Rare is the model railroad that’s built without plywood: it’s strong, flexible, readily available, easy to work with—and misunderstood by many modellers. In order to use plywood to best advantage, here are some things you’ve got to know.
Plywood is made from multiple layers of wood veneers, laminated with the grain in alternating directions. This provides those nice, broad 4×8 sheets we modellers find so inspiring, and gives plywood its amazing strength and dimensional stability. The very nature of plywood, however, virtually guarantees that some internal stresses remain, and those stresses induce warp. “There are two kinds of plywood,” I like to tell people, “pre-warped and ready-to-warp.” That’s just how it is. Let’s examine some common misconceptions about warp:
“Can’t I avoid the problem with thicker plywood?”
No, not really. All plywood warps. By using a thicker material, you’ve made it more difficult to counter the warp, and made the layout heavier to boot.
“Would a better grade of plywood help?”
Somewhat, but even Baltic Birch, which is regarded as a high-grade furniture plywood by the industry, warps—sometimes badly. I’ve seen sheets curled up like potato chips.
“Can I just pick out a flatter sheet at purchase?”
Mmmm…how patient is the guy at your neighborhood lumber yard? Is he willing to pull sheet after sheet off the stack, set each aside, and await your judgment on the flatness of the next one? Don’t count on it. Let’s say for the sake of argument that he’s a saint, you’ve found a perfectly flat, blemish-free sheet two-thirds into the stack, and he’s cheerfully helped you strap it to the car before restacking all the sheets you didn’t buy. Now your perfect sheet of plywood no longer has 1000 pounds resting evenly upon it. Now both sides are exposed to the air, and the ambient humidity. Will it still be flat tomorrow? Don’t count on that, either.
“Painting it on both sides will help, won’t it?”
Paint is not the impermeable shield we imagine—even the best coating is thin and highly porous. A good layer of paint or varnish will slow down the changes in humidity from working their way into the wood, but won’t stop them. Over weeks, months, and seasons, moisture gets alternately drawn in and pulled out of the wood, no matter what the finish. (The French doors to my parlor are a hundred years old, with several thick layers of paint, but they still jam every summer.)
So, you can’t escape plywood warp. How, then, do you confront it?
Brace it well. Your design should include parts that brace plywood against movement across its weakest dimension—its thickness. I like to build small layouts with bracing under the plywood every 12 inches or so, sometimes even closer. Bracing need not be of floor-joist stoutness—straight 1×2 pine is plenty adequate, and even smaller usually suffices. It’s the height, not the thickness, of the bracing, that keeps the plywood flat, so plan to leave a few inches of space under your deck when designing the layout, and brace it up thoroughly.
Check for flatness during construction, and shim if necessary. Keep a large steel carpenter’s square handy as you build, and use it to check the flatness of your subroadbed. Any undesired irregularities should either be fixed now, or dealt with by carefully shimming the track as it’s laid.
Plywood is stronger than you think
Three-quarter-inch-thick plywood is considered suitable for flooring. Why do so many modellers feel they need half-inch plywood to hold up an N scale train? I frequently use three-eighths, which is admittedly overkill, and quarter-inch works just fine. Even eighth-inch material works well if it’s properly braced.
Plywood is never as thick as the label says
Plywood is sold by its nominal thickness—its actual thickness is less than that. A sheet of 3/8 BC plywood, for example, will actually measure about 11/32”. This is important to know when building: that missing 1/32” could mean the difference between a snug joint and a sloppy one, or a module that meets the spec or doesn’t quite. Take actual measurements from the plywood you’ve obtained for your project before making your cuts.
There are other sheet materials besides plywood
The world of sheet materials is a broad one: medium-density fiberboard (MDF), oriented strand board (OSB), particle board, Masonite, extruded Styrofoam, and (yes, for you old-school types) Homasote. Each of these materials is potentially useful in railroad construction. MDF is of particular interest to the furniture-railroad builder. Similar in composition to Masonite, it’s perfectly flat, smooth, machines well, and will not crumble upon casual contact with water (such as during scenery construction). It’s heavier than plywood, however, not as strong, and if exposed to prolonged dampness, will sag where not thoroughly supported.