I am posting a contrarian approach to Homosote. I like and use Homosote, but I did not put plywood under it as usually recommended. I built a layout in the 1980s before forums and I read about Homosote in MR, but not the practice of putting plywood under, so I built the layout with the Homosote directly on the bench structure with no plywood from lack of knowledge. Did not use glue, but screwed down probably on one foot center (cant remember for sure). This was in an attic subject to extreme heat and humidity variation, which theoretically is a recipe for disaster with what I did. The layout was up for two decades without any problem no warpage no change. It worked great for decades. I posted this on this forum years ago and people said I was lucky and this was a bad plan.
Two things contribute, I think. Bench modules were 8’ by 30” (I think), but I did put supports across the 30” on 16” or 24” centers and I did screw down to these cross members too, not just the perimeter. The key thing however is that I painted the Homosote. Top side green finish paint, bottom sealed with old junk paint. I was careful to paint the edges. I think this kept the humidity from warping it (along with screws) and gave a finished appearance to the final layout. Thinking about doing it again on my next layout because it worked.
YMMV
Shields up; incoming fire
Bill
