I have built several sets of risers for both stages and audiences for my theaters. A plywood top on 2x4 sides is the standard all-wood framing technique (although 1x4 can be used to save weight but it is more expensive even for less wood because 2x4 is the typical standard framing wood and is readily available at Home Depot for about $2 an 8' stick - the most important thing is the 4 inch height, not the 2 inch width of a 2x4 and a 1x4 is almost as stable on its 4 inch edge as a 2x4 is).
I typically make these in 4'x4' sections (1/2 sheet of plywood) so they are easier to move/carry by myself. I also make a few 2'x4' sections and I have even made a few 4'x4' half triangle sections so I can put up weird shape stages/platforms.
We have always used a really very simple and elegant method for riser legs that work up to a height of about 36" and are extremely stable. Each riser leg is made out of two pieces of plywood that are notched so that they fit together like those pieces of cardboard you find as dividers in wine cartons. At the top of each piece the notch is much wider to a depth of about 3-1/2" (almost the same as the height of a piece of 2x4). When assembled, the notch at the top carries two pieces of adjacent 2x4 cleanly (or the corner of four platforms). With 4x4 platforms, we use one pylon for each corner and it is amazingly stable. There is no need to screw the platform pieces together (the pylons hold everything totally solid). If the height exceeds 24", I will occasionally run cross bracing between the pylons, but this isn't really necessary. Here is a typed view of the pylons:
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A couple of things to notice are that the long narrow slots (which are used to mate the two pieces, one running down from the top and the other running up from the bottom) are NOT exactly half the height of the piece. In fact they can (and should) be somewhat longer than half the height. When placed on smooth ground, the pylon is more or less self leveling. The long narrow slots need to be pretty close to the thickness of the plywood you use so that the pylon pieces fit together snugly (but not too snug, you have to be able to take them apart too). If you use 3/4" plywood, I would make the narrow slots about 13/16". The top wide slots (call them the "crotch") have to be just over 5.25" wide. 5" is the hypotenuse of a square 3.5" x 3.5" (the square formed by two piece of 2x4 on edge placed side by side. In order to allow for imperfections in matching the platforms together, I cut the crotch slots slightly large.
The pylon height can be anything you want, but the important dimensions are the shoulder of the crotch slots (these MUST be just under 3.5" so they carry the full depth of the 2x4 but dont push up on the plywood top).
You can cut these pylons most easily on a bandsaw (we did them four at a time in a sandwich). The narrow notches can be cut so that at the bottom you just angle in leave a sort of V (just make sure that the notch is absolutely STRAIGHT at least 1/2 of the height of the pylon). This saves time in trying to cut right angles in a notch. The shoulder notches must be flat to carry the 2x4s.
I hope this makes sense. This is a completely NO hardware way to put legs on platforms, only wood (not even screws are needed in the pylons!)
Good luck and if you need more help, you can write me at firstname.lastname@example.org
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