It is always wise to use pressure treated boards when flooring a trailer. Any material that you would brush on the surface would not penetrate very far into the wood. Might help some if you would surface treat it two or three times each year.
The more structure and support you have on the bottom, the thinner you can go. 0.045″ would be the thinnest we recommend. If you don't have a strong support on the bottom or you have beams, then you should go thicker. The most popular thicknesses would be 0.188″ (3/16″) or 0.125″ (1/8″), and even 0.25 (1/4″).
You MUST treat the ends and any cuts with 3-4 coats of preservative AND be sure there's a gap between the boards ! ANY 'standing ' water will eventually rot a board. 3rd. clean the trailer top and bottom !,keep deck tilted,in the sun and wind. If it's dirty and under trees, moisture invites mold and bugs and rot...
Marine grade plywood is great, but it's not intended to span much distance unsupported. Solid wood boards are much more prone to cracking, but will probably be thicker and able to support more load without deflection assuming you get the right kind of boards.
For trailer floors and wagon purposes, marine-grade plywood is recommended due to its resistance to moisture and durability. It's designed to withstand the elements, making it suitable for outdoor use. If budget is a concern, exterior-grade plywood can also be a viable option.
Plywood floors are stiffer than OSB floors by a factor of approximately 10%. As a result, OSB floors are more likely to: squeak due to floor movement; cause hard floor surfaces to crack (such as tile); and.
Do you have an enclosed trailer that needs a little lovin'? Try installing an epoxy floor! An epoxy floor will help keep the trailer looking nice, and since it's so easy to clean, your trailer will stay looking great!
White oak is typically what is used for trailer floors. Not so much for weather resistance, but for strength. Unless he's hauling some really heavy equipment or driving a forklift on the trailer, might be able to use treated lumber. Cheaper and last longer than oak.