Lumber boards can be straight side boards, ship lapped or tongue and grooved, treated with preservative or not and nailed, bolted or screwed to the chassis. There is no standard size, and whatever is available is dependent on what your local mill produces or outlet can source.
Source a piece of lumber that is straight, free of knots or splits and has ideally at least one nice clean machined face. Check that the boards are not cupping, and if they do, position the boards so that the cup is facing downwards onto the chassis. The wider the boards are, the more severe and more likely they are to cup. A good standard board size for decking is 6” x 2.
Thickness is dependent on the type of load to be carried. Transporting horses or stock require boards at least 1 3/8 " thick (a 2" board which has been machined or “dressed” will come in at around 1 1/2") ship lapped (to give the boards additional strength) and free of any defects (splits, knots, warping and twisting).
Transporting machinery or items with a high point loading (heavy machinery with very small weight bearing surfaces or feet) can get away with a minimum of 1 3/8" boards but may require additional chassis support once the load balance points have been found.
For general loads with large weight bearing surfaces and with good chassis support (chassis cross members 18"-20" apart) a good hardwood domestic decking material can be used (3/4"+ thick). The deck will be stronger if it is ship lapped or tongue and grooved (to prevent individual boards bending under weight).
Any hardwood decking will need to be pre-drilled before screwing or nailing down to the trailer, and it will make life a little easier to pre-drill softwood decking material as well unless you have access to an industrial screw machine.
Nailed you ask?? One of the quickest ways of attaching lumber boards to a trailer is to fit 2” x 2” lumber battens to the inside of the cross members with coach bolts and using serrated ring shank or screw shank nails, nail the boards straight to the battens. If using treated lumber boards, use hot dipped galvanized coach bolts, screws or nails for fastening the decking down as the treatment chemicals can prematurely corrode plain steel fastenings. Zinc plated fastenings should not be used as their coating is too thin to offer any protection. Stainless steel fastenings can be used but you may have to take out a small loan out to cover the cost.
Another point to be aware of is that boards will expand when wet and shrink when dry. If you fit wet boards to your trailer, they will shrink once they dry out and you will have gaps either side of the boards, which can trap any loose material. You may also find that the fastenings you used to fix the boards to the trailer may be sitting proud of the deck. All should settle down again when wet but ideally the best time to fit boards to a trailer is when they are relatively dry and leave a 1/8" expansion gap along the sides of the trailer and around 3/16" gap at the tailgate ends.
When fitting boards to the trailer, dry fit the boards across the width of the trailer and measure what gap is left. Divide the gap in half and rip a board down to fit along both sides. Try to rip the boards so that the first and last board are roughly the same width.
Clamp the first board to the trailer (with a 1/8" packer between the trailer side and the board)making sure that the board is almost flush with the front cross member, drill and screw (or nail), clamp the second board up to the first board, drill and screw and repeat.
If the trailer is to be used in the wet or muddy conditions on a regular basis, it may help to coat the underside of the boards with an oil based sealer or similar prior to fitting to the trailer. Most trailer decks will deteriorate and rot from the underside and any additional protection to the deck will help.
Once the deck is fitted, give the top surface a quick skim with a sander to take any high spots off and either seal as per the underside or leave to weather.