Lumber and Foam

The two principle materials used for the structure of my layout are lumber (which I’m using generically as a term for all wood products) and extruded polystyrene (XPS) insulation foam. Many other things are used as well (plaster, sheet styrene, etc) but these two are what hold everything together and provide a firm support. I have some basic carpentry/woodworking skills learned in childhood, but I’m not particularly skilled. My success is more a matter of persistence, and being willing to give up and start over when I make a mistake. But I’ve learned a few useful bits of knowledge along the way, and this page is going to summarize them.

The golden rule of woodworking is “measure twice, cut once”, and this applies to more than just cutting. It’s easy to make a mistake, and stated dimensions may not be as accurate as you’d like. When I buy lumber I clip a tape measure to my belt before I go to the lumberyard and check anything I’m uncertain of. The times I didn’t do this I often regretted it. Before I cut, after I’ve measured and marked the wood for cutting, I go back and check my measurements again before I plug in the saw. It’s easy to catch an error here, and expensive to fix it after you’ve cut the wood.

When handling wood, a good pair of heavy leather gloves or synthetic gloves designed for carpenters are very useful. This will keep you from getting splinters or cutting up your hands on rough wood. You can work without gloves if you’re careful (and I do sometimes), but sooner or later you’ll regret not putting them on (and I do, sometimes). When cutting, safety glasses are an absolute requirement and gloves are a really good idea.

Handling and Storing Materials

There’s more about wood storage on the Choosing Wood page, but the basics are to buy it a few weeks or months before you need it, and store it (flat or vertically, so it won’t bend) in the area where it will be used, so moisture levels in the wood can even out.

Foam has a separate issue: it breaks down from exposure to Ultraviolet light (i.e., sunlight), so keep it away from windows. Older/cheaper fluorescent lights can emit UV, so if you have an old fluorescent shop light, keep it away from that, too (newer fluorescents are usually better about not emitting UV, but not always). Once it’s painted or covered as scenery, UV is no longer an issue.

Wearing good gloves (heavy leather or specialty ones) when handling wood is a good idea; take a pair with you to the lumberyard to avoid splinters.

Dimensional Lumber

This is wood that is cut from a tree to specific dimensions. Boards (like a 1x3 or 1x4) and structural lumber (like a 2x4 or 2x8) are examples of dimensional lumber, and the numbers used in North America (and some other places) are in inches (length is given in feet, usually).

But because of the characteristics of wood these dimensions are used in ways that can confuse a beginner. Wood is cut when wet, and the measurements reflect the cut size. Once wood is dried (typically in a kiln) and sold for use, it will have shrunk. So a 4” wide board will be about 3.5” wide. However lengths (given in feet) will be accurate (to within a quarter inch or so). Wikipedia has an article on lumber with a table, but you can also find more complete tables by googling (like this one). What you’ll actually be able to find in a lumber yard or home store will be a subset of these sizes.

Wood is sometimes priced by the linear foot and sometimes by the “board foot” (which has a formal definition). A linear foot price reflects the length of the material. Thus a 4’ long 1x6 has four linear feet. In contrast, a board-foot number is a volume specification that normalizes width and thickness (to 12” and 1” respectively). Thus the same 4’ 1x6 has two board feet. Many lumberyards will sell woods (typically specialty woods) by the board-foot, and there are even calculators like this one to help you convert units. Mostly you can just use linear feet, but be aware of the difference in case prices are stated (or calculated by the lumberyard) in board feet.

Because lumber is cut from a tree, it may contain knots or other flaws. These are normally not a problem, as long as they aren’t located where you want to drive a screw or cut the wood (knots are hard, and resist drilling and cutting, and often fall out leaving a hole if disturbed). You can pay more for better grades of lumber (see Choosing Wood), but these are usually only needed where the woodgrain will be visible (not typically an issue with model railroad tables). Careful selection can avoid knots. Construction softwoods (what we’d normally use) are graded on a very complex scale intended to describe how well suited they are to specific uses. There are completely different grading systems for plywood and hardwood. But you really don’t need to understand these to build a good layout table. Just look at the wood. That said, in my layout construction I have usually paid a bit more for “select” or “clear” wood to limit the amount of effort I’ll have to spend avoiding knots, but that’s not always the case and in any event it’s a cost/time tradeoff you may do differently. I’m considering using lower-grade wood in the new layout, because I’m going to be buying a lot of it.

Because boards are cut from a tree they contain grain (layers of different density due to seasonal growth variation). When dried, this can cause the wood to curve. It’s important to look along the length of a board on a couple of sides, and see how straight it is. A slight curve may be correctable (or ignorable) depending on what you are doing, but straighter is better. You may not find a perfect board, but you can come close. If the first lumber yard doesn’t have what you think looks good, check a few others out.

Dimensional lumber (in particular pine) is appropriate for certain structural elements in a table. I used it for the side-frames and cross-members of my tables, and for the framework they sit upon. In L-girder benchwork, pine boards are often used for girders and their supports. Table legs are often made from 2x2 lumber, and this is large enough that you can drill a hole in one end and install an adjustable bolt, to allow the layout structure to be made level on an uneven floor (basement floors are rarely perfectly even).

Dimensional lumber will swell in humid weather and shrink in dry weather, and over an extended period a very wet or very dry environment can affect it significantly. Painting to seal the wood with a primer will avoid this problem, and while that isn’t required in many places, if your layout room tends to one extreme or the other, it’s a very good idea. If you do this, remember to paint both sides and the edges, otherwise you aren’t sealing the wood.

However, pine is a softwood (meaning it’s not particularly strong) and only comes in a few standard sizes (you can make others with the right kinds of saws, but that’s more than most people are equipped to do). And it’s rarely perfectly flat. For those reasons many people prefer to cut structural “boards” from plywood instead. And plywood is clearly preferable for large expanses of wood (like table tops). This is particularly common in modular layouts, where exact dimensions and even joints between modules are critical (the same logic applies to sectional layouts like mine, but I used pine).

Engineered Wood Products

Sheet and dimensional materials that don’t occur naturally, but which are made from wood, are called “engineered wood products”. Plywood is the most common example of these used in model railroads, but it’s not the only one. It is important to note that plywood has a number of advantages over the others for structural uses, and you can’t necessarily substitute another sheet product for plywood and have it work well over the long term.

Dimensions of engineered wood products are typically accurate, since they reflect the result of the manufacturing process. Thus 1/2” plywood really is 1/2” thick. Lengths and widths can vary by 1/8” or more (and this caused me some problems with my tables, as my “4-foot” table tops were as much as 1/4” shorter on a couple of tables). As always, measure it first.


Plywood is a manufactured or “engineered” wood product formed of layers of wood glued together. Each layer in a sheet is cut from a tree, but from around the trunk (like peeling an apple) rather than along the length of the trunk like dimensional lumber. Like dimensional lumber it comes in a variety of sizes, typically described by the size of the two larger dimensions in feet, and the thickness in inches. The standard construction size is a 4x8 sheet, but you can often buy it in 2x4 and 2x2 sizes called “panels” (I used 2x4 panels for my table tops). The number of layers will vary by the thickness, and you can get it in “interior” or “exterior” forms (the latter designed to resist water, which is a nice attribute for a table under model railroad scenery, but not needed if you paint the wood). Plywood also has a scale of grades. Because the tops of my tables were going to be visible in several places as the river surface, I wanted to avoid knots or other problems, and have very smooth wood (and I was too lazy to sand it) so I paid extra for pre-sanded higher-grade panels designed to have one side visible.

Plywood’s layers make it resistant to bending, which is why it’s used as a floor material in houses. That also makes it a good structural material for model railroad tables and particularly for any places you need a broad flat surface. Small modular layouts may use 1/4” plywood, while larger ones will use thicker grades. I used fairly strong 1/2” plywood for my table tops, mainly because I wanted it thick enough to drive screws into from below to hold electronics and wire brackets in place.

In L-girder benchwork, plywood is often used as the subroadbed. In modular and sectional layouts it may be used as a single flat surface (as I did), or cut using the “cookie-cutter” approach with some portions raised, as this provides a natural-looking slope if done correctly. For these applications, thicker plywood (even 3/4”) is desirable to reduce the requirement for supports and prevent sagging.

However, “resistant to bending” doesn’t mean it won’t bend. Used as a flat surface it does need some support to avoid sagging (more important for thinner plywoods or those carrying loads) and if stored poorly it can warp. Check that a sheet really is flat before buying it. See the Choosing Wood page for more about the different kinds of plywood and which is applicable for typical model railroad uses.


Hardboard is an engineered wood product made from wood fibers in a glue (resin). It can be made with none, one or both sides smooth (the smooth-sided kind is often called by the brand name “masonite” or the more generic term “tempered hardboard”). Because it is made from individual fibers it has very little resistance to bending, and if used as a flat surface without continuous support, over time it will sag. It’s a poor choice as a structural material (although I’ve used it successfully for small areas with good support).

What it works very well for are vertical surfaces such as backdrops and layout fascias (the front edge of the layout that hides the edges of the wood and foam scenery materials). I used 3/16” tempered hardboard (one side smooth) for my backdrops and layout fascia. Often even thinner material is used, as it can bend around corners. Be sure to rigidly support it even here, as it can bow in or out between supports if there is any give to the support structure.

While you can drill holes in it, hardboard doesn’t work well with screws. I used bolts to attach my fascia (and the bolt heads are very visible), but I glued 1x3 boards to the back of my backdrops with carpenters glue and bolted those to the layout where the bolts wouldn’t be visible. Most people seem to use screws with decorative washers, also known as finishing washers, to attach fascias, so they can be removed. With these the screw simply goes through a hole drilled in the hardboard, and screws into wood behind it, with the washer clamping the hardboard to the support.


When the word “foam” is used in regard to a model railroad layout, it is typically used to mean the kind of large sheets of Extruded Polystyrene (XPS) insulating styrofoam that can be carved into shapes. This differs from the Expanded Polystyrene (EPS) formed of small granules or beads used as a packing material (and often also called styrofoam), sometimes known as “beadboard”. Don’t use “beadboard” for model railroading. Another kind of foam used is “expanded polyurethane” or “foam in a can”, which some people use for building hillsides or other scenic elements.

Extruded Polystyrene

Insulating foam sheets are made by extruding polystyrene in a way that causes it to foam (expand) with a thread-like structure. This means that these sheets are stronger than beadboard, and have a “grain” along their long dimension. It’s much easier to cut this stuff with a knife along the grain than across it, and cutting across it typically requires a saw. Sheets are available in the standard 4x8 size used in construction, but also in 2x8 and 2x2 sections as well. Thicknesses commonly available range from 3/4” to 2”. This kind of foam is typically pink (Corning) or blue (Dow) rather than white, although the foam sold by Woodland Scenics is white and appears to be XPS.

I discovered (the hard way) that dimensions are only approximate. I bought three sheets of the same brand of 2” foam from the same store within a few months of each other, and they ranged in thickness from 1.75” to 2”. This caused me some clearance problems in my subway tunnel. It’s possible that these had been exposed to light at some point, which caused them to shrink. Whatever the reasons, if the thickness is important be sure to measure it.

Since XPS is essentially styrene, it is vulnerable to melting by solvents often used in glues in paints, so you need to ensure that any you use with it are either designed for use with insulation foam or test them. Also, even paints that are safe for plastic models may cause problems with XPS, as it’s more vulnerable to brief exposure to solvents than hard sheet styrene. Foam will degrade if exposed to ultraviolet light (sunlight, or even some fluorescent lighting over a long time) so covering it with scenic materials and storing unused foam away from windows is important.

Because foam isn’t porous, wood glues and similar don’t work very well with it, although they can be used if the material isn’t going to be subject to mechanical stress. Specialty glues also exist, some with the same problems as wood glues. I normally use this in a way that lets me use carpenter’s wood glue. Some commonly-used railroading glues (like Liquid Nails or silicone caulk) may attack foam, although there are usually variants that are foam-safe; check the label.

Foam has several hazards to be aware of. Cutting (or shaping with a rasp) will produce fine dust with an electrostatic charge that makes it stick to things (wear old clothes). This isn’t something you want in your lungs, so wear at least a dust mask (I wear a painter’s respirator with particle filters myself) and keep a shop vac or good vacuum cleaner handy for cleanup (preferably one with a filter on the exhaust to keep all the dust in). It will melt under heat, which makes cutting it with power tools problematic, and normally I use a hand saw if I need to make a large or cross-grain cut. It’s also flammable, and emits a VERY toxic smoke. Don’t expose it to open flame under any circumstances, and storing it indoors where it could be burned in a house fire is a really bad idea. Foam built into a layout and covered with scenic materials is still dangerous in a house fire, but it will take time for it to catch

Expanded Polyurethane

I don’t have any personal experience with this material. Basically you spray it into place on the layout, it expands, and once cured you can cut and shape it like other foam. When curing it will emit gas. The modern kind emits a fluorocarbon gas that’s only harmful in very large quantities (ventilate the layout room while it’s curing to get rid of the smell and it should be perfectly safe). Older forms emitted formaldehyde, but that stuff isn’t around any more.

I’ve heard that this can have problems with the size changing over time, although many modelers have used it successfully.