To Build or Not Build a Frame?

[nachoman]

Question #1 - how thick is your foam, and what type of foam is it? If it is the blue or pink extruded type of foam 1.5 or 2 inches thick, you are probably okay going without a frame at only 3'x5' size. If it is the white foam, or less than 1.5 inches thick, I would recommend a frame. In my experience, thinner foam or the white foam is will sometimes bow slightly and needs support. The white foam isn't nearly as strong, and should have a frame no matter what the thickness.

I would not use plywood for a frame. Instead, I would build a frame out of 1x2 lumber. At that small of a size layout, I would simply build a rectangle with dimensions 2'x4', and glue it underneath the foam, leaving a 6" overhang on all sides. I would upgrade to poplar lumber as opposed to pine. It is straighter and has less knots. Putting an entire sheet of 3/4" plywood underneath the layout as a "frame" would be much heavier and not nearly as strong.