So would you entire layout, be sitting on Homasote, or Fiberboard, after plywood, or bench work???
Only if you want it to.
Solid flat top construction is not my favorite. There are various methods of creating open-top grids to support the subroadbed and/or roadbed at the needed intervals.
Open top grids have the advantages of being cheaper, more conducive to realistic scenery (not flat scenery), and more conducive to track grades and multiple levels. Flat top construction is generally simpler, as there is no cutting of the subroadbed or roadbed. And structures are simpler to place on a flat top.
On many of the Model Railroader project layouts, cookie cutter construction was used. This provided a flat plywood top in the early stages of the layout. Later, the plywood would be cut out with a jig saw where it wasn’t being used to directly support scenery or track (the result looked somewhat like what a cookie cutter does to cookie dough, hence the name). The resulting ribbons of plywood could be bent upwards or downwards to make grades, or supported at different levels.
Since I use cookie cutter or open grid, the plywood and Homasote are ribbons cut to fit under the track.
my thoughts, your choices
Fred W
Flat Table Top?
Eh? What is that!
About half ofmine is on plywood, the other half is on 2" foam board on a grid if 1x2 boards. My friend uses homasote splines attached to his benchwork. Whatever works best for you, man! And before you ask, you find out by trial and error. Why do you think I have two different methods?
My as-yet scenery-free layout looks like the Mount Everest ride in Disney World’s Adventureland. Or, rather, the way that attraction looked before the outer skin was attached - a steel structure with tracks winding through it on various levels without apparent rhyme or reason.
Looking more closely at those tracks, the ones which are supposed to remain unseen in the Netherworld have (from the bottom up) a layer of THIN plywood, a layer of THIN extruded foam, a layer of cardstock track template, ties and rails. Ties and rails may be flex track or wood ties with file-sculpted rail soldered into specialwork. Ballast is absent.
Tracks which are intended to be visible have wider plywood, the foam is sculpted into roadbed contouurs and the track will be ballasted - after the local landforms are reasonably complete.
Most of the area inside the fascia lines is air from deck to overhead - the deck being the concrete floor that holds up the benchwork’s legs. As landforms are added, other flats will be incorporated to support buildings. Those will probably be plywood, with scenic treatment on top and steel angle iron below to insure that the levels STAY level.
Tabletop? Well, at the moment, there’s a temporary worktop (1.5 inch foam) on the joists of a track-free portion of the table that will eventually support two narrow gauge termini and the Haruyama station. Other than that, I have a couple of roller-equipped work cabinets tucked below the L girders.
That’s what happens when you model a place where the scenery stands on edge.
Chuck (Modeling Central Japan in September, 1964)