There are three heating ducts stacked together which means the soffit needs to span 54 inches from the wall to the to the portion coming down from the ceiling joists. The portion coming down from the ceiling only needs to be 12 inches to cover the ducts. The ducts span 40ft across the main load bearing wall. The question is do you build a plywood wall for the 12 inch wall, attach a ledger board and then connect the two with 2X4s or can you build plywood all the way around. What is the better option to give support and reduce potential sag due to weight? Thanks for the help.