ocalicreek said:
I've long wondered about sagging plywood...have you experienced this first hand, or seen it on another layout?
What exactly would cause the ply to warp? Moisture? Train weight?
Also, the foam and ply combo won't sag but laminating the soundboard and ply might?
Sorry to sound like I'm jumping on your case here, I'm really not. It's just that I have always heard stories or rumors of warping plywood (usually 1/2") and have never seen any hard evidence or heard any first-hand accounts of this actually happening. I'd love to be proven wrong here just so the truth can come out.
It's kinda like that homasote swelling/shrinking myth.
(see
http://www.homasote.com/strength/index.html)
Galen
Don't worry about jumping on my case. I like to question information put out as "fact" as well, especially when I'm a little supicious for some reason.
I've worked with plywood in various structures - shelves, model railroads, and boats with thicknesses ranging from 1/4" to 1.5" (2 3/4" pieces laminated together).
Horizontally mounted 1/4" plywood without sufficient closely spaced supports will sag between supports. It will sag more with any kind of weight being put on it, whether by leaning or stuff being put on it. In my experience, once it sags in this application it will not return to flat without being taken up and re-installed. Finally, cheap grades of 1/4" plywood are more prone to warping than 1/2" or thicker plywood due to only 3 plys instead of 5 or more.
I have successfully used 1/2" plywood supported every 16-18" with joists of 1x3 or 1x4 on several layouts. Some had bare plywood, some had Homasote glued to the plywood. My findings concur with Westcott's book on benchwork.
The real problem is sagging of as little as 1/50 of an inch (0.020") is enough of a vertical irregularity to allow model flanges to climb the track under the right conditions. Giving ourselves a safety factor, I allow no more than 0.010" sag under load.
But you really should run sagging experiments for yourself. Get a couple of bricks, place them under a 3" wide, 3ft long strip of your chosen plywood. See how far you can move them apart before you get a 0.010" sag with at least a 50 lb load centered between the bricks (or simply push down hard to simulate the load). The load is to simulate an accidental lean on your part, which will happen sooner or later. I personally believe a 100lb load would be more representational of what could happen. And if you want to be able to walk on your layout, go with 300 lb load.
To avoid sagging with 1/4" thick plywood by itself, I'm willing to wager you are going to need the supports closer than 12".
The sound absorbing board I saw at Home Depot had very little rigidity of its own. The same is true of the Homasote strips I saw. Both were easily bent into curves. I would not count on either to add much strength to my 1/4" plywood unless installed as a filler in a sandwich configuration with 2 layers of plywood. This is how a door gets its incredible rigidity from 1/8" plywood skins. Cardboard filler and wood strips hold the plywood skins over an inch apart. This gives the door most of the bending resistance of an equivalent thickness of solid plywood at a fraction of the weight.
Extruded foam (not beadboard!) sheets or strips of 1" thickness or more have a surprising amount of bending resistance. Check it yourself. So a lamination of foam and 1/4" plywood will combine the bending resistance of both together.
my thoughts, your choices