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namelus, How many shipping containers? In a SHTF were everyone is inside you need a bathroom so you need to build a septic tank. Also you need one or two air supply with filters. Need water tanks to store your water. For cooking I like gas tanks in the ground. Also need to have an other way out of the shopping containers.

The sheet metal roofing needs to go on an 20 to 30 degree angle so the rain or snow goes down.
I would do 12 inches of rock, cement them down.

The 6 inch foam is very good and adding 14 gauge thick steel walls is also very good. On the inside wood is good but try not to use to much drywall. Maybe buy Durock and plaster it like an outside of a house this is water proof so if water gets in there will be no problems. Also Durock is stronger.