Stress Skin SKINs

Taking a poll: what do you skin your stress-skin platforms or 'triscuits' with and why? How do you like to attach your top surface for max hold without putting a thousand holes in your stock platforms.

Our shop has been using MDF for a while, and frankly we're tired of moving that heavy crap around. We also use 1/4" tempered Masonite quite a lot.

Would love to hear what other folk like to do.

Do you mean material for the actual stressed skin? I've never seen anything other than 1/2" and thicker plywood used. I am not an engineer but I wouldn't be comfortable using anything other than a structural panel, which no MDF or hardboard is as far as I know. Use those to cover the platforms for painting, not for strength. Plus the less MDF in your life the happier your muscles and lungs and saw blades will be.

There are other threads discussing process, and Yale if not others have published theirs.

I wish I could help, as I see them discussed here on CB quite often, but I don’t think I’ve ever built one, or used one.

I've gotten away with a one off stress skinned ramp with Luan on top and bottom. Home Depot pink foam in between framing members. Span was maybe 6' long if I remember, and it had to be carried on stage by one actor, so low weight was a priority. The thing worked, but we were all pretty skeptical about it's longevity. Fortunately only one guy walked up it at a time, no dancing on it haha.

Skins were attached with spray on contact adhesive and narrow crown staples...

1/2" A/C. You can top with MDF, 1/4" or otherwise, and it is attached with butt-tons of glue and screws or staples 6"o.c. perimeter and 12" o.c. field. The strength of the 'Stressed Skin' comes from the skinning material. MDF is not designed to take Compression or Tension in the manner it is imposed by a Triscuit.
As you can read in the 'Peeling floor' thread on here MDF can easily, well I was going to say de-laminate but it's not laminated, 'Decompress' maybe? When subjected compression from the faces, MDF is great, when supported adequately. When compressed from the sides or ends the resultant force can push-apart the fibers. MDF is made to not compress, not to hold itself together in such a manner. Because the glue, not the staples or screws, is what is doing most of the work of conveying the compression or tension and the glue only adheres to the outer layer of the material it is on you have to rely on the inherent strength of that material to relay the force to the rest of the material. Plywood uses extremely extremely durable resins to adhere its laminates. It is tested and designed to hold up in compression and tension.

While I think the OP is asking about a finish or top layer over a complete stressed skin, it is possible to use materials other than plywood for the actual skins. Some orchestra shells have been built with just plastic laminate skins. Hardboard as well. Its can be calculated if there is data. I assume someone has measured data for mdf but I have not seen it so can't be sure. A lot would depend upon what the core/framing is and how closely it supports the skins. I don't recall the exact dimensions but using one by framing, I think 1/4" ply was good for framing 12" OC or so; and the further the framing is apart, the thicker the ply has to be. Conversely going to a more or less solid core - like XPS or honeycomb - you can get away with pretty thin material - certainly 3/16" ply like a lot of the major platform manufacturers use - and probably 1/8" (if you can find a quality 1/8" ply - baltic birch being pretty good if you can afford it) IIRC did a lot at Yale with 1/4" bottoms and 3/8 or 1/2" tops. Puncture loads have to be considered - like dropping a pipe on it.

And it's all about the adhesive and good application.

We used 1/2 ply top and bottom with a 1/4 hardboard on top for aesthetics. They are incredibly strong but heavy!

I am indeed referring to ideas about a top/finish skin that would sit above our deck of 4'x4'(ish) stress skin stock units. We switched over from a 4'x8' framed stock unit to the 'Triscuit' design in the Yale Tech Brief many years ago and absolutely love them. For lots of reasons.

We have successfully skinned them with and 1/8" single-sided tempered Masonite hard board which required more brads in the floor, as we would sometimes get buckling from painting/moisture. If we were very careful to allow enough spacing between Maso sheets we could avert that issue but there are still thousands of brads to be pulled out of our stock triscuits after each production.

1/2" even 3/4" MDF has worked extremely well and won't buckle with moisture conditions and because it's so dense, a few brad nails are enough to keep each piece from sliding around and we have been able to illimiate the need to space out the MDF sheets -- the designers love the clean seamless floor. The shop guys however are sick of lugging around the heavy MDF and are on the verge of revolt...

We have even had DECENT success with 1/4" Luan plywood in a pinch though it is NOT durable enough for heavy dancing and tends to crack and start de-laminaing under hard casters under heavy loads.

We will use the same full sheets of WHATEVER material over and over again throughout the year, sacrificing several sheets for contouring to each show's needs. So I'm hoping that maybe there is some fun idea I haven't thought about that would give us durability, not buckle, and not weigh a ton. Or is what we've been doing pretty much on track? Thanks for input so far --

Do you prime both sides and all edges of the 1/8" hardboard?

We do not. I've only used the hardboard that is tempered/smooth on one side. I've never tried painting the rougher underside. Are you suggesting that priming/letting dry both sides and edges with paint before laying each sheet would help prevent warping?

Yes, exactly. The buckling can result from one side expanding faster than the other from humidity. If using hardboard for a stage floor, many experienced folks recommend same - prime both faces and all edges.

You could use double-side tempered hardboard. Sometimes referred to by the brand name Duron. More stable, more expensive.

BTW, there are differences in quality of hardboard, too. We no longer buy it at big box stores and have much better luck at lumber yards for around $16-$18 for single-sided, 1/4".