by Arne » Tue Sep 08, 2009 8:31 am
If you look at the new style beams used in houses, they have 2x3 top and bottom and vertical 1/2"ply in between..
Where I did not need the x member to be a mounting place, I cut the out of 1/2" ply, 1.5" tall and mounted them vertically.
The strength comes from the top and bottom sheets, not the x members.
I did use 2x3 where I wanted to hang the spare tire, mount the galley wall and needed them to bolt the floor to the frame. Also at the front and rear where I had to attach the 1/8" ply roof/ceiling sheets. Looking back, I could have use 2x2 instead of 2x3s.
I think half of the x members are 1/2" ply. The floor has no flex because the 1/8" sheeting on the bottom of the floor would have to stretch, and the load is not heavy enough to cause that to happen. All x members were glued in place.