And please note for the record here, pressure treated lumber chemistry is bad long term for aluminum. I spent a little extra ($80) for a sheet of marine grade plywood and have not regretted it. I also coated it the plywood with 4 coats of vinyl ester fiberglass resin with first coats thinned with xylene for best penetration... That sheet will last another 35 years w/o fail for sure
Please make sure you fabricate a cardboard template to truly catch the entire interface curve of the trailer end, I worked hard to salvage the rotten flooring to use and my old flooring as template led to more air gaps and partial overlap then I expected.