seems meant for now...

There comes a time in every desk's life where someone stands on it for some reason that seems good at the time.