Trailer floors and walls are covered with either Oriented strand board (OSB) or plywood.  OSB is an engineered wood particle board formed by adding adhesives and then compressing layers of wood strands. Due to its lower cost, it is more commonly used in place of plywood. 

OSB has more consistent density and slightly heavier than plywood. While a sheet of plywood might be 5 to 7 plies thick, a sheet of OSB is made from as many as 50 strand layers packed and compressed into the same thickness. It has no weak spots like those that can be left in plywood when knotholes in adjacent plies overlap.

OSB takes longer to get wet than plywood but also takes longer to dry out. This tendency to hold moisture means it can degrade faster than plywood when exposed to chronic leaks.