Sandwich Baseboard:My current layout under construction is primarily an around the walls structure with one island section. The frame is suspended from the walls with no legs. This facilitates clear access underneath for wiring and also keeps the floor clearer. Previously I have used aluminium square tube as a frame and styrofoam as a baseboard. This worked very well but had one limitation which we will discuss later. My current frame is a traditional open grid box frame of 75-100mm x12mm structural plywood. Keeping everything lightweight was important.
On a previous layout and also on a diorama I built, whilst the current house was being completed, I used a sandwich design of styrofoam between boards as the base. This method uses timber on either side of 15mm styrofoam paneling. My first effort was based on 3mm MDF timber as the "bread" around the 15mm stryrofoam sandwich filler. This worked well. I did seal the MDF with paint before installing in case water penetrated the glue of the sandwich.
On my current layout I decided to be more conservative and use 3.5mm plywood, as the bread. Luan plywood or MDF would save money and probably work as well. I started with 2400 x 1200 x 3.5mm plywood, and glued 15mm styrofoam sheets between the plywood sheets. I experimented with several glues. You can buy special glue for styrofoam but in the end I found full strength water soluble PVA white glue very effective. First lay out the first sheet of plywood on a flat surface and coat the whole area with glue. Spread it with a brush. Use water if necessary (sparingly so as not to dilute the glue too much) to help the glue spread. Next step is to lay the foam sheet on top of the plywood. Then spread glue over the styrofoam sheet and lay the top sheet of plywood on top or alternatively spread glue on the second plywood sheet. Use weights to compress the sandwich over 24 hours whilst the glue dries.
The sheets of sandwich material once made could then be cut into the various sections, just like cutting regular plywood. For straight cuts use a circular saw. For shaped cuts use a jig saw with a fine blade. Before installing any sections and after the sections were cut I sanded and then sealed the edges with "No More Gaps". After the No More Gaps dried I painted the whole sandwich section to help seal it. Painting the sandwich an earth colour would be beneficial for future scenery applications.
The sandwich section is slightly lighter than say a 12mm plywood section and just as stable - perhaps more so. The only downside is the sandwich is 21-22mm thick versus a 12 mm plywood base. However if there is a problem such as for sections where track passes underneath and you need maximum clearance you can cut the lower elements of the sandwich and create only a 3.5mm overhead height if required.
I have had this design in use for nearly 10 years and many colleagues also have used it successfully.
There are some advantages over dense styrofoam and plain plywood.
With the sandwich of plywood on both sides and styrofoam in the middle we have timber top and bottom which you can easily screw into. This is handy for mounting small items under the layout like wire hangers and switches. The weight of the sandwich overall is lighter than plywood. Sytrofoam is good but when you want to attach something to it having a plywood base helps. Of course all methods have positives and negatives, but the sandwich model works well for me.
Here is a section laid on some risers. This section is not sealed on the edges with No More Gaps, and is also not painted so you can easily see the construction. The sandwich will accept a wallboard / chipboard screw to attach it to the riser and keep it stable.