Is the old roof one of the multi layer ie: 1/4"plywood, ridged foam, then 1/4" plywood again? If it is I would try to replicate this as opposed to the 2x4 structure. Another option would be to make it with 2x2 lumber to cut down on weight. I would think you may over load the frame by using 2x4's on the roof if its currently built with the multi layer roof.
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~Andrew
99 F250 Super Cab Short Bed 7.3 Powerstroke
84 24' Yellowstone Camino (oldie)
https://rvpdi.com
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