MDF Span For Desk Top

[Lonnie Gallaher]

I am building an office desk that will be 8 feet long. It will have 3 legs. One at each end and one at the center. The end legs are 2 inches thick built-up plywood with a 4 inch extention forming an ell that will be bolted with 1/4 furniture bolts into steel inserts in the top. The center leg is 2 inches thick with 3 inch extensions at each side forming a tee. The connection is the same as the ends. This will provide 2 equal spans of about 38 inches.

I will be using 3/4 MDF for the top. I orginally planned to laminate a perimeter frame about 6 inches wide to make the top 1 1/2 inch thick. The top will be banded with 3/4 x 2 maple. Any thoughts on if the top will stay flat or should I laminate a solid 1 1/2 MDF thick top? Or something else?

Thanks for your help.

[Jerome Stanek]

When we installed desktops for CVS we would use 3/4 MDF and span 32"

[Bruce Wrenn]

A couple of old bed rails screwed to the bottom will solve sagging problems. Screw about every six inches along their length.

[Maurice Mcmurry]

That is large for a structure without a skirt. For a span of 38 inches I would go with the 6 inch wide perimeter frame and a 1x4 or 1x6 center build down too. I would use strong solid wood for the build down. MDF feels strong but has a tendency to sag over time, even at 1 1/2" thick. I always want any non portable table or desk to be able to withstand someone standing in it, which has me thinking about a 4th leg as well.

[Lonnie Gallaher]

Thank you for all of you suggestions.

[Maurice Mcmurry]

This interactive tool gets a mention here at SMC several times a year. It is very helpful.

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

[Michael Burnside]

Personally I would reinforce with a 1x1 steel square tubing and hide with a skirt.

[John TenEyck]

If you have knee clearance for a 3 or 4" high stretcher at mid depth of the top I would add it. Stretchers offer a lot more bending resistance than perimeter frames (by the square of the height), though they too do help.

John

[Howard Rosenberg]

MDF can't support its own weight (it has no grain).

Add a lip to all four edges to create beam strength. Or build a torsion box.

I've built bookcase with eight-foot spans supported inside house joints (dadoes) in the bookcase gables.

Books are heavy. Hundreds of books even more so.

Zero sag.

Works every time - just find good instructions on building a FLAT torsion box.

Or - if you just add a lip around the edges, you'll be fine too.

[Chris Parks]

Build a torsion box using 6mm skins and 3mm internal ribbing and it will never sag and won't need a crane to lift it.

[Richard Daugird]

One more vote for torsion box.

[Justin Rapp]

I built my son's desk from MDF on top and Poplar for the base. The MDF spans the inside area of the apron is 21.5 x 34.5.