"Floating" desk - Avoiding wood sag, aluminium right angle support

Question

I'm building a new desk in my study, and I'm looking for some advice/guidance on structural strength.

The desk will be an L shape, mounted wall to wall along one side of the room, and desk to leg along the other wall of the room. The image I've included shows the measurements. The dotted lines show bracing along the wall (40mm wood, mounted to the wall, screwed to the desktop from the bottom. Yellow lines are solid legs to the ground. (The 500mm space between the wall and the solid leg on the top section is going to be made into a cupboard - all our routers, DVR, etc sit there, so I can't move that much.

The join between the two legs of the desk I was going to do as a mortise, and not glue up the ends to allow for some wood movement without cracking.

The whole desk is going to be made from 40mm laminated rubberwood.

My concerns are: - The wood is bloody heavy. I'm worried about the inner point between the legs of the desk being too weak to hold the weight of it. What do you think? One option is to build a leg down there, but if I can avoid that I'd like to.

• The length of the wood - I'm worried about sag over time. It's a desk, so it'll have computers and books and stuff on it, but also me leaning on it for hours a day. I have thought (and would love some advice here please) to hide a piece of 90 deg. aluminium underneath (cut a slot in the wood, route out slightly to one side to recess), and glue/screw the aluminium in there. Figured that would make it pretty rigid?

Any other suggestions? Is there anything else I should be worrying about?

Thanks, Zak

Answer 1

I would not waste time with an aluminium angle iron. It will being next to nothing in terms of real support for the front edge of your desk. Instead you should look at something like steel rectangular tubing. This you would simply fasten to the underside of desk a little back from the front and make sure it was supported on each end. The steel tubing could be painted black to hide or a standout contrast color. Alternatively you could also install an on-edge apron board in front of the tubing to hide it from view entirely. The apron board itself will add additional support.

For your longer span across the top of your diagram I see a length of 2170mm. Use a tubing with a 50mm width and a 100mm height and a wall thickness of 5mm. Such tubing, if supported on each end, should be able easily handle a center load of 100kg with less than 10mm of vertical deflection.

(I used the deflection calculator here).

In the corner where you have indicated "strength ???" you will want to arrange to join the two rectangular tubes into a "T" like joint. This could be as simple as drilling some through holes and bolting in a short piece of angle iron in the underside corner. Alternatively you could plan things out and weld the joint if you have that type of equipment available.

I would also purge the idea of the mortise joint and the construction complications that brings. Instead use a flat steel plate that lays across the joint and screw that to the bottom side of the desk surfaces. I could easily envision something 75-100mm wide with a 3-5mm thickness. Two rows of holes pre-drilled into the plate at intervals of 75-100mm should do well to support the edges of the desk. Best would be if its length went as long as feasible from the front edge back to the wall.

Answer 2

You've got 40mm wood spanning a gap of 2.1m (or 7 feet)

That will support it's own weight but will bend if you lean on it, and could break if someone sits on it. As you've suggested, you need more depth to make it stiffer and stronger, or another leg to reduce the span.

A real engineer would do the calculations, but I found it easier (and more reassuring) to set up a test. Place a couple of lengths of 50mm (2") thick wood on the ground 2.1m apart, span them with your counter top (or some other wood of similar thickness) and lean on it gently to see how much it bends.

IIRC the equation for bending stiffness uses the depth cubed, so a little more depth makes a much bigger difference than changing the material.

I had a similar bench, and a length of 50mm square wood glued and screwed under the counter top was enough to stiffen it (although it did bend a bit if I sat on it). Aluminium angle would probably need to be a similar size.