Preventing Sag in a Desktop

Advice on constructing a custom desk to stay stiff in service. October 20, 2013

Question (WOODWEB Member) :
An architect I work with has asked me to furnish new work surfaces for his office. Itís spec'd with 1" Europly (exposed edges) laminated with Formica top (gatorskin back). The desktops will be supported with a combination of their file units and adjustable legs. With an unsupported span of 40-45", and a depth of 32", will 1" Europly be adequate? Or should I encourage them to go to a shop plywood core totaling 1.5" (2x.75")? I tried out the Sagulator calc and was a little doubtful of their ratings for fir plywood, and not sure how to compare to birch Europly.

Forum Responses
(Cabinetmaking Forum)
From contributor L:
I don't think there would be much sag in that span. At the same time, if it were me I would want to play it safe and go thicker. I often vacuum press two layers of ply for shelves and counters so I don't have to worry about sag. Two layers of euro birch vacuum pressed is very, very rigid.

From contributor D:
I do not think it would be a problem. Desks tend to not have really large loads for long times like shelves, and the laminated plywood would have a good stiffness to weight ratio so itís not going to sag on its own. I would try to talk them into balancing the panel with a regrind backer of the same thickness.

From Contributor E:
I disagree. For a desk you have to have a backer layer of laminate of equal thickness to the top layer to balance the ply tops or they will warp. There will always be an unbalanced load on the work surface and as a desk there should be a thicker reinforced edge or a thicker cross section. A tubular piece of steel 1" x 3" can be placed further in from the edge so itís not seen. I am an architect and your architect should know the limits of the material. My desk in my office is a solid core door with a 7' span and 2" thick and I can measure 1/2" sag along its length.

From the original questioner:
Yes, we'll definitely go both top/bottom in post-form grade. How long has it taken the seven foot span on your door desk to sag? It seems like you could flip it annually, like a mattress. We will most likely go with a ply core. I'm trying to ascertain how the almost 40" span will do with a computer and some elbows on top of a 1" core, or whether we need something deeper.

From contributor A:
"Doubtful of their ratings for fir plywood, and not sure how to compare to birch Europly." Solid fir vs. solid birch would give you a relative comparison.