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.
Plywood Trailer Flooring. Plywood is a versatile trailer flooring material for a variety of commercial vehicles. Birch is a common wood type for this application because it is wear-resistant, lightweight, and abundant.
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.
Popular options include plywood, aluminum, rubber, composite, and steel. When you need to replace or trailer decking, knowing the characteristics of these materials and the most important factors to consider can help you pick the right choice.
Rough sawn hardwood (oak, etc) treated with a mixture of used motor oil and diesel makes a great trailer deck. A little more labor going in but with regular maintenance (recoat every other year) will last longer then PT softwood.
The wood itself won't expand in length when it gets wet or hot but it will swell slightly in thickness and width when the truck floor is exposed to moisture. A 1/8" gap edge to edge between deck boards is generally recommended for center frame and outside frame trailers.
Your best bet is to use pre-treated lumber, the kind used for decks/porchs available at Home Depot, Lowes, and most lumber yards. This type of wood will hold up better to moisture which your trailer will be exposed to underneath the trailer, weather elements and from your horse's urine.
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.
Do not use pressure treated lumber on an aluminum boat or for trailer bunks. The chemicals in the lumber will react with the aluminum causing corrosion and holes in the boat. I made a small v bunk for the bow of my boat and inadvertently used 2 pieces of pressure treated 2``x6''.
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...