I started on my ceiling this morning and first tried the metal on the existing strapping but it did not work for me. Perhaps the metal that I bought was too light
After trying several things, I settled on screwing 2x3s crosswise over the strapping and running from the front to the back. I wedged the ends into the gaps on the wall framing at the ceiling at the front and the back of the trailer. I had to notch the ends of the second piece in from the side as the gaps narrow and I had to notch under several of the existing "beams" at the front end.
I have done only 2 pieces so far and the roof already feels stronger. I also wedged pieces of 2x in the gaps between the new wood and the roof, using shims where necessary to correct the concave areas. That is very successful.
In total I think I will be putting in 6 of this cross pieces.
I am then putting 1/4 inch plywood over those.
I have no experience to back it up but I just think that using thick plywood would just add more weight
to the ceiling and would not help with re- establishing the curve or correcting areas where it has become concave.
I have had to sacrifice 3/4 inch of head room but my roof was really in bad shape, like someone had been crawling around on it. I live in an area that gets a lot of snow and I really needed to curve the roof as much as possible and to firm it up.
I will take pictures when I am done.