Plywood. If you take a look at plywood, you'll see that it is indeed made up of a number of plies of wood. In other words, several sheets of very thin wood is sandwiched together to make a wooden board. Each ply is set in a 90 degrees direction from the last. The top and bottom plies run the same direction: the long way. So, you'll have the grain running longer then not.
Plywood comes in several different thickness, types and grades.
I'm only going to get into the plywood we use most often.
Plywood comes in standard 4'x8' sheets and is always square. The thickness we use most often are: 1/8", 1/4", 1/2" and 3/4"
  1. 3/4" plywood.
    used most often for covering platforms. it is very strong and is fairly rigid.
  2. 1/2" plywood.
    We use 1/2" for building a lot of props, boxes, furniture parts. It's lighter then 3/4" ply. Cheaper too.
  3. 1/8" & 1/4" plywood.
    Great for making hard covered flats. The thickness depends on how rigid you need the flat to be. you must also take into account how much you need your flats to weigh. If you're flying your flats, you may wish to use the 1/8". That is if you're not going to soft cover them.

Grades of plywood.

Plywood comes in several grades. The grade refers to the surface covering. The better the grade, the nicer the surface. The grades are represented via letters, A, B, C, D, X.

  1. AA is great stuff. It is smooth on both sides with no knot holes. It's used to build nice furniture type objects. Book shelves that are open, thus allowing both sides of the plywood to be seen

  2. AC is more common. One side is the good stuff and the back is ok. We used AC for flooring that is going to been seen by audience members. That is, seen up close and that is not going to be covered with carpet, etc.

  3. CDX is ugly stuff. But it's cheap and works. The surface has big unfilled knot holes on the back (D side). Keep in mind that since this is just the surface ply, it doesn't show through as a knot hole in a piece of wood would. This is perfect plywood to use on platform tops when the platform is also getting covered with something like Homosote.

  4. I'm starting to use OSB. It's ugly and heavy. But it's cheap and strong. As strong as AC ply and as cheap as CDX. It will dull your saw blades faster because of the resin used to glue the Oriented Strands to form the Board (hence OSB) together.

Some do's & don'ts.

  • Anything that is taller then you are can be hard to handle. This stuff can be heavy.
  • Do ask for help when moving heavy plywood.
  • Always have assistance when cutting full sheets with a power saw.
  • Be careful of splinters! Ply tends to give them easily.
  • Ply is much stronger along the grain.
  • Store it either flat or as close to vertical as you can. If you lean it against a wall, at an angle, it'll warp.

The spreadsheet below is copied from thePlatform article written by Michael Powers. Check them out. They're good stuff.

Some Common Sheet Goods Used for Platform Lids in Theatrical Construction
Name Actual Size
Description
Advantages
Disadvantages
Average Cost as of MArch 2000 in US $$
Plywood, 3/4" AC 4' x 8' x 23/32" thick
5 to 7 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. "A" side is smooth and "finish" quality.

Strong, will support heavy loads if properly framed and supportedvailable everywhere.Smooth surface can be painted and become the show surface.

Cost, March, 2000 price is about $32 per sheet. Moderately heavy at 75 pounds per sheet. May have voids inside that can allow point loads to penetrate.

$32.00

Plywood, 3/4" BC 4' x 8' x 23/32" thick 5 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. C side is plugged and rough, D side often has surface voids. Strong, will support heavy loads if properly framed and supported. Available almost everywhere. Smooth surface can be painted and become the show surface. Price is usually $3 to $5 cheaper than AC Plywood. Often very curved, usually has voids. 75 pounds per sheet May have voids inside that can allow point loads to penetrate
$28.00
Plywood, 3/4 CDX
4' x 8' x 23/32" thick 6 layers of wood laminated to the nominal thickness. Layers have grain running 90 degrees to adjacent layers. "B" side is smooth.
About 30% cheaper than AC Plywood

Rarely flat, large voids on D side. Must have some other surface for show side such as Homosote plus masonite or luan.
$22.50

OSB, 3/4"
4' x 8' x 23/32" thick OSB means Oriented Strand Board. Wood is shredded and glued up in a resin with the fibers roughly aligned in the direction of the board. No voids, generally flat, structurally equal to or stronger than 3/4 AC plywood, but about the cost of CDX.

About 10% to 15% heavier than equal thickness of plywood, about 85 lbs. Per sheet. Must have a show surface of masonite or lauan etc. resin dulls tools rapidly.
$22.50
Particle Board, 3/4" 4' x 8' x 23/32" thick, some brands 3/4" true.
Particle board is made of wood that is ground up roughly and glued up in a resin.
No voids, generally flat, but usually less than the cost of CDX. 20% heavier than equivalent thickness of plywood. Structurally weaker than 3/4 AC plywood. Very hard, screws must be pre-drilled for counter sinks to drive flush, edges and corners break off easily. $19.75