Roadbeds made easy with foam

Bill Darnaby presented his ideas for using foam board in layout construction in the June 1994, March ’95, and September ’96 issues of Model Railroader. He also demonstrated how layers of foam could both replace the traditional plywood base and serve as the roadbed under the track. Most new ideas take time to catch on, but now we’re seeing a steady increase in layouts built using foam construction methods.

Extruded foam board

The Styrofoam brand of light blue extruded polystyrene insulation is made by Dow Chemical Co., but it has many competitors that market similar products in other colors. Extruded-foam board is made in rigid 2 x 8 or 4 x 8-foot panels for use in the construction industry. (In warm southern states, finding a retail source of the extruded-foam board can be difficult, but most local dealers can special order it.)

Foam board is sold in building supply stores in standard thicknesses of ¾”, 1″, 1½”, 2″, 2½”, and 3″. This product is inexpensive, lightweight, easy to cut and shape with ordinary woodworking tools, and it’s strong enough in compression strength to be suitable for many model railroad purposes.

Don’t confuse the stiff extruded foam board with the white bead board type of molded Styrofoam used to make ice chests and many types of common packaging materials. This bead board is much softer, has little strength, and has limited uses for model railroad applications.

Selecting the right material for the application is the key to success in building a layout. For all of its advantages, extruded foam board isn’t suitable in every instance, and it does have some disadvantages. It’s best suited for model railroads where the track is relatively flat. While it can be used in layouts that have grades, the construction of grades using foam board is much more involved. For short grades, you may wish to use Woodland Scenics foam riser incline sections. However, for major grades on a layout, plywood is a better alternative.

Since foam has little shear strength, extra steps must be taken if you want to attach operating accessories, like switch motors or gate mechanisms, to it. The most common complaint is some applications of foam board transmit more sound than plywood. While this may be true, I’ve found things quiet down once the scenery and ballast are in place.

Safety first

Common sense dictates that reasonable personal safety precautions should be taken when working with foam. Whenever cutting or shaping tasks create foam dust, wear a dust mask or respirator and eye protection. If you use a hot wire foam cutter, wear a respirator to protect against the fumes from the melted foam. And it’s always best to work in an open area that has good ventilation.

If the only support is an open grid structure, I recommend using a minimum of 2″ thick foam. If the foam is laminated on top of a board, the wood provides the primary support, so thinner (¾”, 1″, or 1½”) foam may be used. Even if the thicker foam isn’t needed for strength on top of the wood, you may still want to use it to provide enough depth to carve out streams and scenery effects below track level.

2. Working with foam board

There are a variety of ways to cut the foam depending on its thickness. For the thicker panels (1½” or 2″), I prefer a saber saw with a fine tooth blade. If you use the saw, wear eye protection and a dust mask to avoid inhaling the fine foam dust.

The trick to sawing the thicker panels is to go slowly and let the blade do the cutting. Avoid pushing the saw. If the blade overheats it will melt the edges of the foam, making a mess of the blade and producing a rough cut.

The thinner (¾” or 1″) panels can also be cut with the saber saw or by using a sharp utility knife and a metal straight edge to deeply score the panel. (A dull knife chews up the edges of the cut.) I align the cut line in the foam over a straight edge, or a 2 x 4 standing on edge, and bend the foam down to snap it off in a clean straight line.

3. Foam board adhesive

For the best results, use the proper foam board adhesive to attach the panels to the benchwork. Most home centers sell a number of brands, but be careful to look for the foam board adhesive labeling on each tube. There are a lot of different types of adhesives in similar looking tubes, so it’s easy for browsing customers to mix them up in the bins. Checking the label will ensure that the adhesive you get is compatible with the foam board (solvent-based adhesives dissolve the foam).

4. Open grid benchwork

For open grid applications (above), I glue the foam board in place by running a ¼” bead of adhesive along the top edges of the grid.

Next, I carefully align the foam board as I gently set it in place and press the foam into the adhesive (below, left), making sure there aren’t any gaps.

Finally, I clamp and weigh the foam down (above) to obtain a tight joint and let the adhesive cure undisturbed overnight.

5. Shelf application

Attaching foam to a shelf is easy. I apply ¼” beads of adhesive in a grid spaced 2″-3″ apart across the wood shelf.

Then I seat the foam into the adhesive, clamp or weight the foam to secure it, and allow the adhesive to cure overnight.

6. Adding cork roadbed

Gluing cork roadbed to the foam is easy. I spread the adhesive caulk (above) into a thin layer along the track’s center line, press the cork into the caulk, and allow it to cure. I also use the caulk to secure the roadbed and track.

Some situations, such as lightly used branches and industrial spurs, don’t have much ballast, so I model this trackage by gluing the track directly to the foam with ordinary white glue.

White glue is stronger than most people think, so a thin bead is all that’s needed to secure the track. Just be aware that white glue will reactivate with the use of water when ballast is applied, so it may temporarily release its hold on the track.

7. Shaping land contours

Prototype track may be level, but the ground around it includes shallow undulations and depressions. Although anything that will cut the foam will work, my tool of choice is a small belt sander and coarse sandpaper.

By holding most of the sander’s weight in my hand I can gently work it over the foam with a lot of control and leave a smooth finished surface. Be sure to wear a dust mask or respirator during this step. Extending scenery upward from a foam base is a matter of cutting and shaping chunks of foam and gluing them to the base.

A modeler may either glue rough blocks to the surface and shape them later on, or carve a block to shape first and then glue the sloping block in place (right).

8. Finishing the surface

Fitting the various panels together or adding elevation with blocks of foam normally leaves some open gaps between the pieces. Filling these 1⁄8″ to ¼” gaps (above) is relatively easy using joint compound (drywall mud) smoothed with a wet paintbrush. After the mud dries, the top layers of scenery will completely conceal these joints.

Once I’m satisfied with the contours and have the gaps filled, I paint the foam prior to applying my final layer of scenery. I’ve found this base color isn’t critical, but I do try to match the basic scenery color that will follow. I use either acrylic craft paint (left) or household latex paint for this step.

9. Installing accessories

Mounting hardware on the foam requires a little planning, as foam board won’t hold screws. Some other provision must be made to mount switch motors and other devices under the layout. My solution is to glue small pads of wood to the bottom of the foam at the locations where I plan to mount the accessories. I use ¼” thick untempered hardboard or medium-density fiberboard (MDF) for the pads. Larger pads help spread out the pressures and their larger gluing surface means they’re less likely to pull loose.

I spread a liberal amount of foam board adhesive on a pad, push it into place against the foam and tape it securely until the adhesive dries (usually overnight). Once the adhesive hardens, the pad is ready for any mounting screws that may be needed.

The switch motor shown was installed on a 3″ x 4″ rectangle of ¼” hardboard that I drilled with mounting holes for the screws. Once the pad was installed, I used four mounting screws to attach the Tortoise switch motor.

Foam board Q&A

Here are a few answers to the most common questions I get about using extruded foam bench tops:
Q: Is foam board construction a viable method to use for bench tops?
A: Yes. It’s seeing more widespread use and is an excellent substitute for plywood in the right situations.
Q: What thickness should I use?
A: If the only support is one of the open grid styles of benchwork shown on page 57, the 2″ thickness should be used. If the underlying support is a shelf (such as hollow core door or a 1 x 12 plank) any thickness can be used.
Q: If I place the foam board over open grid or L-girder benchwork, what spacing should I use for the joists?
A: Use 16″ (or less) centers.
Q: How do I cut the foam?
A: Thicker (1½” or 2″ thick) panels can be cut using a saber saw with a fine tooth blade. Thinner foam can be cut with a utility knife.

Lance Mindheim is a frequent contributor to Model Railroader and our annual magazines. He lives in Silver Spring, Md., with his wife, Cathy, and son, Zachary. He owns the Shelf Layouts Co., Inc. (www.shelflayouts.com), a custom layout building and design firm.