Do you have a scale ruler? With the foam being 1.5 inches thick ( 20 feet in N scale ) and with your roadbed the drop below grade should end up being around 25 feet in N scale. If more drop is wanted below the bridge you could always cut out a section of the subbase and fill in under the cut with left over foam and carve the creek bed out of that below grade. Or if you want add a another layer of foam and start again.
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fluff wrote:need some opinions. i have built several small n scale layouts and never even came close to finishing one. i always used 1 inch foam and 2 layers of it. i now have what i have always wanted, 12x10 around the walls varying from 16 inches to 24 inches wide. i used 3 layers this time with 1/2 inch foamboard on top of 1/2 plywood. that leaves a max of 1 1/2 inches below the track. is that enough or should i add another 1/2 in layer? it looks "shallower" than i expected when i cut out for a bridge. i like the 1/2 in stuff because for me its easier to vary different levels below track. i also didnt realize how much everything costs now or i probably would have used the 1 in to save on costs. still cant post pics....thanks
For those areas that you need much deeper, just cut out the plywood and block down to the base of whatever the scenery is to be. In n scale, I doubt that a river or even a smaller ravine would need much more than an additional inch or two.
The only thing to keep in mind is the position of your joists. Locate these cut out scenery items between the framework.
If you're concerned about the structural integredy of the layout for a fairly large or long cut out, then make sure that you glue and screw blocking and the plywood to bridge the gap.
This is one advantage to L girder, where the elevations can drop feet if you need to.
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