What Is a Torsion Box?

What Is a Torsion Box

The dictionary will tell you that torsion refers to a twisting.  This article has nothing to do with torsion as twisting, but it is about some real woodworking that you might not be aware of yet.

Think hollow-core doors, workbench tops, or assembly tables, or even outdoor decks.  In fact, it is the outdoor deck we will use to illustrate the benefits and strengths of a torsion box, with a passing mention of a workbench for your home woodworking shop.

Torsion boxes refer to flat, sturdy, and fairly lightweight surfaces that, when joined as the bread of a sliced chicken sandwich, are strengthened by the sliced chicken.  Of course, we don’t mean sliced chicken literally; rather, we refer to a grid-type construction that the bread attaches to, creating a greater strength than the bread would otherwise have on its own.

The core material is less important than the configuration of the grid with which it is built.  The grid, or something close to the grid, provides strength in supporting the flat surfaces.  In the case of a piece of plywood as the flat surface, the grid upon which it lays gives it a strength that is many times stronger than the plywood alone.

Examples of a Torsion Box

It is the grid construction that makes the torsion box what it becomes.  The grid is built separate from the flat-surfaced “bread” that will lay upon it when completed, and when completed, the “bread” is attached by screws, nails, brads, and a bit of glue.  The method of attachment is immaterial (pun intended) to the strength of the assembly, as long as it is a stable connection.

Home construction.  We’ll start with a larger example and work our way down to the shop’s workbench.  The floor joists run provide the span of support from floor to floor as the house is being built.  They are usually set 16” apart on center and are somewhere between 2” x 6” to 2” x 12” in dimension.

Then, approximately 16” in length, joiners are nailed from one joist to the next along the span, creating a grid effect in construction.  Subflooring plywood is then nailed onto the joists that have been joined in that grid pattern, and the flooring sits on the subflooring, whether it be padding and carpet, or hardwood, or laminate flooring, etc.

This may be a bit bigger example of a torsion box, but the principle is the same.  It provides greater strength in construction than simply nailing the plywood, the “bread” onto the joists without the joiners, and that is the basic advantage of the torsion box concept.

Outdoor decks.  One of our recent projects was the construction of a 16’ x 16’ deck behind our house.  The very same method of construction was used in building out the deck.  

The outer frame of 2” x 6” pressure-treated lumber, supported against and by 4” x 4” x 12” legs, was built; joists were attached to the frame from side to side;  2” x 6” joiners were then added between the joists along their entire spans; and, a grid was created.

The decking 2” x 6” lengths were then screwed to that grid.  Those decking boards were made stronger by the support offered by the grid.  If you’ve built decks before, you understand this principle and method for deck construction.  The decking was the “bread,” if you will, in the grid sandwich in this instance.

A Woodworking Shop Workbench with Torsion Box Construction

Torsion Box

Continuing with the ever smaller builds, we turn now to the shop workbench.  The principle remains the same.

You might start with a piece of lesser-grade plywood, or some leftover scraps of 1” x 2” stock, or whatever you have lying around in your shop.  When cut into the length of the bench, you’re building, they become the “joists” in the grid.  Then smaller pieces the length of the distance between these “joists” are cut and nailed, perhaps with brads, to the “joists” to create a grid pattern.

The core material you use to create the grid does not have to be of high quality since it will never be seen.  It’s the sliced chicken in the sandwich, hidden by the bread.  The grid pattern is what provides the structural integrity and will serve the torsion box well, not the top layer of the sandwich.

You can use scraps of odd plywood pieces by simply cutting them to the dimension to create the grid along the span of the joists.  The grade can be poor, with knots and holes and flaws.  Or, scraps of MDF, even, will work well in creating the grid. The core material is immaterial, again, because the strength and structural integrity come from the grid pattern.

The strength of the top and bottom pieces, the “bread,” will be enhanced by the grid, and in the shop arena, where heavy power tools or other objects may rest on it, the bread will not sag or bow.  

In the case of a workbench, you’d start with the base frame, probably of 2” x 4” construction.  Assuming you’ve taken care to ensure the frame was level, you’d lay a piece of plywood on top of the frame and double-check for level again.  Shims, as needed, will give you that level bottom piece of “bread.”

The grid would be built on that bottom piece of plywood out of whatever you had handy in the shop, and as we said, the core material of the torsion box doesn’t need to be of high quality or grade.  The grid pattern provides more so than the core material.  

The top piece of plywood, the top piece of bread for the sandwich, lays atop the grid and becomes many times stronger than it is on its own.  It will be able to handle heavy tools, or a heavy toolbox, or whatever you place on it, with the grid supporting the weight well and evenly distributed on the plywood.

The Advantages of A Torsion Box

This is the advantage of torsion box construction method.  Adding strength for short dollars and just a little bit of extra work.  You might see it in corrugated cardboard boxes, where a piece of cardboard has been folded like an accordion and spread out between the outer and inner pieces of cardboard to add a small measure of additional strength to the box.  It is the same principle of added strength to make the outer and inner layers stronger than they would be on their own.

We watched a handful of torsion box workbench construction videos, most of which were in the 22 – 38 minute length.  Although they were all helpful in discussing the principle of torsion box construction, we did find a shorter video that touched upon all the high points to illustrate our descriptions above.  This is the one we settled on for you.

In the video, you will hear the woodworker mention the high points of torsion box construction in the first couple of minutes:

  • Strength
  • Perfectly flat surface

Building one is a bit of extra work, but for strength and a perfectly flat surface, isn’t a little bit of extra work worth the effort?  After all, we’re woodworkers, and isn’t that what we do anyway?  Building a torsion box is not difficult at all, and the results are solid.

So why not invest in that work and get the results that will serve you well for a long time?