[This is the fourth post in a month-long series on stage decking and platforming. Check out this article in the December issue of Stage Directions, as well as this discussion forum.]

As I discussed last week, a stressed skin panel derives its strength from the cross-sectional I-beam shapes the combination of stringers and skins create. What would happen, though, if you could take one of the webs of one of those I beams and somehow make it the full width of the panel but not increase the weight significantly? That’s the foundation design principle of the stressed-skin panel: create a single unit panel whose lightweight, rigid core resists the compression and tensile stresses generated by a bending load.

Like those for stressed-skin panels, the calculations for determining the allowable load of a sandwich-core panel can be a little intimidating, but they are, once again, based on simple algebra. Using a spreadsheet program to run the calculations can save time and help you avoid costly errors.

Based on my spreadsheet, a 4’x8’ panel comprised of a ½” BC plywood (5-ply) top and bottom lid and a 2” extruded polystyrene core (i.e., pink or blue rigid foam insulation board) can carry a load of 41 pounds per square foot, provided it is supported every four feet along its length, roughly equivalent to our 2x4-framed 4x8 platform. (The governing load is deflection, or how much the panel will flex between the supports—all other tests exceed a 50 psf load. A load of 50 psf will cause the panel to deflect slightly more than 1/8”, which while greater than the standard acceptable deflection in this situation, is probably not a deal breaker.)

Material costs:

Unlike rigid-framed platforms, sandwich-core panels require the use of stud wall supports, increasing the overall cost of materials for a deck. In some situations, this may overtake any savings to be had over material costs for individual platforms/panels. Using 4x8 panels will require the same number of stud walls as rigid-framed platforms (which require supports every 4 feet) and 4x4 stressed-skin panels.

Construction Labor

4x4 panel and 4x8 panel

Approximately 2 person-hour, moderately-skilled (requires precise planing and shaping of stringers, counter-boring holes for installation bolts). However, dry time must be accounted for, as the process requires the use of special adhesives which make material-breaking strength bonds between the plywood and the foam core. These glues require significant drying time (sometimes up to 24 hours) and must set up under pressure.

Load-in Labor

4x4 panel

Typically can be carried easily by one person

When using stud walls, can be very quick—four bolts/lags in the corners

4x8 panel

Best carried by two people, though in a pinch can be carried by a single worker

When using stud wells, can be very quick—four bolts/lags in the corners

Long-term “save-ability”

4x4 and 4x8 panels

Very durable, especially is care is taken in the assembly process, and in setting counter-bored holes for installation.

Rep-ability

4x4 panels

Swiftly shifting a large deck is relatively simple as only one person is required to carry a panel.

4x8 panels

Swiftly shifting a large deck requires multiple people (2 people are needed to carry each platform); weight of 4x8 panels can lead to fatigue

Weight

Volume/Size

4x4 panel: 4’x4’x0.25’ = 4 cubic feet

4x6 panel: 4’x8’x0.25’ = 8 cubic feet

Other considerations

Sandwich-core panels offer two distinct advantages over either rigid-framed platforms or stressed-skin panels: they inherently deaden sound, and they are readily adapted to organic shaped deck profiles. When built with a rigid-foam core material, sandwich-core panels soften that “reverberant drum” effect that plagues many stage decks, because the core breaks up and absorbs sound waves. And, because they are homogeneous across their width and through their length, you can run a long-bladed jig saw along an organic-shaped profile, slicing through lids and core at the same time and be assured that there won’t be a need to add filler supports to catch the lids.

Like stressed-skin panels, sandwich-core panels can be used in short overhang situations, where the stud wall supports are held back from the edge of the deck by about a foot or so.

Like rigid-framed platforms, supporting point loads that do not fall on stringers or stud walls is relatively easy, because the core material is consistent throughout the entire panel: simply add a support under the panel wherever a point load lands.

Like both rigid-framed platforms and stressed-panels, having a stock of sandwich-core panels makes building a deck relatively inexpensive, as all you really will need to purchase materials for will be stud wall supports and any suicide panels that are required to complete a particular shape.

The benefits of sandwich-core panels are pretty obvious: they are much lighter than either of their cousins (compare 54 pounds for a 4x4 sandwich-core panel to 95 pounds for a 4x4 stressed-skin panel or 95 pounds for a 4x8 platform); they handle point-loads and organic shapes more easily; they offer inherent sound-deadening characteristics. Additionally, they take up less space than 2x4-framed platforms (compare 4 or 8 cubic feet to 10 cubic feet)—though they are slightly larger than similar stressed-skin panels.

However, there are disadvantages to using sandwich-core panels: they are more expensive than either of their cousins ($44 for a 4x4 panel, compared to $21 for a 4x4 stessed-skin panel; $57 for a 4x8 compared to $45 for a 2x4-framed platform). The level of construction experience required isn’t significantly greater than that required for a 2x4-framed platform; however, acquiring the proper adhesive, achieving proper clamping pressure, and waiting through extended dry times make building sandwich-core panels more time consuming and complicated than either of their cousins. As with stressed-skin panels, if I’m in a situation where I don’t have a lot of dedicated construction time, I’m going to go with 2x4-framed platforms; however, given the time and materials, I lean toward sandwich-core over 2x4-framed platforms any day. Choosing between sandwich-core and stressed-skin panels is relatively easy for me as well, provided I have the shop time to make the sandwich-core panels happen: they are lighter and offer advantages over stressed-skin panels that I prefer.

Next week: a look at building a deck out of plywood and stud walls, and summing up all this data.