At age 12 I helped the old man build a trailer for transporting 1/2 tonne bins of spuds on the farm. The trailer was made by cutting the cab off an old Chev truck, probably about a 5 tonne unit. A heavy drawbar was fitted where the engine used to site and the front axle ripped out.
The deck was made by surrounding it with inverted 3" angleiron. The deck was laid with 1 1/2" Jarrah (Eucalyptus Maculata) a very very durable aussie hardwood. The Jarrah was oiled heavily with Linseed and diesel.
The Jarrah boards are simply captured at the ends under the angleiron...no screws or bolts on the ceck, fully free floating. If you need to replace a board you just unbolt the angle at one end and take the jarrah out.
A few weekends ago I was down the farm looking at that trailer with my nephew who is now the "farmer" in the family. He was in awe when I told him that the chasis was older than me, and that it had only been a farm trailer for 31 years in its second life. That Jarrah deck has only been replaced once in all that time and it takes a hell of a beating.
The old Chev diff is still going round and round too
BTW, if you are bolting a timber deck down to a steel frame, elongate the bolt holes in the timber to allow for longitudinal expansion.