How do I keep a wide, wall-mount desk from sagging?

Question

I currently have a desk made from a 18mm 4x8 sheet of plywood cut in half lengthways and sandwiched to make double thickness at 36mm. It has 4 hairpin legs (one at each corner) and an Ikea set of Alex drawers in the middle for support and storage (see my child's drawing below for illustration).

I have relocated the desk recently by rotating it 90 degrees, and it fits with a few mill to spare between the walls of my office room - So my plan is to ditch the legs and mount the desk to the wall.

My plan is to secure battens to the wall on the sides and back of where the table will sit.

My question now is: What would be a good way to stop/reduce any potential sagging/bowing of the top while in situ?

• Perhaps a front brace, stretching between the wall battens, with internal bracing, kind of like a shed/decking base?
• Some C-channel metal like is used to stop warping of joined boards?

Any help is much appreciated.

Also, if you think weight will be an issue, I was thinking of also stripping it down a a single sheet and keeping a strip at the front to give the illusion of one thick piece.

I probably should have done that in the first place, rather than ruin a half a piece of birch :'(

Answer 1

• Put the drawer unit back in the middle as suggested by Tim.

  • For an 8' length, this is probably your best bet as it will provide wide support in the middle of the span, leaving 2 notably shorter spans which will each, inherently, have less tendency to sag.

• Add a vertical support to the front edge.

  • A strip of solid wood, roughly 18x36mm, with the 36mm dimension vertical, mounted to the front edge of the desk with glue and screws will provide notable sag resistance as well as nicely hiding the edge of the plywood.
  • Note that this is sag resistance, not prevention. You'll probably still get some sagging going on.
  • This will impact "slide under" space and could interfere with chair arms, snag clothes, etc. However, if it's nicely finished, it shouldn't be a problem for snagging pants or shirts.

• A C-channel of aluminum, steel, brass, or other fashionable metal wrapped around the front edge of the desk will provide sag resistance and dress up the front edge, as well.

  • The thicker the metal, the more sag resistant it will be.
  • Choose a metal/finish to compliment the room and add and "industrial-chick" look.
  • It will create a lip at the front edge that has the bonus of keeping pens from rolling off the desk, but the drawback of possibly being right where your elbows and/or forearms may rest on the edge of the desk, possibly being uncomfortable and/or leaving marks on your arms from resting on this edge.

• Put two legs, side-by-side, in the center of the span so they look something like this quickie ASCII art.

  • This will provide a center-span brace, but won't work as well as having the drawers in the middle because it will leave longer spans on either side of the support, and it won't support as much of the middle of the desk surface as the drawers would.

 \   ||   /
  \  ||  /
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• Use a series of manufactured wall angle brackets to support the bottom of the desk along the long back wall.

  • Metal shelf-support brackets can be had (in the US, at least) for shelves up to 15" or 18" (38cm or 45cm). Use the deepest ones you can find in your area. Even a series of 12" (30cm) brackets should provide significant weight support.
  • Put one bracket on each stud in the long wall (roughly every 16" (40cm) if you've got a brick/masonry/etc wall without studs - use appropriate fasteners).
  • Supporting at least the 1/2 - 2/3 of the desk's span from the wall, these should take a considerable amount of weight since shelf supports that deep also tend to be designed for heavy shelf loads.
  • Depending on design, they may interfere with knee space, so choose carefully.
  • You could make brackets out of dimensional lumber (such as 2x4, or the metric equivalent) and you could custom tailor the "front" end to minimize the impact on knee-space under the desk.

• Use any combination of the above to provide suitable support and meet your aesthetic demands.

Note that any treatment to support only the front edge (the metal channel or a wood skirt board), will only provide sag resistance and not sag prevention. Eight feet is a long way to go without any additional support transferring the load to the wall behind or the floor below. You could, most likely, get a dead-level-for-decades front edge by using a fairly thick steel c-channel, but it might be far too thick for comfort or appearance, leaving you with a really big shelf that you don't want to use as a desk.

Answer 2

At 36mm it will be pretty stiff. I've used 40mm thick chipboard doors as workshop benchtops and they were plenty stiff enough, plywood should be stiffer.

However if it's sagging in the middle you need to support it in the middle. a heavy duty shelf bracket or two screwed to studs under the shelf, might be the way to go.

Or you could add thickness at the front to stiffen it up, 41% thicker will make it twice as stiff.

Answer 3

Similar question was asked recently. On that one, they wanted one corner to be free-floating, which made the design harder, but most of the information there is applicable to your project.

I'd say these are your choices - you can do more than one for extra strength:

1. Heavy-duty metal brackets attached to wall studs. Something designed to hold up shelves or floating countertops.
2. A leg, or legs.
3. An apron, 3-4 inches tall, like you normally see on a wooden table.